Cholecystokinin-induced formation of inositol phosphates in pancreatic acini

For cholecystokinin (CCK) and the partial agonist CCK-JMV-180 [Boc-Nle28,31,CCK-(27-32)-2-phenylethyl ester], we examined their abilities to stimulate the accumulation of inositol phosphates (IP), mobilize intracellular calcium, and stimulate enzyme secretion in rat pancreatic acini. CCK-8 caused an increase in [3H]IP2 and [3H]IP3 at 10 s and a slower increase in [3H]IP1. High-pressure liquid chromatography separation demonstrated that at 10 s 100% of the increase of [3H]IP3 was IP3(1,4,5). CCK-JMV-180 caused no increase in [3H]IP3 at 10 s and only 28% of the maximal increase seen with CCK-8 at 15 min. CCK-8 caused an 11-fold increase in calcium outflux, whereas CCK-JMV-180 was only 45% as effective and 3,000 times less potent. CCK-JMV-180 antagonized the CCK-8-stimulated increase in [3H]IP3 and mobilization of intracellular calcium. CCK-8 caused an 81-fold increase at 2.5 s in IP3(1,4,5) measured by a mass radioreceptor assay and half-maximal stimulation occurred at 2 nM, whereas CCK-JMV-180 only caused a 3-fold increase. Analysis of the ability of CCK-8 or CCK-JMV-180 to stimulate enzyme secretion demonstrated that at low concentrations, each peptide stimulates enzyme secretion without causing detectable calcium mobilization, whereas at increasing peptide concentrations calcium mobilization occurs without detectable accumulation of IP3(1,4,5), but at still higher concentrations IP3(1,4,5) accumulation is finally detected. These results demonstrate that peptides that stimulate enzyme secretion by interacting with CCK receptors can cause maximal stimulation with minimal changes in calcium mobilization and maximal changes in calcium mobilization occur with minimal changes in IP3(1,4,5), suggesting marked amplification.