Bicarbonate and fluid secretion evoked by cholecystokinin, bombesin and acetylcholine in isolated guinea-pig pancreatic ducts

1. HCO3- secretion was investigated in interlobular duct segments isolated from guinea-pig pancreas using a semi-quantitative fluorometric method. Sec retagogue-induced decreases in intracellular pH, following blockade of baso lateral HCO3- uptake with a combination of amiloride and DIDS, were measure d using the pH-sensitive fluoroprobe BCECF. Apparent secretory HCO3- fluxes were calculated from the initial rate of intracellular acidification. 2. In the presence of HCO3-, stimulation with secretin (10 nM) or forskolin (5 muM) more than doubled the rate of intracellular acidification. This ef fect was abolished in the absence of HCO3-. It was also abolished in the pr esence of HCO3- when DIDS and NPPB were applied to the luminal membrane by microperfusion. We therefore conclude that the increase in acidification ra te is a useful index of secretagogue-induced HCO3- secretion across the lum inal membrane. 3. Secretin, cholecystokinin (CCK) and bombesin each stimulated HCO3- secre tion in a dose-dependent fashion. They evoked comparable maximal responses at about 10 nM and the EC50 values were 0.5 nM for secretin, 0.2 nM for CCK and 30 pM for bombesin. Acetylcholine (ACh) was also effective, with a max imum effect at 10 muM. 4. The stimulatory effect of CCK was blocked completely by the CCK1 recepto r antagonist devazepide but not by the CCK2 receptor antagonist L365,260. T he CCK analogue JMV-180 (Boc-Tyr(SO3H)-Nle-Gly-Trp-Nle-Asp-phenylethyl este r), which is an agonist of the high-affinity CCK, receptor but an antagonis t of the low-affinity receptor, also stimulated HCO3- secretion but with a smaller maximal effect than CCK. JMV-180 partially inhibited the response t o a high concentration of CCK but not to a lower concentration, suggesting that both high- and low-affinity states of the CCK1 receptor evoke HCO3- se cretion. 5. The stimulatory effect of bombesin was blocked completely by the gastrin -releasing peptide (GRP) receptor antagonist D-Phe(6)-bombesin(6-13)-methyl ester (BME) but not by the neuromedin B (NMB) receptor antagonist D-Nal-cy clo[Cys-Tyr-D-Trp-Orn-Val-Cys]-Nal-NH2 (BIM-23127). 6. Secretagogue-evoked fluid secretion was also examined using video micros copy to measure the ZD rate of swelling of ducts whose ends had sealed duri ng overnight culture. Secretin, CCK1 bombesin and ACh all evoked fluid secr etion with maximal rates of approximately 0.6 nl min(-1) mm(-2), and with c oncentration dependences similar to those obtained for HCO3- secretion. 7. We conclude that CCK, bombesin and ACh stimulate the secretion of a HCO3 --rich fluid by direct actions on the interlobular ducts of the guinea-pig pancreas and that these responses are mediated by CCK, receptors, GRP recep tors and muscarinic cholinoceptors, respectively.