Effects of minoxidil and nitroprusside on reflex increases in myocardial contractility.

1 The effects of nitroprusside and minoxidil on increases in myocardial contractility resulting from carotid artery occlusion were investigated in anaesthetized dogs. The results were compared with those produced by intravenous influsion of noradrenaline. 2 Nitroprusside and minoxidil attenuated the pressor responses produced by carotid artery occlusion. 3 Nitroprusside, but not minoxidil, attenuated the maximal myocardial contractility resulting from carotid occlusion. 4 The pressor and contractility responses to noradrenaline infusion were unaffected by either agent. 5 Nitroprusside failed to alter the myocardial responses produced by dimethylphenylpiperazinium. 6 These results, in conjunction with those of other investigators who have demonstrated that nitroprusside does not affect the release of noradrenaline from adrenergic neurons, suggest that nitroprusside may inhibit sympathetic nervous system reflex activity via an afferent and/or central component.