Endocrine responses to exogenous bombesin and gastrin releasing peptide in conscious calves.

The effects of I.V. infusions of synthetic amphibian bombesin and porcine gastrin releasing peptide (GRP), at a dose of 5 pmol/kg. min for 30 min, have been investigated in conscious calves 3-6 weeks after birth. The protocols produced a closely similar rise in the bombesin-like immuno-reactivity of the arterial plasma of 208 +/- 14 pmol/l (bombesin) and 210 +/- 32 pmol/l (GRP) which fell exponentially with a half-life of about 3 min when the infusions were terminated. Neither peptide produced a discernible change in mean heart rate or aortic blood pressure, or in the mean arterial plasma concentrations of enteroglucagon, gastric inhibitory peptide (GIP), gastrin or cholecystokinin (CCK). GRP, but not bombesin, produced a small but significant rise in the mean plasma somatostatin concentration. Both peptides produced a significant rise in mean plasma pancreatic glucagon and pancreatic polypeptide concentration and proved to be exceptionally potent insulinotropic agents. These responses were associated with a rise in plasma glucose concentration which could not be attributed to a direct action of GRP on the liver. The distribution of bombesin-like immunoreactivity in the gastrointestinal tract was consistent with the findings of other workers who have concluded that it is restricted to nerve terminals. However, our other findings show that GRP is capable of acting as a true hormone.