Cardioacceleratory action of tachykinin-related neuropeptides and proctolin in two coleopteran insect species

Several cardioactive peptides have been identified in insects and most of t hem are likely to act on the heart as neurohormones. Here we have investiga ted the cardioactive properties of members of a family of insect tachykinin -related peptides (TRPs) in heterologous bioassays with two coleopteran ins ects, Tenebrio molitor and Zophobas atratus. Their effects were compared wi th the action of the pentapeptide proctolin. We tested the cardiotropic act ivity of LemTRP-4 isolated from the midgut of the cockroach Leucophaea mode rae, CavTK-1 and CavTK-II isolated from the blowfly Calliphora vomitoria. T he semi-isolated hearts of the two coleopteran species were strongly stimul ated by proctolin. We observed a dose dependent increase in heartbeat frequ ency (a positive chronotropic effect) and a decrease in amplitude of contra ctions (a negative inotropic effect). In both beetles the TRPs are less pot ent cardiostimulators and exert lower maximal frequency responses than proc tolin. LemTRP-4 applied at 10(-9)-10(-6) M was cardiostimulatory in both sp ecies inducing an increase of heart beat frequency. The amplitude of contra ctions was stimulated only in Z. atratus. CavTK-I and CavTK-II also exerted cardiostimulatory effects in Z. atratus at 10(-9) -10(-6) M. Both peptides stimulated the frequency, but only CavTK-II increased the amplitude of the heart beat. In T. molitor, however, the CavTKs induced no significant effe ct on the heart. Immunocytochemistry with antisera to the locust TRPs LomTK-I and LomTK-II w as employed to identify the source of TRPs acting on the heart. No innervat ion of the heart by TRP immunoreactive axons could detected, instead it is possible that TRPs reach the heart by route of the circulation. The likely sources of circulating TRPs in these insects are TRP-immunoreactive neurose cretory cells of the median neurosecretory cell group in the brain with ter minations in the corpora cardiaca and endocrine cells in the midgut. In conclusion, LemTRP-4, CaVTK-I and CavTK-II are less potent cardiostimula tors than proctolin and also exert stimulatory rather than inhibitory actio n on amplitude of contractions. The differences in the responses to proctol in and TRPs suggest that the peptides regulate heart activity by different mechanisms. (C) 2001 Elsevier Science Inc. All rights reserved.