Actions of GTP on molluscan buccal, cardiac, and visceral smooth muscle

Abstract

Octopus bimaculoides oesophageal, crop and stomach smooth muscle preparations responded to 10 −4M GTP with prompt contractions and superimposed rapid twitch activity. These responses were similar to those induced by equimolar 4AP application to oesophagus and crop. None of the preparations responded significantly to 10 −4M ATP. The threshold for GTP induction of rapid twitch activity was 5 × 10 −5M and that for significant tonic force was 10 −4M, although there was some variation in sensitivity to GTP among different types of visceral muscle preparation, the crop circular smooth muscle being strongly excited at 10 −5M GTP. The systemic ventricle of Eledone was excited by GTP in a dose-dependent manner but contractions were weak compared to those elicited by FMRFamide or by 50 mM K + depolarization. Aplysia crop preparations responded to ATP with contractions similar to those induced by glutamate but these two agents were far less effective than GTP in inducing tonic force and rapid twitch activity. GTP possessed clear tonotropic actions on Aplysia oesophagus, crop and stomach smooth muscle strips and on the ventricle where it also accelerated spontaneous beating. The similarity of GTP and 4AP actions here suggest that GTP may effect EC coupling indirectly via cAMP leading to inhibition of a K + conductance and subsequent cell depolarization, the latter having been reported previously. Alternatively GTP/receptor binding may modulate ion-gating channels which regulate cation entry also leading to the previously noted depolarization.