Regulation of K secretion across the porcine gallbladder epithelium

Porcine gallbladder epithelium from the neck and the fundus of the organ was stripped of serosal muscle and mounted in Ussing chambers to investigate the mechanisms of K secretion. The sensitivity to K channel blockers and regulation by norepinephrine (NE), adenosine 3',5'-cyclic monophosphate (cAMP), and increases in intracellular Ca concentration ([Ca]) were studied. The porcine gallbladder secretes K (~0.8 μeq/cm² · h) under basal conditions. Mucosal tetraethylammonium (TEA) produced a concentration- dependent increase in short-circuit current (I(sc)) and inhibited the unidirectional serosal-to-mucosal ⁸⁶Rb flux J(sm)/(Rb), resulting in a >60% reduction in net Rb secretion. In contrast, serosal Ba produced a concentration-dependent decrease in I(sc) and stimulated J(sm)/(Rb), resulting in a >200% increase in net Rb secretion. NE inhibited J(sm)/(Rb) and net Rb secretion in both regions. In the fundic region the mucosal-to- serosal Rb flux (J(ms)/(Rb)) was also significantly increased, suggesting that active K absorption was activated. Exogenous cAMP increased J(sm)/(Rb) and net Rb secretion by >85% in both regions. This increase in net Rb secretion was blocked by mucosal TEA but unaffected by NE. The Ca ionophore ionomycin also increased J(sm)/(Rb) and net Rb secretion and reduced the I(sc) by ~50%. Neither mucosal TEA nor Ba blocked changes in steady-state Rb secretion induced by ionomycin. Although both serosal Ba and ionomycin produced significant reductions in I(sc), the effects of Ba were blocked by ionomycin pretreatment. These findings indicate that basal K secretion occurs through TEA-sensitive apical K channels and is regulated by intracellular cAMP. NE likely reduces K secretion by decreasing intracellular concentration of cAMP. Elevations in intracellular [Ca] stimulate K secretion through an additional, TEA-insensitive, apical K permeability. Additionally, elevated intracellular [Ca] may inhibit basolateral K efflux.