Effects of endogenous and exogenous secretin on plasma pancreatic polypeptide concentrations in dogs

The effects of exogenous and endogenous secretin with or without intravenous glucose infusion upon islet hormone secretion were studied in four conscious mongrel dogs fitted with a duodenal fistula. Intravenous infusion of secretin for 1 h at doses of 0.5 and 4 U/kg raised plasma secretin concentrations to physiological and pharmacological levels respectively, without affecting plasma insulin and pancreatic polypeptide concentrations. In contrast, bolus injections of secretin at high concentrations produced significant increases of plasma insulin at 0.5 U/kg and 4 U/kg and of pancreatic polypeptide at 4 U/kg. Plasma glucagon did not change during intravenous infusion of low dose secretin (0.5 U · kg^-1 · h^-1), but decreased during infusion of 4 U · kg^-1 · h^-1 or bolus injection of secretin (0.5 U/kg). Intravenous infusion of glucose together with secretin (0.5 U/kg and 4 U/kg) did not affecf plasma insulin, glucagon, or pancreatic polypeptide levels significantly compared with the changes caused by glucose infusion alone. Intraduodenal instillation of HCl, which produced plasma secretin concentrations similar to those evoked by intravenous infusion of secretin (4 U · kg ^-1 · h^-1), led to a rise in plasma pancreatic polypeptide. It is concluded that the stimulatory effects of secretin on insulin and pancreatic polypeptide and the inhibitory effect on glucagon are pharmacological, and that increase of plasma pancreatic polypeptide after intraduodenal infusion of HCl is not mediated by endogenous secretin.