Apical and basolateral Ca2+ channels modulate cytosolic Ca2+ in gallbladder epithelia

Gallstone formation is associated with altered gallbladder (GB) ion transport and increased concentration of GB bile Ca2+. Recent studies show that increased cytosolic Ca2+ ([Ca2+](i)) stimulates GB Cl(-)secretion. However, the mechanism by which extracellular Ca2+ ([Ca2+](e)) enters the cytosol remains unclear. We tested the hypothesis that entry of [Ca2+](e) into cytosol occurs via apical and basolateral membrane Ca2+ channels. Prairie dog GBs were mounted in Ussing chambers, standard electrophysiologic parameters were recorded, and unidirectional Cl- fluxes (J, mu Eq . cm(-2). hr(-1)) were measured using Cl-36 at various mucosal Ca2+ in the absence or presence of mucosal lanthanum (La3+), a non-diffusible Ca2+ channel blocker. Serosal [Ca2+](e) was maintained at trace levels, In the absence of mucosal La3+, short circuit current (Isc) showed a positive correlation with mucosal [Ca2+](e) as represented by a second order polynomial equation (y = 4.1 + 2.5x - 0.73x(2), r = 0.68, P < 0.001). In contrast, unidirectional mucosa to serosa Cl- flux (J(ms)(Cl)) was inversely correlated with [Ca2+](e) (y = 47.9 - 8.7x + 0.9x(2), r = 0.51, P < .05) Addition of 1 mM mucosal La3+ blunted the effects of [Ca2+](e) on electrophysiologic parameters and J(ms)(Cl). However, basolateral repletion with 5 mM Ca2+ reverses the blocking effects of La3+ on J(ms)(Cl). These data suggest that [Ca2+](e) enters the cytosol via apical and basolateral Ca2+ channels. We conclude that GB apical Ca2+ channels may represent a pathway for biliary Ca2+ entry into the cell and therefore may represent an important regulatory pathway for GB ion transport during gallstone formation. (C) 1996 Academic Press, Inc.