Activation of the calcium-sensing receptor attenuates TRPV6-dependent intestinal calcium absorption

Calcium (Ca²⁺) homeostasis is vital to many physiological functions and is thus tightly regulated by altering Ca²⁺ transport across intestine, kidneys, and bone. It has been appreciated for some time that endocrine hormones, including parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D₃ (1,25-[OH]₂ D₃), alter Ca²⁺ transport across the intestine and kidneys or aid mobilization from bone (1–4). However, more recently, the homeostatic mechanisms permitting direct sensing of extracellular Ca²⁺ by the nephron or bone and subsequently altering tubular Ca²⁺ reabsorption or bone remodeling were delineated (5, 6). This direct sensing of extracellular Ca²⁺ occurs, at least in part, by the 7-transmembrane G protein–coupled Ca²⁺ sensing receptor (CaSR) (7).

PTH release from the parathyroid gland increases plasma Ca²⁺ levels through direct effects on the nephron and bone and indirect effects on the intestine via stimulation of renal CYP27B1 activity, which catalyzes the synthesis of 1,25-[OH]₂ D₃ (8–11). PTH secretion is regulated by the CaSR, where increased extracellular Ca²⁺ activates the receptor, inhibiting release of PTH (12–14) and hence formation of 1,25-[OH]₂ D₃. In the thick ascending limb (TAL), blood Ca²⁺ concentration is also sensed by the basolateral CaSR, which directly signals to decrease Ca²⁺ reabsorption in that nephron segment (15–18). Conversely, PTH stimulates Ca²⁺ absorption from the TAL (18, 19) and transcellular Ca²⁺ reabsorption from the distal convoluted tubule (DCT) and connecting tubule (CNT) (20–22). These studies highlight how the renal tubule both responds to endocrine regulation, and directly senses extracellular Ca²⁺, to amend Ca²⁺ reabsorption, thereby preventing hypercalcemia.

The duodenum, cecum, and proximal colon are sites of significant intestinal transcellular Ca²⁺ absorption (23, 24). Transcellular Ca²⁺ transport is a unidirectional, ATP-driven process mediated, at least in part, by the apically expressed channel TRPV6, the intracellular Ca²⁺-buffering protein calbindin-D_9k (CABP9K), and the basolateral Ca²⁺-extruding proteins plasma membrane Ca²⁺ ATPase 1b (PMCA1b) and Na⁺/Ca²⁺-exchanger (NCX1) (23). Hypocalcemia leads to increased PTH secretion, which stimulates the production of 1,25-[OH]₂ D₃ and thus increases intestinal Ca²⁺ transport (8–11). 1,25-[OH]₂ D₃ increases intestinal Ca²⁺ absorption by increasing the expression of TRPV6, a phenomenon that correlates with intestinal Ca²⁺ absorption (24–26). The resulting increased Ca²⁺ influx in turn enhances the expression of CABP9K (27–29). Conversely, hypercalcemia inhibits PTH release and consequently reduces intestinal Ca²⁺ uptake, by limiting active 1,25-[OH]₂ D₃ synthesis. However, this latter regulatory mechanism would be rather slow with respect to attenuating hypercalcemia.

The CaSR is expressed throughout the intestine (30–32), where it regulates fluid, sodium, and chloride secretion (32–34). However, a direct role in Ca²⁺ homeostasis has not been reported (7, 33). We hypothesized that the intestinal CaSR has a functional role in maintaining Ca²⁺ homeostasis, where it detects extracellular Ca²⁺ levels and directly alters transcellular Ca²⁺ absorption across the sensing intestinal epithelium in response. To test our hypothesis, we first examined the expression of transcellular Ca²⁺-transporting proteins following chronic CaSR activation and found decreased expression of genes known to facilitate transcellular Ca²⁺ absorption across the intestine. We further observed that acute pharmacological or physiological activation of a basolateral CaSR in intestinal epithelium ex vivo attenuated transcellular Ca²⁺ absorption. Moreover, this attenuation was absent in transgenic mice expressing functionally inactive TRPV6 Ca²⁺ channels. Together, our results demonstrate that basolateral activation of an intestinal CaSR directly inhibits local Ca²⁺ absorption from that intestinal segment via TRPV6.

Activation of an intestinal CaSR decreases expression of genes involved in transcellular Ca²⁺absorption. The expression of genes mediating transcellular Ca²⁺ absorption was measured on intestinal tissue from FVB/N mice fed a low (0.01%), normal (0.6%), or high (2%) Ca²⁺ diet for 21 days. Trpv6 mRNA expression was increased in mice fed a low-Ca²⁺ diet, with the greatest, greater than 30-fold increase, observed in the proximal colon (Figure 1A). A high-Ca²⁺ diet suppressed Trpv6 expression in the duodenum, cecum, and proximal colon, perhaps because of low 1,25-[OH]₂ D₃, although a direct inhibitory effect of plasma Ca²⁺ cannot be excluded. The same relationship, between increased dietary Ca²⁺ content and reduced gene expression, was observed for S100g, which encodes the intracellular Ca²⁺-buffering and -shuttling protein CABP9K. The mRNA expression of the basolateral Ca²⁺ efflux transporters NCX1 (Slc8a1) and PMCA1b (Atp2b1) were unaltered in all tissues under different dietary calcium–containing conditions (Figure 1, A–C).

