Stimulation of somatotropin secretion following peripheral administration of the tripeptide, syndyphalin 33 in sheep, pigs and rats

Abstract In the present study, a simple tripeptide alkylamine, syndyphalin 33 (SD33, Tyr-DMet(O)-Gly-methylphenethylamide) was shown to stimulate somatotropin (GH) secretion in sheep, hogs and rats following peripheral administration. Intravenous (i.v.) administration of SD33 at doses of 0.05, 0.1 and 0.2 μmol/kg stimulated a significant increase in circulating GH levels in sheep within 5 minutes post-injection. This response was not attenuated following repeated i.v. injections of SD33 (0.05 μmol/kg) administered at 2 hour intervals. In addition, plasma GH levels were significantly stimulated following either subcutaneous (s.c.) or oral administration of SD33 in hogs and rats. Subcutaneous administration of SD33 at doses of 0.5, 1.0 and 2.0μmol/kg stimulated a significant increase in plasma GH concentrations within 30 minutes of injection in both species. Oral administration of SD33 at 1.0, 10 or 100 μmol/kg in rats resulted in a significant elevation in plasma GH levels which peaked at 30 minutes post-gavage. In the pig, circulating GH levels were significantly increased within 30 minutes post-ingestion and remained elevated for at least 2 hours at the 2.0 μmol/kg dose level. The ability of naloxone to block SD33-stimulated GH secretion suggests that this peptide acts via mu opiate receptors.