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The mechanism by which clonidine suppresses renin release was investigated in conscious rats. This suppression was studied by means of selected autonomic interventions in conjunction with changes in sodium balance. Serum renin activity and direct arterial pressure were monitored. Clonidine administration suppressed basal (by 68-85%), diuretic-induced (by 89%), and sympathetic nervous system-mediated (by 75-100%) renin release. Cholinergic, ganglionic, and peripheral sympathetic neuronal blockade did not prevent this inhibitory effect of clonidine. These results indicate a peripheral site of action for suppression of renin release by clonidine. The alpha-adrenergic blocking drug phentolamine prevented clonidine suppression of renin release in sodium-depleted rats and was partially effective in normal rats. Phentolamine blocked the decrease in renin caused by clonidine in ganglion-blocked rats. Clozapine, a new neuroleptic agent with alpha-adrenergic blocking activity, or phenoxybenzamine blocked the effect of clonidine on renin release in both sodium-depleted and normal rats. After ganglionic blockade in sodium-depleted rats, clonidine caused a significantly greater suppression of renin release than did an equipressor dose of methoxamine. These data, combined with hemodynamic correlates, suggest that clonidine inhibits renin release by activation of an intrarenal alpha-adrenergic receptor.
Pettinger et al. (Sat,) studied this question.