The effects of two types of pyrethroid on rat skeletal muscle.

The mechanical and electrophysiological effects of two synthetic pyrethroids, cismethrin (Type I) and deltamethrin (Type II) have been studied in whole rat gastrocnemius muscle and in isolated perfused rat diaphragm. In both directly or indirectly stimulated preparations deltamethrin but not cismethrin had a positive inotropic effect which in the case of the whole animal could be reversed by increasing the stimulation frequency. In the isolated diaphragm the deltamethrin effect could be prevented by TTX suggesting it is primarily the result of an increased sodium conductance. Deltamethrin also reversed a partial curare blockade in the diaphragm. Electrophysiological studies showed that the inotropic effect of deltamethrin is accompanied by repetitive muscle action potentials. In addition deltamethrin significantly increased muscle fibre input resistance, a novel finding which may underlie the curare reversal effect. It is concluded that the effectiveness of deltamethrin compared with cismethrin in causing repetitive firing in the muscle membrane is attributable to the greater duration of the after-depolarization produced by deltamethrin.