Figure 1

Relative intestinal mRNA expression of transcellular Ca²⁺ transport mediators under altered extracellular Ca²⁺ conditions. (A–C) Relative mRNA expression of transcellular Ca²⁺ transport mediators TRPV6 (Trpv6), CABP9K (S100g), NCX1 (Slc8a1), or PMCA1b (Atp2b1), normalized to 18S rRNA expression in mice on high-, normal- (Con), or low-Ca²⁺ diet for 21 days (n = 7 for each diet). (D–F) Relative mRNA expression in animals treated with 1,25-[OH]₂ D₃ (VD) or vehicle (Veh) (n = 8 for each). (G–I) Relative mRNA expression in animals treated with cinacalcet (Cin) or control (Veh) diet (n = 8 for each). All data are presented as the mean ± SEM, normalized to the mice on the normal/control diet. Asterisks indicate a statistically significant difference from the normal/control mice by 1-way ANOVA (all genes in A and Slc8a1 and Atp2b1 in B and C), Brown-Forsythe test (S100g in B), Kruskal-Wallis test (Trpv6 in B and C), or Student’s unpaired t tests (D–I); *P < 0.05, **P < 0.01, ***P < 0.001.

The serum Ca²⁺ of mice fed altered-Ca²⁺ diets was not different from that of mice fed a normal-Ca²⁺ diet (15). This was likely the result of altered 1,25-[OH]₂ D₃ production induced by varying Ca²⁺-containing diets (28, 35, 36). To assess the effect of 1,25-[OH]₂ D₃ on the intestinal expression of genes mediating transcellular Ca²⁺ transport, mice were directly administered (via intraperitoneal injection) 1,25-[OH]₂ D₃ (500 pg/g body weight) for 5 days and the studies were repeated (15). This increased expression of Trpv6 and S100g (Figure 1, E and F). These data are consistent with the observation that 1,25-[OH]₂ D₃ enhances intestinal Ca²⁺ absorption via increased expression of TRPV6 (37). Interestingly, the increase was less pronounced than what was observed on a low-Ca²⁺ diet, even though serum 1,25-[OH]₂ D₃ levels were increased to a greater extent (15). However, the 1,25-[OH]₂ D₃–injected mice were markedly hypercalcemic, potentially attenuating the increased expression induced by 1,25-[OH]₂ D₃ (15).

To examine the effect of CaSR activation on Trpv6, S100g, Slc8a1, and Atp2b1 expression, we administered the calcimimetic cinacalcet (1 mg/g body weight) for 5 days. Trpv6 expression was reduced in cinacalcet-treated mice (Figure 1, G–I). Importantly, PTH and serum Ca²⁺ were substantially lower in cinacalcet-treated mice; however, 1,25-[OH]₂ D₃ levels were not altered by cinacalcet (15). In addition, expression of S100g and Atp2b1 was reduced in the cecum of cinacalcet-treated mice (Figure 1H), and all genes involved in transcellular Ca²⁺ absorption were decreased in the proximal colon of cinacalcet-treated animals (Figure 1I). Together, these results are consistent with direct Ca²⁺ sensing by the intestine decreasing transcellular Ca²⁺ absorption via decreasing the expression of genes mediating transcellular Ca²⁺ absorption.

Extracellular Ca²⁺ inhibits transcellular Ca²⁺ absorption in the proximal colon. Next, we sought to determine whether a direct Ca²⁺-sensing mechanism regulates intestinal Ca²⁺ absorption independent of calciotropic hormones. To do so, we measured net Ca²⁺ flux across the proximal colon ex vivo in Ussing chambers (i.e., net Ca²⁺ flux = unidirectional apical-to-basolateral Ca²⁺ flux – unidirectional basolateral-to-apical Ca²⁺ flux from the same segment and animal). We chose to study this segment initially as it had the largest changes in expression (Figure 1), in addition to significant 1,25-[OH]₂ D₃–mediated regulation of transcellular Ca²⁺ absorption, as well as greater sojourn time and thus Ca²⁺ availability (29, 38, 39). Importantly, measurements of Ca²⁺ flux made in Ussing chambers enabled us to avoid the confounding effects of calciotropic hormones. The buffer bathing the tissue contained equal concentrations of Ca²⁺, and the transepithelial voltage was clamped to 0 mV. This eliminated a net driving force for paracellular Ca²⁺ movement, enabling us to attribute net flux to movement through the transcellular Ca²⁺ transport pathway. The net Ca²⁺ flux obtained under condition A (control) was compared with the one obtained under condition B (i.e., bilateral application of cinacalcet, Figure 2). Figure 3A displays a typical short-circuit current trace recorded from a single channel. Bilateral cinacalcet administration significantly reduced net Ca²⁺ absorption (Figure 3B), consistent with the proximal colon sensing increased extracellular Ca²⁺ and attenuating Ca²⁺ absorption in response.

