Effect of a smooth muscle relaxant from the stonefish, Synanceia trachynis, on KCl-induced responses in the guinea-pig vas deferens.

Stonefish skin secretion contains a smooth muscle relaxant, synancein II, that selectively inhibits tonic responses of KCl-induced contractions of the guinea-pig vas deferens. Synancein II (5-50 micrograms/ml) and nifedipine (10(-8)-10(-7) M) reduced tonic responses equally. Over these concentrations, synancein II inhibited phasic responses more than did nifedipine. Rapid inhibition of phasic responses by synancein II (low concentrations) but its inability to block this response at high concentrations suggests that more than one extracellular and/or intracellular Ca2+ source may contribute to the generation of the phasic response. Some calcium antagonists selectively inhibit tonic responses induced by KCl in many different organs, whilst others inhibit phasic responses. Whereas we previously reported that synancein II selectively inhibits phasic rather than tonic responses in the guinea-pig ileum, the present results reveals a reversal of this selectivity.