Effects of substance P on cardiovascular regulation in the rabbit

The effect of substance P on blood pressure and aortic reflex was investigated in rabbits. Microinjections of substance P and Sar9, Met(O2)11-SP (a selective NK1-receptor agonist) into the floor of the fourth ventricle led to a dose-dependent increase of blood pressure and a sharp enhancement of the baroreflex. These effects were abolished by pretreatment with SR 140333 (a selective NK1-receptor antagonist). Intraventricular injection of the antagonist alone significantly decreased the amplitude of the aortic reflex. After bivagotomy, the amplitude of the parasympathetic component of the baroreflex decreased dramatically and substance P injections were no longer effective. Our results demonstrate that substance P activation of NK1 receptors plays a major role in the modulation of the parasympathetic component of the baroceptor reflex.