Investigations on the effects of the neuropeptide proctolin on muscle contraction and intramuscular signalling pathways in the marine isopod Idotea emarginata (Crustacea, Isopoda)

The neuropeptide proctolin is contained in motoneurones of the marine isopod Idotea emarginata and modulates contraction of dorsal extensor muscle fibres by exerting pre- and postsynaptic effects. The aim of this study was to further investigate the postsynaptic effects of the neuropeptide proctolin on the extensor muscle fibres. Therefore, the first part of the study was to characterise a 30 kDa protein, that shows an increased degree of phosphorylation after treating the fibres with proctolin. A thin filament associated 30 kDa protein was identified, which was phosphorylated at a serine residue and detected by an antiserum against the thin filament regulatory protein troponin I. The isoelectric points of the 30 kDa protein were in the range of isoelectric points of troponin I isoforms already identified in other muscles. The phosphorylation of a thin filament associated regulatory protein by proctolin is a so far unknown modulation pathway of this peptide. In the second part of study the proctolin induced intracellular signalling pathways in the extensor muscle fibres of Idotea were further investigated. No proctolin-induced increase of cAMP-concentration was measured. Potentiation of caffeine contractures in the presence of proctolin showed that proctolin increases the intracellular Ca2+-concentration or the Ca2+-sensitivity of intramuscular proteins. Additionally, a proctolin-induced decrease of the intracellular cGMP-concentration was measured in the muscle fibres. This proctolin-induced decrease of cGMP-concentration is mediated by a PKC and is independent of the activation of cGMP-specific phosphodiesterases. These results suggest that other effects of proctolin might also be mediated by an activation of PKC