Interaction between secretin and cholecystokinin-octapeptide in the exocrine rat pancreas in vivo and in vitro.

This study investigates the interaction between physiological doses of the synthetic gut hormones, cholecystokinin-octapeptide (CCK8) and secretin on pancreatic juice secretion in the anaesthetized rat and on amylase secretion and Ca2+ and Mg2+ mobilization in isolated pancreatic segments and acinar cells. CCK8 (150 pmol kg-1 h-1) and secretin (100 pmol kg-1 h-1) evoked marked time course increases in pancreatic juice flow, total protein output and amylase secretion in the anaesthetized rat when administered separately compared to saline controls. Simultaneous intravenous infusion of CCK8 and secretin did not yield either an additive response or a potentiation but instead it caused a decrease in secretory responses. Administration of either polymyxin B (10(-8) mol kg-1 h-1) or staurosporine (10(-8) mol kg-1 h-1), two protein kinase C inhibitors, simultaneously with both CCK8 and secretin caused a further decrease in all secretory parameters. Superfusing pancreatic segments with either CCK8 (10(-11) M) or secretin (10(-11) M) elevated amylase output compared to the smaller response with a combination of CCK8 and secretin. Combining staurosporine (10(-6) M) with CCK8 and secretin resulted in a further decrease in amylase output. CCK8 (10(-11) M) evoked a large increase in radiolabelled Ca2+ influx into pancreatic segments and elevated cytosolic free Ca2+ concentration ([Ca2+]i) in acinar cells loaded with the fluorescent dye, Fura-2. Secretin (10(-11) M) alone had no significant effect on Ca2+ mobilization but it markedly attenuated the increases in radiolabelled Ca2+ influx and [Ca2+]i elicited by CCK8. In superfused pancreatic segments CCK8 (10(-11) M) evoked a net efflux of Mg2+ whereas secretin (10(-11) M) induced a net uptake of Mg2+. Combining secretin with CCK8 also resulted in a net uptake of Mg2+. The results indicate that both Ca2+ and Mg2+ mobilization may be associated with the interaction between CCK8 and secretin in the rat pancreas.