Secretin Promotes Osmotic Water Transport in Rat Cholangiocytes by Increasing Aquaporin-1 Water Channels in Plasma Membrane

Abstract

Although secretin is known to stimulate ductal bile secretion by directly interacting with cholangiocytes, the precise cellular mechanisms accounting for this choleretic effect are unknown. We have previously shown that secretin stimulates exocytosis in cholangiocytes and that these cells transport water mainly via the water channel aquaporin-1 (AQP1). In this study, we tested the hypothesis that secretin promotes osmotic water movement in cholangiocytes by inducing the exocytic insertion of AQP1 into plasma membranes. Exposure of highly purified isolated rat cholangiocytes to secretin caused significant, dose-dependent increases in osmotic membrane water permeability (P f ) (e.g. increased by 60% with 10 ؊7 M secretin), which was reversibly inhibited by the water channel blocker HgCl 2 . Immunoblotting analysis of cholangiocyte membrane fractions showed that secretin caused up to a 3-fold increase in the amount of AQP1 in plasma membranes and a proportional decrease in the amount of the water channel in microsomes, suggesting a secretin-induced redistribution of AQP1 from intracellular to plasma membranes. Both the secretin-induced increase in cholangiocyte P f and AQP1 redistribution were blocked by two perturbations that inhibit secretin-stimulated exocytosis in cholangiocytes, i.e. treatment with colchicine and exposure at low temperatures (20 and 4°C). Our results demonstrate that secretin increases AQP1-mediated P f in cholangiocytes. Moreover, our studies implicate the microtubule-dependent vesicular translocation of AQP1 water channels to the plasma membrane, a mechanism that appears to be essential for secretin-induced ductal bile secretion and suggests that AQP1 can be regulated by membrane trafficking.Bile is formed primarily by hepatocytes and secreted at the bile canaliculus; subsequently, its volume and composition are modified in the lumen of bile ducts as a result of the transport of water and solutes by cholangiocytes (1, 2). While this ductal bile secretion results from the osmotically driven movement of water, the regulatory and mechanistic aspects are obscure. We recently reported that cholangiocytes (unlike hepatocytes) express the water-selective channel protein aquaporin-1 (AQP1) 1 and proposed that ductal bile secretion results from the movement of water across this protein (3, 4). Based on studies in renal epithelial cells, it is currently thought that AQP1 is constitutively inserted into plasma membranes and is not hormone responsive (5, 6).Secretin is known to stimulate ductal bile secretion via specific receptors on cholangiocytes (7). We and others recently proposed that secretin-induced bile secretion was associated with the microtubule-dependent exocytic insertion of cytoplasmic vesicles into the cholangiocyte plasma membrane (8 -10). Interestingly, hormone-regulated exocytic movement of transporters has been demonstrated in other cell types (11). For example, in renal collecting tubule cells the water channel aquaporin-2 moves to and from the apical plasma membrane in the presence...

Medical and surgical treatment options for polycystic liver disease1

“…Biliary epithelia maintain a cAMP-dependent Cl and HCO3 secretion that facilitates fluid secretion. 45,46 Intravenous administration of secretin in ADPKD patients increased fluid secretion in hepatic cysts. 47 This suggests that increased cAMP drives fluid secretion in hepatic cysts.…”

An emerging consensus on aquaporin translocation as a regulatory mechanism

“…For example, early studies demonstrated that secretin, a hormone involved in osmoregulation, promotes osmotic water movement in cholangiocytes by inducing exocytic insertion of AQP1 into the cell membrane (Marinelli et al, 1997). Quantitative immunoblotting assays revealed an inverse correlation between the presence of AQP1 at the plasma membrane and within the microsomal intracellular compartment in response to secretin.…”

“…Colchicine-induced disruption of microtubules suggested secretin-induced AQP1 translocation was mediated by the microtubule network (Marinelli et al, 1997). The microtubule network has been further implicated in the induced translocation of AQP1 from intracellular compartments to the cell membrane following diverse stimuli (Conner et al, 2010, Tietz et al, 2006.…”

Secretin activation of the apical Na+-dependent bile acid transporter is associated with cholehepatic shunting in rats

“…Cholangiocyte plasma and microsomal membranes were isolated as previously described. 16 Cholangiocytes were sonicated in 0.3 mol/L sucrose containing 0.01% soybean trypsin inhibitor, 0.1 mmol/L phenylmethylsulfonyl fluoride, and 0.1 mmol/L leupeptin. The plasma membrane fraction was obtained by centrifugation at 200,000g for 60 minutes on a discontinuous 1.3 mol/L sucrose gradient.…”