The neuropeptide proctolin potentiates contractions and reduces cGMP concentration via a PKC-dependent pathway

As in many other arthropods, the neuropeptide proctolin enhances contractures of muscles in the crustacean isopod Idotea emarginata. The enhancement of high K+-induced contractures by proctolin (1 micromol l-1) was mimicked upon application of the protein kinase C (PKC) activator phorbol-12-myristate 1-acetate (PMA) and was inhibited by the PKC inhibitor bisindolylmaleimide (BIM-1). The potentiation was not inhibited by H89, a protein kinase A (PKA) inhibitor. Proctolin did not change the intracellular concentration of 3',5'-cyclic adenosine monophosphate (cAMP) whereas it significantly reduced the intracellular concentration of 3',5'-cyclic guanosine monophosphate (cGMP). The reduction of cGMP was not observed in the presence of the PKC inhibitor BIM-1. 8-Bromo-cGMP, a membrane-permeable cGMP analogue, reduced the potentiating effect of proctolin on muscle contracture. We thus conclude that proctolin in the studied crustacean muscle fibres induces an activation of PKC, which leads to a reduction of the cGMP concentration and, consequently, to the potentiation of muscle contracture.