SECRETIN PROMOTES OSMOTIC WATER TRANSPORT IN RAT CHOLANGIOCYTES BY INCREASING AQUAPORIN-1 WATER CHANNELS IN PLASMA-MEMBRANE - EVIDENCE FOR A SECRETIN-INDUCED VESICULAR TRANSLOCATION OF AQUAPORIN-1

Although secretin is known to stimulate ductal bile secretion by direc tly interacting with cholangiocytes, the precise cellular mechanisms a ccounting for this choleretic effect are unknown, We have previously s hown that secretin stimulates exocytosis in cholangiocytes and that th ese cells transport water mainly via the water channel aquaporin-1 (AQ P1), In this study, we tested the hypothesis that secretin promotes os motic water movement in cholangiocytes by inducing the exocytic insert ion of AQP1 into plasma membranes, Exposure of highly purified isolate d rat cholangiocytes to secretin caused significant, dose-dependent in creases in osmotic membrane water permeability (P-f) (e.g. increased b y 60% with 10(-7) M secretin), which was reversibly inhibited by the w ater channel blocker HgCl2, Immunoblotting analysis of cholangiocyte m embrane 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 secreti n-induced redistribution of AQP1 from intracellular to plasma membrane s, Both the secretin-induced increase in cholangiocyte P-f and AQP1 re distribution were blocked by two perturbations that inhibit secretin-s timulated exocytosis in cholangiocytes, i.e. treatment with colchicine and exposure at low temperatures (20 and 4 degrees C). Our results de monstrate that secretin increases AQP1-mediated P-f in cholangiocytes. Moreover, our studies implicate the microtubule-dependent vesicular t ranslocation of AQP1 water channels to the plasma membrane, a mechanis m that appears to be essential for secretin-induced ductal bile secret ion and suggests that AQP1 can be regulated by membrane trafficking.