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Blood pressure response to intravenous noradrenaline is influenced by substance P in normotensive and hypertensive rabbits.

Substance P (SP) applied to the cerebral ventricles elevates arterial blood pressure (BP). This elevation is mediated by an increased sympathetic tone and, moreover, could be attenuated by intravenous (i.v.) SP. To check the assumption that a change of adrenoceptor efficiency might contribute to the attenuation, SP (5 ng/kg per min) was continuously given i.v. and 20 micrograms SP injected intracerebroventricularly (i.c.v.) in anaesthetized normotensive as well as hypertensive rabbits. The BP response to 5 micrograms noradrenaline (NA) was checked within a control period before SP was given and rechecked 30 min after i.c.v. SP with continued i.v. infusion. Under these circumstances SP altered the BP response to NA qualitatively and quantitatively. An initial BP drop was followed by a rise which was remarkably smaller in magnitude than that obtained within the control period. The BP response to phenylephrine (PHE), however, was not altered by the identical SP protocol. Substance P application only i.v. or only i.c.v. was ineffective. Moreover, no differences could be revealed between normotensive and hypertensive animals in the BP response to NA. It is concluded that the beta-adrenoceptor efficiency changed similarly in the normotensive and hypertensive rabbits with i.v. SP, provided sympathetic tone had previously increased. In this manner i.v. SP might modify an exaggerated sympathetic tone which might be implicated in the pathogenesis of arterial hypertension.