Mode of action of an inhibitory neuropeptide SchistoFLRFamide on the locust oviduct visceral muscle

The possible mode of action of an inhibitory neuropeptide SchistoFLRFamide (PDVDHVFLRFamide) on locust oviduct visceral muscle was studied by examining its ability to inhibit calcium ionophore A23187-, caffeine- or phorbol ester-induced oviduct contractions. A23187-induced muscle contractions included two components: one which required the influx of extracellular Ca2+ through Ca2+ channels (blocked by cobalt ions), the other not blocked by cobalt ions. SchistoFLRFamide inhibited the cobalt-sensitive components, but not the latter. Caffeine induced two phases of contractions, a phasic contraction which was probably due to the release of intracellular Ca2+ and a tonic contraction which required the influx of extracellular Ca2+. SchistoFLRFamide inhibited the tonic contraction in a dose-dependent manner but did not influence the phasic contraction. The phorbol 12-myristate 13-acetate-induced muscle contraction required the presence of extracellular Ca2+, with Ca2+ probably entering through ligand-gated channels, but the contraction was not inhibited by SchistoFLRFamide. Based upon these results, it appears that SchistoFLRFamide inhibits muscle contraction by preventing the accumulation of free intracellular Ca2+ from extracellular stores. SchistoFLRFamide is incapable of inhibiting contraction induced by the release of intracellular stores of Ca2+. It is possible that SchistoFLRFamide closes or blocks voltage-gated and some ligand-gated Ca2+ channels in the plasma membrane.