To explain the role of the Thr5 residue of proctolin (Arg-Tyr-Leu-Pro-Thr) in the myotropic activity of this insect neuropeptide, we synthesized two groups of its analogues: 1) Arg-Tyr-Leu-Pro-X-OH with X = Val (1), D-Val (2), Ile (3), D-Ile (4), Ala (5), D-Ala (6), Asn (7), Gln (8), Ser (9), Pro (10), Phe (11), Asp (12), Glu (13), Arg (14), D-Arg (15), Lys (16) and Gly (17) and 2) Arg-Tyr-Leu-Pro-R', where R' = isobutylamine (18), S-l-methyl-1-phenylmethylamine (19), R-1-methyl-1-phenylmethylamine (20), R-2-amino-1-propanol (21), S-2-amino-1-propanol (22), R-1-amino-2-propanol (23), S-2-amino-1-propanol (24), 3-amino-1-propanol (25). Decapeptide proctolylproctolin (H-Arg-Tyr-Leu-Pro-Thr-Arg-Tyr-Leu-Pro-Thr-OH) (26) was synthesized. Syntheses of these peptides were carried out by solid-phase method. All peptides were bioassayed in vitro on the semi-isolated hearts of Tenebrio molitor using a cardioexcitatory test and on the foregut of locust (Schistocerca gregaria). Peptides 1, 3, 5, 9, 13, 14, 16, 22, and 23 retained about 30-50% of the cardioexcitatory activity in T. molitor. Analogues 1 and 3 preserved about 50% and analogue 8 about 80% of the myotropic activity, whereas compound 4 and 9 showed a very weak contractile activity in S. gregaria.</p
