MUSCARINIC STIMULATION OF GALLBLADDER EPITHELIUM .1. ELECTROPHYSIOLOGY AND SIGNALING MECHANISMS

To understand the effects of acetylcholine (ACh) on fluid-absorbing ep ithelia, we carried out experiments on Necturus gallbladder epithelium . Binding studies with 1-quinuclidinyl[phenyl-4(N)-H-3]benzilate (QNB) demonstrated that Necturus gallbladder epithelial cells express high- affinity muscarinic receptors. The effects of ACh and carbachol were e xerted from the basolateral surface and consisted of a transient hyper polarization of both cell membranes and a concomitant decrease in the apparent fractional resistance of the apical membrane. Atropine blocke d both effects. ACh also elicited transient elevations of inositol 1,4 ,5-trisphosphate and intracellular free calcium ([Ca2+]i) levels, the latter by both release from intracellular stores and basolateral influ x. The phospholipase C antagonist U-73122 inhibited the effects of ACh , whereas inhibition of prostaglandin and guanosine 3',5'-cyclic monop hosphate synthesis with indomethacin or methylene blue, respectively, had no effect. In conclusion, Necturus gallbladder epithelium expresse s muscarinic receptors in the basolateral membrane. Receptor activatio n stimulates phospholipase C and elevates cellular levels of inositol 1,4,5-trisphosphate and [Ca2+]i. The elevation in [Ca2+]i activates K channels but apparently not Cl- channels.