Figure 2

Protocol used to measure unidirectional Ca²⁺ fluxes across intestinal preparations. The transepithelial voltage across tissue preparations (y axis) was clamped to 0 mV for the duration of the experiment (x axis). The voltage spikes along the x axis correspond to 2-mV pulses applied and used to determine the transepithelial resistance (TER). We added 0.1 μM tetrodotoxin (TTX) basolaterally first and allowed the resulting short-circuit current to stabilize. At time 0, the solutions were exchanged for fresh ones with 1 side spiked with ⁴⁵Ca²⁺. Asterisks indicate the time points when samples were taken for radioactivity measurements. Two gray horizontal lines represent 15-minute time intervals, where unidirectional ⁴⁵Ca²⁺ flux was calculated for each condition. We added 10 μM forskolin at the end of the experiment to confirm tissue viability.

Figure 3

The effect of altering extracellular Ca²⁺ on Ca²⁺ fluxes across mouse proximal colon. (A) An example of the short-circuit current (I_SC) recorded throughout protocol 1. TTX was added and I_SC allowed to stabilize. The I_SC spikes occurred in response to 2-mV pulses. At the second arrow, solutions were exchanged, with 1 side only containing ⁴⁵Ca²⁺. The tissue was deemed viable if the I_SC increased more than 3 times with forskolin administration at the end of the experiment. (B) Changes in the net Ca²⁺ flux (net J_Ca²⁺) between condition A, pretreatment (Pre Rx), and condition B, vehicle (ethanol) or 10 μM cinacalcet (n = 6 each treatment). (C) The change in net J_Ca²⁺ between condition A, high Ca²⁺ (2.5 mM), and condition B, low-Ca²⁺ (0.5 mM), or the converse (n = 6 each). Raw values are presented; asterisks indicate a statistical difference between the conditions (Student’s paired t tests; *P < 0.05, and **P < 0.01).

To implicate physiological changes in extracellular Ca²⁺ regulating transcellular Ca²⁺ absorption, we again examined net Ca²⁺ flux across proximal colon ex vivo in Ussing chambers before and after changing the Ca²⁺ concentration in the buffers simultaneously under voltage clamp conditions (i.e., buffers in both chambers were exchanged from bilaterally containing solutions with high Ca²⁺ (2.5 mM) to solutions with low Ca²⁺ (0.5 mM) to eliminate a transepithelial electrochemical gradient for calcium under both conditions). When the extracellular Ca²⁺ concentration was decreased, net Ca²⁺ flux increased, and conversely, when the extracellular Ca²⁺ concentration was increased, net Ca²⁺ flux decreased (Figure 3C). These results further support the idea that the proximal colon directly senses extracellular Ca²⁺ and acutely alters transcellular Ca²⁺ absorption to maintain plasma Ca²⁺ within physiological limits.

Increased basolateral extracellular Ca²⁺ attenuates transcellular Ca²⁺ absorption. The CaSR is expressed throughout rodent and human intestine (32, 40–42), including in both the apical and basolateral membranes of proximal colonocytes (30–32). We confirmed intestinal CaSR expression by measuring mRNA via quantitative real-time PCR (Figure 4A). Next, to determine whether apical and/or basolateral Ca²⁺ sensing mediates decreased transcellular Ca²⁺ absorption, we measured net Ca²⁺ flux as above but applied cinacalcet to either the basolateral or the apical hemichamber. Apical application of cinacalcet did not alter net Ca²⁺ flux (Figure 4B). In contrast, basolateral treatment significantly decreased net Ca²⁺ flux (Figure 4C). Moreover, basolateral application of cinacalcet attenuated net Ca²⁺ flux across the duodenum and the cecum, other sites of transcellular Ca²⁺ absorption (Supplemental Figure 1; supplemental material available online with this article; https://doi.org/10.1172/jci.insight.128013DS1). These data are consistent with basolateral CaSR signaling decreasing transcellular intestinal Ca²⁺ absorption.

Figure 4

Expression of the CaSR and effect of apical or basolateral CaSR activation. (A) Relative mRNA expression of the CaSR throughout mouse intestine (n = 12), normalized to duodenum. (B and C) Changes in the net J_Ca²⁺ in the proximal colon of wild-type mice between condition A, pretreatment, and condition B, apical or basolateral 10 μM cinacalcet application (n = 7 each application in B; n = 6 in C). Raw values are presented; asterisks indicate a statistical difference between the conditions (Student’s paired t tests; *P < 0.05).

TRPV6-mediated Ca²⁺ absorption is attenuated by CaSR activation. To identify the channel regulating transcellular Ca²⁺ flux in response to increased basolateral extracellular Ca²⁺, we repeated the Ca²⁺ flux studies on wild-type (TRPV6^WT/WT) and TRPV6^D541A/D541A-knockin mice. These animals express TRPV6 with mutation D541A in the pore loop, rendering it nonfunctional (43). Interestingly, TRPV6^WT/WT mice had significantly greater net Ca²⁺ flux across the proximal colon under control conditions (condition A), compared with TRPV6^D541A/D541A mice (Figure 5A). Moreover, TRPV6^WT/WT mice reduced net Ca²⁺ flux in response to basolateral cinacalcet treatment, in contrast with TRPV6^D541A/D541A mice, where net Ca²⁺ flux was unchanged (Figure 5A).

