Changes in neuromuscular transmission of guinea pig vas deferens produced by decamethrin treatment

Responses of the isolated vas deferens of guinea pig to clonidine (inhibition of contractions to field stimulation at 2.5 Hz), tyramine (inhibition of contractions to field stimulation at 10 Hz), prostaglandin E2 (inhibition of contractions to field stimulation at 10 Hz), and noradrenaline (contraction of longitudinal muscle) were determined after administration of decamethrin (18 mg kg-1, ip) once a day for 3 consecutive days. Treatment with decamethrin produced a subsensitivity of the prejunctional alpha 2-adrenoceptor system as evidenced by the fact that the alpha 2-agonist clonidine was less effective in decreasing nerve-stimulated induced contractions of the vas deferens. In addition, the presynaptic action of tyramine on postganglionic motor transmission was impaired. However, no detectable changes in the inhibition by prostaglandin E2 of twitch responses were produced by decamethrin. Decamethrin treatment had a significant effect on noradrenaline responsiveness, causing an increase in the maximum contractile response, indicative of an enhanced postreceptor mechanism. The present results suggest that decamethrin treatment reduces peripheral presynaptic adrenoceptor sensitivity. This reduction will lead subsequently to increased noradrenaline release and postsynaptic adrenoceptor upregulation.