ACTIONS OF GTP ON MOLLUSCAN BUCCAL, CARDIAC, AND VISCERAL SMOOTH-MUSCLE

Octopus bimaculoides oesophageal, crop and stomach smooth muscle prepa rations responded to 10(-4)M GTP with prompt contractions and superimp osed rapid twitch activity. These responses were similar to those indu ced by equimolar 4AP application to oesophagus and crop, None of the p reparations responded significantly to 10(-4)M ATP. The threshold for GTP induction of rapid twitch activity was 5 x 10(-5)M and that for si gnificant tonic force was 10(-4)M, although there was some variation i n sensitivity to GTP among different types of visceral muscle preparat ion, the crop circular smooth muscle being strongly excited at 10(-5)M GTP. The systemic ventricle of Eledone was excited by GTP in a dose-d ependent 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 glutama te bat these two agents were far less effective than GTP in inducing t onic force and rapid twitch activity. GTP possessed clear tonotropic a ctions on Aplysia oesophagus, crop and stomach smooth muscle strips an d 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 p reviously. Alternatively GTP/receptor binding may modulate ion-gating channels which regulate cation entry also leading to the previously no ted depolarization.