Peptide-Induced Ca2+ Movements in a Tonic Insect Muscle: Effects of Proctolin and Periviscerokinin-2

Abstract

Although most of the characterized insect neuropeptides have been detected by their actions on muscle contractions, not much is known about the mechanisms underlying excitation-contraction coupling. Thus we initiated a pharmacological study on the myotropic action of the peptides periviscerokinin-2 (PVK-2) and proctolin on the hyperneural muscle of the cockroach Periplaneta americana. Both peptides required extracellular Ca2+ to induce muscle contraction, and a blockage of sarcolemmal Ca2+channels by Mn2+ or La3+inhibited myotropic effects. The peptides were able to induce contractions in dependence on the extracellular Ca2+ concentration in muscles depolarized with high K+ saline. A reduction of extracellular Na+, K+, or Cl− did not effect peptide action. Nifedipine, an L-type Ca2+-channel blocker, partially blocked the response to both peptides but to a much lesser extent than contractions evoked by elevated K+. Using calcium imaging with fluo-3, we show that proctolin induces an increase of the intracellular Ca2+ concentration. In calcium-free saline, no increase of the intracellular Ca2+concentration could be detected. The inhibiting effect of ryanodine, thapsigargin, and TMB-8 on peptide-induced contractions suggests that Ca2+ release from the sarcoplasmic reticulum plays a major role during peptide-induced contractions. Preliminary experiments suggest that the peptides do not employ cyclic nucleotides as second messengers, but may activate protein kinase C. Our results indicate that the peptides induce Ca2+ influx by an activation or modulation of dihydropyridine-sensitive and voltage-independent sarcolemmal Ca2+ channels. Ca2+-induced Ca2+ release from intracellular stores, but not inositol trisphosphate-induced Ca2+ release, seems to account for most of the observed increase in intracellular Ca2+. Additionally, both peptides were able to potentiate glutamate-induced contractions at threshold concentrations.