Proctolin and related N-methylated pentapeptides selectively contract locust foregut but not rat ileum

Proctolin contracts locust, but contrary to a previous report, not rat gut tissue, in a dose-dependent manner. [N-Me-Thr]- and [N-Me-Leu]-, but not [N-Me-Tyr]-proctolin were also agonists on locust foregut. We propose that proctolin contains a trans-Pro and exists in a cyclic conformation based upon solution NMR studies.