Large and prolonged in vivo response of pancreatic secretion to phenylethylamide and phenylethylester derivatives of Boc-[Nle-Nle]CCK(26–33) in the rat

Supramaximal doses of cholecystokinin (CCK) induce in vitro submaximal biological responses (i.e., smaller by 50% than the response to a maximal dose of CCK), desensitization and residual stimulation, and in vivo secretory inhibition and edematous pancreatitis. It has been reported previously that supramaximal doses of Boc-[Nle²⁸-Nle³¹]CCK(27–32)/-phenylethylester (JMV180) do not produce these effects. The aim of this study was to analyze the in vivo response of pancreatic secretion of the rat to a wide dose range of Boc-[Nle²⁸-Nle³¹]CCK(26–33) (JMV118), an analog of CCK8 with the same activity spectrum as CCK8, to JMV180 and to Boc-[Nle²⁸-Nle³¹]CCK(27–32)-phenylethylamide (JMV170). The three peptides were administered as intravenous infusions and as bolus intravenous injections. In the case of infusions, the same maximal effect was observed with all three peptides. It was obtained with 22.5 pmol/kg · min of JMV118; JMV180 and JMV170 were about 700 times less potent. In the case of bolus injections, the maximal response to JMV118 was observed with 450 pmol/kg, and the response peaked 10–15 min after the injection. Higher doses of JMV118 induced a secretory peak that was smaller and delayed relative to the moment of injection. JMV180 and JMV170 were about 500 times less potent: the maximal response was observed with 218700 pmol/kg and peaked 10–15 min after the injection. Larger doses of JMV180 and JMV170 produced neither supramaximal inhibition nor a delayed peak response, but induced a sustained stimulation of pancreatic secretion that could last more than 3 h after the injection. These data indicate that single large doses of JMV180 and JMV170 can produce a large and long-lasting stimulation of pancreatic secretion in vivo, a goal that cannot be reached with JMV118 or CCK8.