Figure 5

Effect of extracellular Ca²⁺ on Ca²⁺ fluxes across proximal colon from TRPV6^WT/WT or TRPV6^D541A/D541A mice. (A) Change in net J_Ca²⁺ between condition A, pretreatment, and condition B, basolateral 10 μM cinacalcet application (n = 6 each). (B) Change in net J_Ca²⁺ between condition A, high Ca²⁺ (2.5 mM), and condition B, low Ca²⁺ (0.5 mM) (n = 6 each). Raw values are presented; asterisks indicate a statistical difference between conditions (Student’s paired t test for within genotype comparisons or unpaired t tests for between genotype comparison; *P < 0.05, and **P < 0.01).

Because TRPV6^D541/D541A mice do not display net Ca²⁺ flux at baseline, we sought to stimulate net Ca²⁺ flux by exposing proximal colon to high–extracellular Ca²⁺ buffer and then lowering extracellular Ca²⁺. Again, TRPV6^WT/WT mice had significantly greater net Ca²⁺ flux at baseline compared with the TRPV6^D541A/D541A mice (Figure 5B). As observed for wild-type FVB/N mice, when proximal colon from TRPV6^WT/WT mice was switched from a high- to a low-Ca²⁺ buffer, net Ca²⁺ flux increased. This was in contrast with TRPV6^D541A/D541A mice, where no change in net Ca²⁺ flux was observed. These results implicate TRPV6 in mediating transcellular Ca²⁺ absorption across the proximal colon in response to changes in basolateral extracellular Ca²⁺.

CaSR expression is sufficient for TRPV6 to respond to extracellular Ca²⁺. To understand how the CaSR may confer acute inhibition of Ca²⁺ flux through TRPV6, we sought to reconstitute the system in vitro. To this end, we expressed human TRPV6 and the CaSR in Xenopus oocytes and measured the Ca²⁺ current (I_Ca). We chose this system to study the effect of CaSR activation on TRPV6 activity because it lacks endogenous G protein–coupled receptors that are often present in mammalian cell culture models. Oocytes expressing TRPV6 alone failed to decrease I_Ca after incubation with cinacalcet (Supplemental Figure 2 and ref. 44). In contrast, oocytes coexpressing the CaSR and TRPV6 displayed a significant reduction in I_Ca after cinacalcet treatment (Figure 6A). These results are consistent with our ex vivo observation that CaSR activation inhibits Ca²⁺ flux through TRPV6.

Figure 6

Effect of phospholipase C inhibition on CaSR-mediated inhibition of Ca²⁺ absorption in in vitro and ex vivo. (A) Effect of CaSR activation on Ca²⁺-induced currents (I_Ca) in TRPV6 expressing oocytes in the presence and absence of cinacalcet and/or U73122, a phospholipase C (PLC) inhibitor (n = 6 each). Mean I_Ca values obtained from TRPV6- and CaSR-expressing oocytes were normalized to vehicle I_Ca values from TRPV6-expressing oocytes ± SEM; asterisks indicate a statistically significant difference between the conditions (multiple-comparisons Kruskal-Wallis test; *P < 0.05; **P < 0.01). (B) Effect of cinacalcet and/or U73122 on the plasma membrane expression of TRPV6 and CaSR in oocytes determined by immunoblot. As a loading control, β-actin was blotted (bottom). (C) Quantification of surface TRPV6 expression, normalized to total TRPV6 (n = 3 each). (D) Effect of basolateral cinacalcet (10 μM) and vehicle (DMSO) or PLC inhibitor U73122 (10 μM) on mouse proximal colon (n = 6 each). Raw values are presented; asterisks indicate a statistical difference between the conditions (Student’s paired t tests; *P < 0.05).

PLC regulates Trpv6 in vitro (45–49). We therefore measured normalized I_Ca in TRPV6- and CaSR-expressing oocytes in the presence of U73122 (5 μM), a PLC inhibitor, or in the presence of U73122 and cinacalcet. The PLC inhibitor increased I_Ca even in the absence of the CaSR (Supplemental Figure 2). Further, PLC inhibition increased I_Ca in the absence and presence of cinacalcet, implicating PLC inhibition in the CaSR-mediated decrease in TRPV6 activity (Figure 6A and Supplemental Figure 3). We next examined the effects of cinacalcet and U73122 on total and surface expression of TRPV6 and the CaSR in Xenopus oocytes and found that membrane expression was not altered by either drug (Figure 6, B and C; see complete unedited blots in the supplemental material). Together, these data implicate the PLC pathway in the inhibition of TRPV6 channel activity by the CaSR.

CaSR activation inhibits transcellular Ca²⁺ absorption via PLC activation. Finally, the involvement of PLC in CaSR-mediated regulation of TRPV6 was investigated in the proximal colon ex vivo. To this end, we again used the PLC inhibitor U73122 in combination with cinacalcet in Ussing chambers. For these experiments, we had a similar condition A (control condition), but for condition B, we administered either cinacalcet plus vehicle (DMSO) or cinacalcet plus U73122. The cinacalcet/vehicle–treated group displayed a significant decrease in net Ca²⁺ flux (Figure 6D). However, co-incubation with the PLC inhibitor prevented the inhibitory effect of cinacalcet (Figure 6D). These data are in agreement with our in vitro data (Figure 6) and together imply that basolateral CaSR activation decreases transcellular Ca²⁺ transport through TRPV6 via a CaSR-induced activation of PLC in the proximal colon.

