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

1HCO3−secretion was investigated in interlobular duct segments isolated from guinea‐pig pancreas using a semi‐quantitative fluorometric method. Secretagogue‐induced decreases in intracellular pH, following blockade of basolateral HCO3−uptake with a combination of amiloride and DIDS, were measured using the pH‐sensitive fluoroprobe BCECF. Apparent secretory HCO3−fluxes were calculated from the initial rate of intracellular acidification.2In the presence of HCO3−, stimulation with secretin (10 nm) or forskolin (5 μm) more than doubled the rate of intracellular acidification. This effect was abolished in the absence of HCO3−. It was also abolished in the presence of HCO3−when DIDS and NPPB were applied to the luminal membrane by microperfusion. We therefore conclude that the increase in acidification rate is a useful index of secretagogue‐induced HCO3−secretion across the luminal membrane.3Secretin, cholecystokinin (CCK) and bombesin each stimulated HCO3−secretion in a dose‐dependent fashion. They evoked comparable maximal responses at about 10 nmand the EC50values were 0.5 nmfor secretin, 0.2 nmfor CCK and 30 pmfor bombesin. Acetylcholine (ACh) was also effective, with a maximum effect at 10 μm.4The stimulatory effect of CCK was blocked completely by the CCK1receptor antagonist devazepide but not by the CCK2receptor antagonist L365,260. The CCK analogue JMV‐180 (Boc‐Tyr(SO3H)‐Nle‐Gly‐Trp‐Nle‐Asp‐phenylethyl ester), which is an agonist of the high‐affinity CCK1receptor but an antagonist of the low‐affinity receptor, also stimulated HCO3−secretion but with a smaller maximal effect than CCK. JMV‐180 partially inhibited the response to a high concentration of CCK but not to a lower concentration, suggesting that both high‐ and low‐affinity states of the CCK1receptor evoke HCO3−secretion.5The stimulatory effect of bombesin was blocked completely by the gastrin‐releasing peptide (GRP) receptor antagonist d‐Phe6‐bombesin(6‐13)‐methyl ester (BME) but not by the neuromedin B (NMB) receptor antagonist d−Nal−cyclo[Cys−Tyr−d−Trp−Orn−Val−Cys]−Nal−NH2(BIM−23127).6Secretagogue‐evoked fluid secretion was also examined using video microscopy to measure the rate of swelling of ducts whose ends had sealed during overnight culture. Secretin, CCK, bombesin and ACh all evoked fluid secretion with maximal rates of approximately 0.6 nl min−1mm−2, and with concentration dependences similar to those obtained for HCO3−secretion.7We 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 CCK1receptors, GRP receptors and muscarinic cholinoceptors, respectively.