Proctolin analogues and a proctolin metabolite as antagonist of the peptide proctolin

The locust oviduct bioassay was used to assess a variety of proctolin analogues as possible agonists and antagonists of the peptide proctolin. Both [alpha-methyl-L-tyrosine2]proctolin and [N-methyl-L-tyrosine2] proctolin were antagonists of proctolin with thresholds of 5 x 10(-9) M. Interestingly, at these threshold doses the analogues were antagonists when applied along with proctolin, being capable of shifting the dose-response curve for proctolin an order of magnitude to the right. Of the three tripeptides tested Tyr-Arg-Thr and Arg-Tyr-Thr showed no agonistic effects and were incapable of antagonizing proctolin-induced contractions. The third tripeptide, Leu-Pro-Thr, showed minimal agonistic effects and when applied with proctolin, significantly decreased the maximum response and increased the ED50 values of the parent compound. Interestingly, this tripeptide is a degradation product of proctolin. Cycloproctolin possessed no agonistic activity up to 10(-5) M but did antagonize proctolin's response in a dose-dependent manner with 2 x 10(-5) M cycloproctolin shifting the proctolin curve nearly two orders of magnitude to the right. Simultaneous application of 10(-9) M [alpha-methyl-L-tyrosine2]proctolin and 10(-5) M cycloproctolin showed some synergistic effect as the maximum response to the peptide was decreased by 21.6% and the dose-response curve shifted further to the right. These proctolin antagonist will be useful tools for examining the physiological importance of proctolin in insects as well as helping to identify receptor subtypes.