The two individual cell membranes of epithelia are functionally coupled, so that changes in apical membrane conductance are paralleled by changes in basolateral K+ conductance. However, the signal that regulates basolateral K+ conductance, thereby coupling the two membranes, is unknown. We tested the hypothesis that the cellular calcium concentration, [Ca2+]c, may regulate basolateral K+ conductance in canine tracheal epithelium, a Cl- -secreting epithelium that shows marked membrane coupling. Three findings support the hypothesis. First, the intracellular Ca2+ antagonist 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate hydrochloride (TMB-8) attenuated the secretory response. Second, the secretagogue epinephrine increased [Ca2+]c, as measured with quin-2. Third, we found a K+ channel that was activated by Ca2+ on the cytosolic side of the membrane. Thus, cytosolic Ca2+ regulates the basolateral K+ conductance and may be the signal responsible for functional coupling of the two cell membranes.