Apical and Basolateral Ca2+Channels Modulate Cytosolic Ca2+in Gallbladder Epithelia

Abstract

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+]einto 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, μEq·cm−2·hr−1) were measured using36Cl at various mucosal Ca2+in the absence or presence of mucosal lanthanum (La3+), a non-diffusible Ca2+channel blocker. Serosal [Ca2+]ewas maintained at trace levels. In the absence of mucosal La3+, short circuit current (Isc) showed a positive correlation with mucosal [Ca2+]eas represented by a second order polynomial equation (y= 4.1 + 2.5x− 0.73x2,r= 0.68,P< 0.001). In contrast, unidirectional mucosa to serosa Cl−flux (JClms) was inversely correlated with [Ca2+]e(y= 47.9 − 8.7x+ 0.9x2,r= 0.51,P< .05) Addition of 1 mMmucosal La3+blunted the effects of [Ca2+]eon electrophysiologic parameters and JClms. However, basolateral repletion with 5 mMCa2+reverses the blocking effects of La3+on JClms. These data suggest that [Ca2+]eenters 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.