The CaSR is expressed throughout the intestine; however, a direct role for the intestinal CaSR in maintaining Ca²⁺ homeostasis has not been described (7, 33). Alterations in plasma Ca²⁺ indirectly regulate plasma Ca²⁺ via altering PTH secretion and consequently 1,25-[OH]₂ D₃ production (37, 50, 51). In general, adjustment of intestinal Ca²⁺ absorption has been thought to occur by reducing circulating 1,25-[OH]₂ D₃, secondary to a decrease in PTH secretion induced by lower blood Ca²⁺ levels. However, such a mechanism would be slow to respond to acute elevations in serum Ca²⁺. We therefore tested whether the intestine can directly adjust Ca²⁺ absorption in response to extracellular Ca²⁺. Herein, we report that the intestine has a direct extracellular Ca²⁺-sensing mechanism, which alters transcellular Ca²⁺ absorption through TRPV6. This is predominantly based on 3 observations: (a) both increased extracellular Ca²⁺ and a calcimimetic decreased transcellular Ca²⁺ absorption in Ussing chambers ex vivo; (b) this alteration in transcellular Ca²⁺ absorption is driven by TRPV6 because TRPV6^WT/WT, but not TRPV6^D541A/D541A mice, alter transcellular Ca²⁺ flux in response to changes in extracellular Ca²⁺; and (c) extracellular Ca²⁺ in the presence of the CaSR, but not in its absence, inhibits Ca²⁺-mediated TRPV6 currents in oocytes, a process involving PLC in vitro and ex vivo. Taken together, these results reveal a mechanism in the bowel whereby alterations in plasma Ca²⁺ are detected by a basolateral CaSR, which amends Ca²⁺ absorption via a TRPV6 pathway to maintain Ca²⁺ homeostasis (Figure 7).

Figure 7

Proposed model of CaSR-mediated inhibition of Ca²⁺ absorption. (A) Increased plasma Ca²⁺ is sensed by the CaSR expressed in kidneys, bone, intestine, and parathyroid glands. In response, the kidneys decrease Ca²⁺ reabsorption, there is decreased bone resorption, and parathyroid hormone (PTH) secretion is decreased. We show herein that intestinal Ca²⁺ absorption is inhibited. In concert, this reduces plasma Ca²⁺. Importantly, PTH acts to increase plasma Ca²⁺ by increasing bone Ca²⁺ resorption, renal tubular reabsorption, and indirectly by increasing intestinal Ca²⁺ absorption by increasing 1,25-[OH]₂ D₃ synthesis in the kidneys. (B) High plasma Ca²⁺ is detected by the intestinal epithelial basolateral CaSR, which inhibits TRPV6-mediated transcellular Ca²⁺ transport via PLC.

PTH increases production of 1,25-[OH]₂ D₃, which acts on the intestine to increase Ca²⁺ absorption (24–26). Consistent with this, our data show that mice fed a low Ca²⁺ diet had increased plasma 1,25-[OH]₂ D₃, but maintained normal plasma Ca²⁺ (15), and had increased expression of transcellular Ca²⁺ absorption mediators. In addition, direct administration of 1,25-[OH]₂ D₃ increased expression of intestinal transcellular Ca²⁺ absorption mediators. However, the degree of increased expression observed was less in the 1,25-[OH]₂ D₃–injected group than the mice on a low-Ca²⁺ diet. Interestingly, the mice administered 1,25-[OH]₂ D₃ also had increased plasma Ca²⁺, which could have attenuated gene expression via a direct effect on the intestinal CaSR (15). Conversely, a high-Ca²⁺ diet decreased expression of these mediators of transcellular Ca²⁺ absorption. This may be due to decreased secretion of PTH and therefore decreased activation of 1,25-[OH]₂ D₃ (15). However, it might also be a result of chronic activation of the basolateral intestinal CaSR directly altering expression of transcellular Ca²⁺ absorption mediators. Consistent with this, administration of the calcimimetic cinacalcet suppressed plasma PTH levels and Trpv6 and S100g expression, without altering plasma 1,25-[OH]₂ D₃ (15). Reduced circulating PTH could decrease 1,25-[OH]₂ D₃ levels and consequently reduce the expression of transcellular Ca²⁺ absorption mediators. However, cinacalcet-treated mice did not have reduced circulating 1,25-[OH]₂ D₃ (15). Thus, decreased Trpv6 and S100g expression are not a result of PTH-dependent reduction in 1,25-[OH]₂ D₃ but instead are potentially due to a direct activation of an intestinal CaSR. Interestingly, cinacalcet appears to suppress Trpv6 and S100g expression to a greater extent than a high-Ca²⁺ diet (Figure 1). This is likely due to greater activation of the CaSR by the calcimimetic than the high-Ca²⁺ diet as reflected in the greater suppression of PTH by this intervention (15). It is noteworthy that we and others observed CaSR expression along the intestine (31, 32). Together, the data are consistent with the bowel altering transcellular Ca²⁺ absorption via transcriptional downregulation directly in response to increased extracellular Ca²⁺, independent of 1,25-[OH]₂ D₃.

