Structure-activity relationship for grayanotoxin derivatives in frog skeletal muscle.

In the presence of biologically active grayanotoxins, the frog skeletal muscle fiber generated a spontaneous action potential which was followed by a sustained depolarization. This depolarization was due to the specific increase in the membrane permeability to sodium, as it was abolished by withdrawal of sodium from the external medium or application of 1 x 10(-6) M tetrodotoxin. Potencies of biological activity of 34 grayanotoxin analogs were estimated from the measurement of critical concentrations for spontaneous generation of a prolonged action potential. It was found that the biological activity of grayanotoxins was largely dependent on both stereospecificity and hydrophobicity of the molecules. The essential groups in the grayanotoxin molecules for the biological activity were determined to be 3 beta-OH or 2 beta, 3 beta-epoxy group, 5 beta-OH, 6 beta-OH and 10 beta-methyl groups. There was an optimal hydrophobicity of the molecule for the maximal biological activity. Either increase or decrease in the number of OH groups from the optimal number of five reduced the biological activity. Four essential groups corresponding to those in the grayanotoxin molecules are also present in the molecules of veratridine, batrachotoxin and aconitine, all of which are known to keep the sodium channel open.