Two Functionally Distinct Cholecystokinin Receptors Show Different Modes of Actions on Ca2' Mobilization and Phospholipid Hydrolysis in Isolated Rat Pancreatic Acini

A new hepatapeptide cholecystokinin (CCK) analog, JMV-180 (Boc-Tyr(SO:)-Nle-Gly-Trp-Nle-Asp-2phenylethylester), acts as an agonist at high affinity CCK receptors on rat pancreatic acini to stimulate amylase release but unlike cholecystokinin octapeptide (CCKS) does not act on low affinity CCK receptors to inhibit amylase release (Galas, M. D., Lignon, M. F., Rodriguez, M., Mendre, C., Fulcrand, P., Laur, J., and Martinez, J. (1988) Am. J. Physiol. 254, G176-G188). To investigate the biochemical mechanisms initiated by CCK acting on each class of CCK receptor, the effects of JMV-180 and CCKS on amylase release, Ca2+ mobilization, and phospholipid hydrolysis were studied in isolated rat pancreatic acini. When acini were loaded with the intracellular Ca2+ chelator BAPTA, amylase release stimulated by both JMV-180 and CCKS was reduced. Measurement of 46Ca2+ efflux and cytosolic free calcium concentration ([Ca”‘]i) by the fluorescence of fura-a-loaded acini in a stirred cuvette showed that JMV180 induced a concentration-dependent increase but with a maximal response only two-thirds that induced by CCKS. When [Ca2+]i of individual fura-2loaded acinar cells was measured by microspectrofluorometry, all concentrations of JMV-180 (1 nM-10 j.bM) induced repetitive transient [Ca2+li spikes (Ca2+ oscillations). By contrast, stimulation with a high concentration of CCKS (1 nM) caused a large increase in [Ca2+]i followed by a small sustained elevation of [Ca2+]i. The measurement of inositol trisphosphate (IP3) production by both [3H]inositol labeling and 1,4,5-IP3 radioreceptor assay showed that JMV-180 had only minimal effects at 10 ~.LM in contrast to the large increase induced by high concentrations of CCK8 (more than 1 nM). JMV-180 blocked the effect of a high concentration of CCK8 on both [Ca2+lj and I,4,5-IP3 productions but did not affect the response to carbamylcholine. JMV180 caused a delayed monophasic stimulation of 1,2-diacylglycerol (DAG) sustained to 60 min without the early increase in DAG observed in response to CCKS. Furthermore, JMV-180 stimulated the release of [3H]choline metabolites, primarily phos-