The current model of transcellular Ca²⁺ absorption suggests a significant role for TRPV6 (28, 35, 52). TRPV6 is transcriptionally regulated by 1,25-[OH]₂ D₃ and estrogen (8–11, 36). Here, we report alterations in Trpv6 expression in response to extracellular Ca²⁺, in the absence of altered 1,25-[OH]₂ D₃, adding intestinal CaSR activation to the list of transcriptional regulators. It should be noted that because CABP9K expression is regulated by cytosolic Ca²⁺, the corresponding changes in CABP9K expression observed likely reflect decreased Ca²⁺ absorption, and therefore, decreased cytosolic Ca²⁺, rather than a direct transcriptional response to CaSR activation (35, 36).

Not only have we observed a chronic transcriptional effect of extracellular Ca²⁺ on TRPV6 expression, but we also identified an acute, direct regulatory role of extracellular Ca²⁺ on TRPV6 activity. Decreased net Ca²⁺ flux was observed across proximal colon of TRPV6^WT/WT mice, but not TRPV6^D541A/D541A-mutant mice, following basolateral CaSR activation. Similarly, the increased net intestinal Ca²⁺ absorption observed in TRPV6^WT/WT mice in response to lower extracellular Ca²⁺ was not observed in TRPV6^D541A/D541A mice. These observations directly implicate TRPV6 in mediating altered transcellular Ca²⁺ absorption in response to CaSR activation. This was confirmed in vitro with Xenopus oocytes. CaSR activation in oocytes expressing TRPV6 and the CaSR decreased TRPV6-mediated Ca²⁺ currents. Previous work found evidence of CaSR-mediated alterations in paracellular Ca²⁺ permeability in colonic and renal epithelium (15, 34, 53). However, our experimental setup allowed us to eliminate the driving force for passive paracellular Ca²⁺ transport (i.e., a transepithelial electrochemical gradient). Thus, our results reflect changes in the net Ca²⁺ flux via an active transcellular pathway. Together, these data strongly support the presence of an acute regulatory effect of the CaSR in modifying cellular Ca²⁺ uptake, and thus transcellular Ca²⁺ absorption, via TRPV6.

Acute regulation of epithelial membrane channels can be accomplished by alterations in channel function or membrane expression. Membrane expression of TRPV5, a close family member of TRPV6, is altered in the DCT/CNT, thereby regulating channel activity (54, 55). Therefore, we assessed whether CaSR-mediated TRPV6 regulation was the result of alterations in membrane expression. This was not the case. In Xenopus oocytes expressing TRPV6 and the CaSR, cinacalcet had no effect on membrane expression of TRPV6. Unlike the changes in intestinal expression of Trpv6 mediated by chronic cinacalcet administration, acute changes in TRPV6-mediated Ca²⁺ flux are likely due to a CaSR-mediated regulation of TRPV6 activity, rather than expression.

Activation of the CaSR stimulates a network of cell-signaling pathways. In colonocytes, CaSR activation alters fluid absorption via PLC (32, 34). Consistent with this, PLC inhibition prevented decreased Ca²⁺ flux through TRPV6 in response to activation of the CaSR both in vitro and ex vivo. PLC is a membrane-bound phospholipase that catalyzes phosphatidylinositol 4,5-bisphosphate (PIP₂) into diacylglycerol and inositol triphosphate (IP3), and IP3 increases intracellular Ca²⁺ (56), a signaling pathway used by the CaSR in the parathyroid (34). TRPV6 activity is upregulated by PIP₂ and downregulated by intracellular Ca²⁺ (57). Extracellular Ca²⁺ inhibits TRPV6 via PIP₂ hydrolysis in whole-cell patch clamp experiments and everted duodenal gut sac ⁴⁵Ca²⁺ transport assays (46, 47). Furthermore, increased intracellular Ca²⁺, another consequence of PLC activation, directly inhibits TRPV6, providing another molecular explanation for how CaSR activation could inhibit TRPV6 (57, 58). Regardless of the exact downstream mechanism, our data provide evidence of intestinal CaSR-mediated PLC regulation of TRPV6 function.

