The Effect of an Intestinal Polypeptide Fraction on Insulin Release in the Ratin Vitroandin Vivo

The effect of insulin release in the rat in response to a partially purified intestinal polypeptide fraction, “Intestinal Insulin Releasing Polypeptide (IRP),” has been studied in vivo and in vitro. It potentiated the release of insulin in response to 16.7 mM glucose and 16.7 mM leucine in rat pancreas pieces incubated in vitro, but had no effect on insulin release in the presence of 3.3 mM glucose. IRP produced a small increase in the insulin secretory response to the intravenous administration of an amino acid mixture, but the effect was not statistically significant. A mixture of secretin and pancreozymin, 1 U/kg of each, potentiated the plasma insulin response to intravenous glucose, but to a smaller degree than IRP and did not accelerate glucose disappearance. Secretin studied in a similar manner snowed no insulin releasing effect at doses up to 2 U/kg. IRP contains less than 4 U/mg secretin activity; no detectable pancreozymin or cholecystokinin activity and no significant contamination by GLI. It is concluded, therefore, that the insulin releasing activity of this preparation is not due to any of these factors—separately or in combination— but to a previously uncharacterised polypeptide now isolated from the small intestine. (Endocrinology93: 1323, 1973)