Peptide-induced Ca2+ movements in a tonic insect muscle: effects of proctolin and periviscerokinin-2

Although most of the characterized insect neuropeptides have been detected by their actions on muscle contractions, not much is known about the mechan isms underlying excitation-contraction coupling. Thus we initiated a pharma cological study on the myotropic action of the peptides periviscerokinin-2 (PVK-2) and proctolin on the hyperneural muscle of the cockroach Periplanet a americana. Both peptides required extracellular Ca2+ to induce muscle con traction, and a blockage of sarcolemmal Ca2+ channels by Mn2+ or La3+ inhib ited myotropic effects. The peptides were able to induce contractions in de pendence on the extracellular Ca2+ concentration in muscles depolarized wit h high K+ saline. A reduction of extracellular Na+, K+,or Cl- did not effec t peptide action. Nifedipine, an L-type Ca2+-channel blocker, partially blo cked the response to both peptides but to a much lesser extent than contrac tions evoked by elevated K+. Using calcium imaging with fluo-3, we show tha t proctolin induces an increase of the intracellular Ca2+ concentration. In calcium-free saline, no increase of the intracellular Ca2+ concentration c ould be detected. The inhibiting effect of ryanodine, thapsigargin, and TMB -8 on peptide-induced contractions suggests that Ca2+ release from the sarc oplasmic reticulum plays a major role during peptide-induced contractions. Preliminary experiments suggest that the peptides do not employ cyclic nucl eotides as second messengers, but may activate protein kinase C. Our result s indicate that the peptides induce Ca2+ influx by an activation or modulat ion of dihydropyridine-sensitive and voltage-independent sarcolemmal Ca2+ c hannels. Ca2+-induced Ca2+ release from intracellular stores, but not inosi tol trisphosphate-induced Ca2+ release, seems to account for most of the ob served increase in intracellular Ca2+. Additionally, both peptides were abl e to potentiate glutamate-induced contractions at threshold concentrations.