The currently accepted model of intestinal Ca²⁺ absorption is that the duodenum, cecum, and proximal colon are capable of both transcellular and paracellular Ca²⁺ absorption while the jejunum and ileum contribute only paracellular Ca²⁺ absorption and/or secretion (23, 59, 60). There has been greater emphasis on the duodenum as a site of Ca²⁺ absorption and regulation recently (61); however, a significant role for the proximal large bowel in mediating intestinal Ca²⁺ absorption in humans and rodents has been appreciated for decades (62–65). In addition, multiple studies support the presence of 1,25-[OH]₂ D₃–mediated regulation of transcellular Ca²⁺ absorption from the proximal large bowel (38, 39, 64–66). Thus, the contribution of this segment to overall Ca²⁺ homeostasis should be considered. Our work provides further evidence the proximal colon plays a regulatory role in Ca²⁺ homeostasis. We have identified a potentially novel regulatory mechanism present in the proximal large bowel, which includes a Ca²⁺-sensing mechanism that detects altered extracellular Ca²⁺ and amends Ca²⁺ absorption to restore plasma Ca²⁺. We hypothesize that the luminal Ca²⁺ that is not absorbed from the duodenum and distal small bowel is likely subjected to fine regulation by the proximal large bowel, which senses the body’s extracellular Ca²⁺ and fine-tunes Ca²⁺ absorption and consequently fecal excretion to maintain plasma Ca²⁺ within the physiological range. Interestingly, a similar Ca²⁺-handling mechanism is observed in renal tubules. After significant paracellular reabsorption from the proximal tubule and the TAL, urinary Ca²⁺ excretion is fine-tuned in the more distal DCT/CNT segments by a transcellular pathway analogous to the one observed in the proximal large bowel (54, 67). Our results reveal that these pathways share a similar regulatory mechanism, a direct Ca²⁺-sensing mechanism that affects Ca²⁺ transport. Further, the DCT/CNT and the large bowel have both been estimated to contribute 10% of Ca²⁺ reabsorption in their respective organs (61, 68). Together, our findings highlight a substantial Ca²⁺ regulatory role in the proximal large bowel and challenge the prevailing contention that this segment is not important for Ca²⁺ homeostasis.

The administration of cinacalcet to dialysis patients often causes hypocalcemia (69–71). This has been attributed to hungry bone syndrome, via rapid lowering of plasma PTH. Our work provides an alternative explanation for this observation. Cinacalcet administration would not only attenuate release of PTH from the parathyroid but also inhibit Ca²⁺ absorption from the intestine, thereby lowering plasma Ca²⁺ levels.

In conclusion, we demonstrate a Ca²⁺-sensing mechanism present in the proximal large bowel that regulates Ca²⁺ absorption through a transcellular pathway, both acutely and chronically. The transcellular pathway mediating this effect relies on apical Ca²⁺ influx through TRPV6 because this effect was absent in TRPV6^D541A/D541A-mutant mice. The CaSR appears to be the sensor of extracellular Ca²⁺ because the pathway can be reconstituted in vitro by coexpressing the CaSR and TRPV6 in Xenopus oocytes. The cellular mechanism contributing to acute CaSR modulation of TRPV6 function involves PLC activation, which ultimately results in TRPV6 inactivation. This might be via a decrease in PIP₂ levels or an increase in intracellular Ca²⁺. These studies contribute to our understanding of Ca²⁺ homeostasis, providing evidence that the proximal large bowel can sense extracellular Ca²⁺ and adjust intestinal Ca²⁺ absorption to maintain plasma Ca²⁺ levels.

Mice. Wild-type FVB/N mice (Jackson Laboratory) and Trpv6^D541A/D541A-knockin mice (43) were housed in virus-free conditions and maintained on a 12-hour light/12-hour dark cycle. The TRPV6^D541A/D541A mice were backcrossed to FVB/N for more than 5 generations. Standard pelleted chow (PicoLab Rodent Diet 5053: 21% wt/wt protein, 5.0% wt/wt fat, 0.81% wt/wt Ca²⁺, and 2.2 IU/g vitamin D₃) and drinking water were available ad libitum. The experiments with respect to chronic altered Ca²⁺-containing diets (21 days) and treatment with 1,25-[OH]₂ D₃ (5 days) and cinacalcet (Santa Cruz Biotechnology) (5 days) were performed and described previously (15).

Real-time quantitative PCR. Following euthanasia, the duodenum, cecum, and proximal colon were collected as previously described (25). Total RNA was isolated using TRIzol Reagent and reverse-transcribed into cDNA using Random Primers and SuperScript II reverse transcriptase (all from Invitrogen). Primers and probes (Integrated DNA Technologies) designed for TRPV6 (Trpv6), CABP9K (S100g), NCX1 (Slc8a1), PMCA1b (Atp2b1), and CaSR (CaSR) were used to quantify expression levels with an ABI Prism 7900 HT Sequence Detection System (Applied Biosystems).

Ussing chamber experiments. ⁴⁵Ca²⁺ flux across the duodenum, cecum, and proximal colon of 8- to 12-week-old FVB/N, Trpv6^WT/WT, and Trpv6^D541A/D541A mice was performed essentially as previously (72). Following euthanasia, whole-wall duodenal, cecal, and proximal colonic intestinal sections of FVB/N, Trpv6^WT/WT, and Trpv6^D541A/D541A mice were dissected, linearized, and transversely cut into 3-mm segments. NB: Whole-wall intestinal sections used as sections with seromuscular layer stripped did not behave differently (72). These segments were mounted in an Ussing chamber (EM-CYS-4 system with P2400 chambers and P2407B sliders, Physiologic Instruments) and incubated with 4 ml Ringer’s solution consisting of 115 mM NaCl, 2.5 mM K₂HPO₄, 40 mM KH₂PO₄, 1.2 mM MgCl₂, 1.2 mM CaCl₂, and 25 mM NaHCO₃, bubbled with 5% vol/vol CO₂, 95% vol/vol O₂ at 37°C on both sides. Apical and basolateral solutions contained 10 mM mannitol, 10 mM glucose, and 2 μM indomethacin (bilateral, MilliporeSigma). The basolateral solution also contained 0.1 μM tetrodotoxin (Alomone Labs). The transepithelial potential difference was clamped to 0 mV by a VCC MC6 Multichannel Voltage/Current Clamp (Physiologic Instruments) and the resulting short-circuit current recorded with Acquire & Analyze software (Physiologic Instruments) through Ag-AgCl electrodes and 3 M KCl agarose bridges. The TER was calculated using Ohm’s law, following the measurement of the current generated in response to 2-mV pulses lasting 2.5 seconds, applied every 100 seconds.

