In Vitro and In Vivo Biological Activities of PG-KII, a Novel Kassinin-Like Peptide from the Skin of the Australian Frog, Pseudophryne Güntheri

Abstract

We compared the in vitro and in vivo biological activities of PG-KII (pGlu-Pro-Asn-Pro-Asp-Glu-Phe-Val-Gly-Leu-Met-NH 2 ), a new peptide belonging to the tachykinin family, related to kassinin, isolated and sequenced from extracts of the skin of the Australian myobatrachid frog Pseudophryne güntheri , with those of the well-known tachykinins [substance P (SP), neurokinin A (NKA), neurokinin B (NKB), and kassinin (KASS)] to study its pharmacological and receptor profile. PG-KII always proved inactive in the in vitro and in vivo (gastric emptying) NK 2 bioassays. It resulted equipotent to SP and more potent than KASS, NKA, and NKB in all in vitro smooth muscle preparations preferentially activated by the NK 1 -selective agonists. On an in vivo NK 3 receptor-mediated function, gastric acid secretion, PG-KII had a potency similar to that of NKB. In contracting guinea pig ileum, which contains NK 1 , NK 2 , NK 3 , and also new tachykinin receptor subtypes, PG-KII was more potent than SP, NKB, and NKA. The cholinergic antagonist, atropine, significantly reduced the guinea pig contractile activity of both PG-KII and NKB but not that of SP or NKA. Pretreatment with the NK 1 -selective antagonist, CP 96,345, and with the NK 2 -selective antagonist, MEN 10,376, modified neither the in vivo nor the in vitro effects of PG-KII. These findings indicate that PG-KII is neither an NK 1 nor an NK 2 receptor agonist but has a spectrum of biological actions close to that of the NK 3 receptor agonists. PG-KII elicits strong contractile activity in guinea pig ileum. Administered centrally in the rat it regulates inhibition of gastric acid secretion.