Isolation, primary structure, and effects on α-melanocyte-stimulating hormone release of frog neurotensin

Neurotensin (NT) was isolated in pure form from the small intestine of the European green frog, Rana ridibunda, and its primary structure was established as pGlu-Ala-His-Ile-Ser-Lys-Ala-Arg-Arg-Pro-Tyr-Ile-Leu. This sequence contains five amino acid substitutions (Leu²→Ala, Tyr³→His, Glu⁴→Ile, Asn⁵→Ser, and Pro⁷→Ala) compared with human NT. A peptide with identical chromatographic properties was identified in an extract of frog brain. Synthetic frog NT produced a concentration-dependent increase in αMSH release from perifused frog pars intermedia cells, with an ED⁵⁰ of 5 x 10 ⁹ M. A maximum response (276.3 ± 45.5% above basal release) was produced by a 10^-8-M concentration. Repeated administration of NT to melanotrope cells revealed the occurrence of a rapid and pronounced desensitization mechanism. The data are consistent with a possible role for the peptide as a hypophysiotropic factor in amphibians.