Seven proctolin analogues (I-VII) modified in position 2 of the peptide chain by Phe (p-guanidino) (I), Phe (p-OEt) (11), Tyr (3'-NH2) (III), Tyr (3'-N02) (IV), Afb (p-OH) (V) (Afb = 3-amino-4-phenyl-~-butyric acid), Afb (p-NH2) (VI), Afb ( p-NO2) (VII), and the tetrapeptide Tyr (3'-NH2)-Leu-Pro-Thr (VIII) were synthesized by the classic liquid-phase method. The biological effects of the peptides were investigated in cardioexcitatory tests on two insect species, the cockroach Periplaneta americana L., and the yellow mealworm, Tenebrio molitor L. Within physiological concentrations (lo-' -M) peptides 11, 111, and IV stimulated the heart action of P. arnericana like proctolin itself. Under identical conditions, in the case of T. molitor, only peptide 111 showed cardiostimulatory properties, whereas other compounds (including I1 and IV) were inactive at concentrations up to M. Results reported here reflect, with reference to the analogues I-VII, selective recognition of receptors on myocardium of both insect species. The tetrapeptide VIII revealed a weak deacceleratory effect on P. uniericanu and T. molitor heart action.Key words: cardioexcitatory effect on insects; insect neuropeptide proctolin; structure-function relationship; synthesis of new proctolin analogues Address:
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