Unidirectional Ca²⁺ fluxes (i.e., apical to basolateral or basolateral to apical) were measured using the protocol shown in Figure 2. At time 0, either the apical or basolateral solution was exchanged for a fresh solution of the same composition spiked with 5 μCi/ml ⁴⁵Ca²⁺. Three samples (50 μl each) were taken from both chambers at 15-minute intervals throughout each experimental condition (condition A: sample taken at 20, 35, and 50 minutes; condition B: samples taken at 75, 90, and 105 minutes). After the third sample was collected under condition A, the buffers were immediately changed and/or treatments applied (i.e., 10 μM cinacalcet hydrochloride [cinacalcet] in ethanol or 10 μM U73122 in DMSO), and the tissue was incubated for another 20 minutes before sampling for condition B. Radioactivity was measured with an LS6500 Multi-Purpose Scintillation Counter (Beckman Coulter), and unidirectional Ca²⁺ fluxes in opposite directions were paired to calculate net Ca²⁺ flux (net apical-to-basolateral flux). All Ussing chamber fluxes were normalized to surface area (cm²) before analysis. A total of 4 pairs were made per animal, and only pairs with less than 25% difference in TER were considered (changes in TER are shown in Supplemental Table 1).

Xenopus oocyte expression and 2-electrode voltage clamp. The preparation of Xenopus oocytes and the 2-electrode voltage clamp experiments were performed as previously described (73). Capped RNA of human TRPV6 (accession number NM_018646, generated using in vitro transcription with mMESSAGE mMACHINE kit by Ambion) and human CaSR cDNA (Origene; catalog RC211229) were injected into Xenopus oocytes. Two days after injection, whole-cell Ca²⁺ currents of oocytes were recorded at room temperature in a standard extracellular solution containing 100 mM NaCl, 2 mM KCl, 1 mM MgCl₂, and 10 mM HEPES (pH 7.5) with 5 mM Ca²⁺. Baseline current was determined by using the solution above but without 5 mM Ca²⁺. The 2 electrodes (capillary pipettes; Warner Instruments) impaling an oocyte were filled with 3 M KCl to form a tip resistance of 0.3–2 MΩ. A Geneclamp 500B amplifier and Digidata 1322A AD/DA converter (Molecular Devices) were used to obtain the currents. pClamp 9 software (Axon Instruments) was used for data acquisition and analysis. Currents and voltages were digitally recorded at 200 ms/sample and filtered at 2 kHz through a Bessel filter. Sigma Plot 14 (Systat Software) was used for plotting data.

Oocytes’ surface protein expression was determined with a biotinylation assay as previously described (73). In short, the oocytes were incubated with 0.5 mg/ml sulfo-NHS-SS-Biotin (Pierce) for 30 minutes at room temperature, and nonreacted biotin was quenched with 1 M NH₄Cl. After a wash, oocytes were harvested in ice-cold CellLytic M lysis buffer (MilliporeSigma) with a 1 times proteinase inhibitor mixture (Thermo Fisher Scientific). The surface proteins were absorbed by 100 μl streptavidin (Pierce) at 4°C overnight and subjected to SDS-PAGE. Mouse primary anti-CaSR monoclonal antibody (1:2000, Gentex, catalog GTX19347), in-house–generated anti-TRPV6 polyclonal antibody (1:1000) (74), mouse primary anti–β-actin monoclonal antibody (1:1000, Santa Cruz Biotechnology, catalog sc-47778), and horseradish peroxidase–coupled secondary antibody (1:5000, Santa Cruz Biotechnology, catalog sc-2005) were used for immunoblotting. The immunoblots were quantified using ImageJ software (NIH).

Statistics. Data are presented as mean ± SEM, and all data reported are based on measurements made on more than 6 animals (minimum 3 males and 3 females). A Shapiro-Wilk test was performed to assess for normal distribution. One-way ANOVA, Brown-Forsythe test, Kruskal-Wallis test, and Student’s unpaired or paired 2-tailed t tests (GraphPad) were carried out to determine statistical significance as appropriate, and P values less than 0.05 were considered statistically significant.

Study approval. All animal experiments were approved by the Animal Care and Use Committee for Health Science of the University of Alberta (protocol 213 for mouse and 234 for frog) and followed the Guide for the Care and Use of Laboratory Animals (National Academies Press, 2011).