The effects of sodium nitroprusside and 8-bromo-cyclic GMP on electrical and mechanical activities of the rat tail artery.

The effects of sodium nitroprusside and 8-bromo-guanosine 3':5'-cyclic monophosphate (8-bromocyclic GMP) on the electrical and mechanical activities of the rat tail artery were compared. The inhibitory effects of sodium nitroprusside on the contractions induced by noradrenaline, phenylephrine, KC1 and clonidine were mimicked by 8-bromo-cyclic GMP. Sodium nitroprusside and 8-bromo-cyclic GMP increased the resting membrane potential only in preparations with low initial resting membrane potentials. In tissues previously contracted and depolarized with noradrenaline, KC1 and clonidine, both sodium nitroprusside and 8-bromo-cyclic GMP caused relaxation without significantly affecting the membrane potential. Both sodium nitroprusside and 8-bromo-cyclic GMP abolished neurally-mediated contractions without any significant effect on the electrical responses. These results suggest that the actions of sodium nitroprusside and 8-bromo-cyclic GMP are not related to membrane hyperpolarization or inhibition of membrane excitability.