Comparison of the myotropic activity of position-2 modified analogues of proctolin on the hindgut of Periplaneta americana and the oviduct of Locusta migratoria

Abstract

The largest series of position-2 modified proctolin analogues to have been examined to date were tested for their ability to mimic the basal contraction induced by proctolin on hindgut of the cockroach, Periplaneta americana, and oviduct of the locust, Locusta migratoria. Twelve analogues of proctolin (Arg-Tyr-Leu-Pro-Thr), differing in the substituent (H, OMe, OEt, OPr, F, Cl, Br, I, NO 2, NH 2, N 3, Me) located at the para-position of the aromatic amino acid, caused dose-dependent contractions of both tissues at concentrations quite similar to proctolin. Seven showed greater or equal potency on the hindgut but, with one exception, they were less active on the oviduct than proctolin. The rank order of potency of the analogues depends on the tissue, lending more support to the notion that insects have more than one type of proctolin receptor. No relationship was observed between myoactivity and lipophilic, steric, electron donating or electron withdrawing properties of the substituents at the para-position of the aromatic amino acid. This may be the result of more than one sub-type of proctolin receptor on the specific tissue with differing structural requirements for optimum activity.