Properties of the luminal membrane of isolated perfused rat pancreatic ducts

Abstract

The aim of the present study was to investigate by what transport mechanism does HCO ₃ ⁻ cross the luminal membrane of pancreatic duct cells, and how do the cells respond to stimulation with dibytyryl cyclic AMP (db-cAMP). For this purpose a newly developed preparation of isolated and perfused intra-and interlobular ducts of rat pancreas was used. Responses of the epithelium to inhibitors and agonists were monitored by electrophysiological techniques. Addition of HCO ₃ ⁻ /CO₂ to the bath side of nonstimulated ducts depolarized the PD across the basolateral membrane (PD_bl) by about 9mV, as also observed in a previous study [21]. This HCO ₃ ⁻ effect was abolished by Cl⁻ channel blockers or SITS infused into the lumen of the duct: i. e. 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB, 10⁻⁵ M) hyperpolarized PD_bl by 8.2±1.6 mV (n=13); 3′,5-dichlorodiphenylamine-2-carboxylic acid (DCl-DPC, 10⁻⁵ M) hyperpolarized PD_bl by 10.3±1.7 mV (n=10); and SITS hyperpolarized PD_bl by 7.8±0.9 mV (n=4). Stimulation of the ducts with dbcAMP in the presence of bath HCO ₃ ⁻ /CO₂ resulted in depolarization of PD_bl, the ductal lumen became more negative and the fractional resistance of the luminal membrane decreased. Together with forskolin (10⁻⁶ M), db-cAMP (10⁻⁴ M) caused a fast depolarization of PD_bl by 33.8±2.5 mV (n=6). When db-cAMP (5×10⁻⁴ M) was given alone in the presence of bath HCO ₃ ⁻ /CO₂, PD_bl depolarized by 25.3±4.2 mV (n=10). In the absence of exogenous HCO ₃ ⁻ , db-cAMP also depolarized PD_bl by 24.7±3.0 mV (n=10). The present data suggest that in the luminal membrane of pancreatic duct cells there is a Cl⁻ conductance in parallel with a Cl⁻/HCO ₃ ⁻ antiport. Dibutyryl cyclic AMP increases the Cl⁻ conductance of the luminal membrane. Taking together our present results, and the recent data obtained for the basolateral membrane [21], a tentative model for pancreatic HCO ₃ ⁻ transport is proposed.