Ca2+ Calmodulin Regulates Basal Gallbladder Absorption

Background: Gallbladder absorption is altered during gallstone formation, a phenomenon that may be partly the result of elevated biliary Ca2+ levels. Recent studies suggest that changes in gallbladder absorption are mediated by intracellular Ca2+ ([Ca2+]ic). However, the mechanisms by which [Ca2+]ic regulates gallbladder ion transport are not known. Calmodulin is a Ca2+ receptor protein in the Ca2+ messenger system that modulates ion transport in the small intestine. We hypothesized that Ca(2+)-calmodulin mediates the effects of [Ca2+]ic on gallbladder absorption.

Methods: Prairie dog gallbladders were mounted in Ussing chambers, and standard electrophysiologic parameters were recorded. Unidirectional Na+, Cl-, and water fluxes were measured before and after mucosal exposure to 5 x 10(-5) mol/L trifluoperazine, a potent calmodulin antagonist. In addition, the ion transport effects of increased extracellular calcium and theophylline were determined in the presence of calmodulin inhibition.

Results: Inhibition of calmodulin resulted in an increase in net Na+ and water absorption and converted the gallbladder from a Cl- absorptive state. Similar results were obtained during exposure to two other calmodulin antagonists that differ only in their affinity for calmodulin but not in their hydrophobicity, suggesting that the observed changes were caused by specific calmodulin inhibition. Effects of trifluoperazine were reversed by increasing luminal [Ca2+] or theophylline exposure.

Conclusions: The effects of calmodulin inhibition are directly opposite of the effects of the Ca2+ ionophore. We conclude that Ca(2+)-calmodulin regulates gallbladder absorption at basal [Ca2+]ic. Further studies are needed to determine whether altered calmodulin activity is responsible for increased gallbladder absorption during gallstone formation.