Background: Vascular smooth muscle cells (VSMCs) exhibit phenotypic plasticity, promoting vascular calcification and increasing cardiovascular risk. Changes in VSMC intracellular calcium ([Ca²⁺]_i) are a major determinant of plasticity, but little is known about changes in [Ca²⁺]_i in chronic kidney disease (CKD). We have previously demonstrated such plasticity in aortas from our rat model of CKD and therefore sought to examine changes in [Ca²⁺]_i during CKD progression.

Materials and Methods: We examined freshly isolated VSMCs from aortas of normal rats, Cy/+ rats (CKD) with early and advanced CKD, and advanced CKD rats treated without and with 3% calcium gluconate (CKD + Ca²⁺) to lower parathyroid hormone (PTH) levels. [Ca²⁺]_i was measured with fura-2.

Results: Cy/+ rats developed progressive CKD, as assessed by plasma levels of blood urea nitrogen, calcium, phosphorus, parathyroid hormone and fibroblast growth factor 23. VSMCs isolated from rats with CKD demonstrated biphasic alterations in resting [Ca²⁺]_i: VSMCs from rats with early CKD exhibited reduced resting [Ca²⁺]_i, while VSMCs from rats with advanced CKD exhibited elevated resting [Ca²⁺]_i. Caffeine-induced sarcoplasmic reticulum (SR) Ca²⁺ store release was modestly increased in early CKD and was more drastically increased in advanced CKD. The advanced CKD elevation in SR Ca²⁺ store release was associated with a significant increase in the activity of the sarco-endoplasmic reticulum Ca²⁺ ATPase (SERCA); however, SERCA2a protein expression was decreased in advanced CKD. Following SR Ca²⁺ store release, recovery of [Ca²⁺]_i in the presence of caffeine and extracellular Ca²⁺ was attenuated in VSMCs from rats with advanced CKD. This impairment, together with reductions in expression of the Na⁺/Ca²⁺ exchanger, suggest a reduction in Ca²⁺ extrusion capability. Finally, store-operated Ca²⁺ entry (SOCE) was assessed following SR Ca²⁺ store depletion. Ca²⁺ entry during recovery from caffeine-induced SR Ca²⁺ store release was elevated in advanced CKD, suggesting a role for exacerbated SOCE with progressing CKD.

Conclusions: With progressive CKD in the Cy/+ rat there is increased resting [Ca²⁺]_i in VSMCs due, in part, to increased SOCE and impaired calcium extrusion from the cell. Such changes may predispose VSMCs to phenotypic changes that are a prerequisite to calcification.