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

1 HCO 3 − 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 HCO 3 − uptake with a combination of amiloride and DIDS, were measured using the pH‐sensitive fluoroprobe BCECF. Apparent secretory HCO 3 − fluxes were calculated from the initial rate of intracellular acidification. 2 In the presence of HCO 3 − , stimulation with secretin (10 n m ) or forskolin (5 μ m ) more than doubled the rate of intracellular acidification. This effect was abolished in the absence of HCO 3 − . It was also abolished in the presence of HCO 3 − 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 HCO 3 − secretion across the luminal membrane. 3 Secretin, cholecystokinin (CCK) and bombesin each stimulated HCO 3 − secretion in a dose‐dependent fashion. They evoked comparable maximal responses at about 10 n m and the EC 50 values were 0.5 n m for secretin, 0.2 n m for CCK and 30 p m for bombesin. Acetylcholine (ACh) was also effective, with a maximum effect at 10 μ m . 4 The stimulatory effect of CCK was blocked completely by the CCK 1 receptor antagonist devazepide but not by the CCK 2 receptor antagonist L365,260. The CCK analogue JMV‐180 (Boc‐Tyr(SO 3 H)‐Nle‐Gly‐Trp‐Nle‐Asp‐phenylethyl ester), which is an agonist of the high‐affinity CCK 1 receptor but an antagonist of the low‐affinity receptor, also stimulated HCO 3 − 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 CCK 1 receptor evoke HCO 3 − secretion. 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−cyclo[Cys−Tyr− d −Trp−Orn−Val−Cys]−Nal−NH 2 (BIM−23127). 6 Secretagogue‐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 −1 mm −2 , and with concentration dependences similar to those obtained for HCO 3 − secretion. 7 We conclude that CCK, bombesin and ACh stimulate the secretion of a HCO 3 − ‐rich fluid by direct actions on the interlobular ducts of the guinea‐pig pancreas and that these responses are mediated by CCK 1 receptors, GRP receptors and muscarinic cholinoceptors, respectively.