The effects of selected proctolin analogues on contractions of locust (Locusta migratoria) oviducts

Abstract We have examined the ability of some selected proctolin analogues to mimic the basal contraction induced by proctolin (Arg-Tyr-Leu-Pro-Thr) on a locust oviduct. The criterion of agonist-induced basal contraction was used to construct dose-response relationships for these selected analogues. Three of the analogues were modified in the [Tyr 2 ]-position, two were modified in the [Arg 1 ]-position, and one was a tetrapeptide lacking the N-terminal arginine, Tyr(3′-NH 2 )-Leu-Pro-Thr. All were chosen because of their differing rank orders of activity on two other insect preparations. [ l -Dopa 2 ]proctolin, [Phe(pNH 2 ) 2 ]proctolin and [Phe(pNMe 2 ) 2 ]proctolin were all capable of mimicking proctolin, as too were [Lys 1 ]proctolin and [homo-Arg 1 ]proctolin, although with varying thresholds, half-maxima and maxima. The tetrapeptide was a very poor agonist, resulting in only a minor basal contraction at the high dose of 10 −5 M. The rank orders, based upon doses required for half-maxima and maximal contraction were different from either a cockroach heart or a mealworm heart. The results support the suggestion for subtypes of proctolin receptors within insects, and indicate that these analogues may be useful tools for comparing these sub-types.