ROLE OF THE HEART AND PERIPHERAL RESISTANCE IN THE REFLEX EFFECT OF GROUP-I AFFERENT-FIBERS ON BLOOD-PRESSURE

Experiments were done on anesthetized and curarized cats to see whethe r the increase in blood pressure caused by electrical stimulation of g roup I afferent fibers is related to a direct reflex effect on the hea rt. The reflex effect of electrical stimulation of group I afferent fi bers from the gastrocnemius-soleus muscles on the arterial pressure, t he left ventricular pressure, the inotropic state of the left ventricl e (dP50/dt) and the heart rate were compared before and after beta-blo ckade with propranolol (0.1 mg/kg intravenously) to reduce a possible direct effect on the heart. The same comparison was made before and af ter alpha-blockade with phentolamine (2.5 mg/kg intravenously) to keep the peripheral resistance constant. Electrical stimulation of group I afferent fibers caused an increase in the blood pressure, the left ve ntricular pressure and, to some extent, the inotropic state of the lef t ventricle and the heart rate. The beta-blockade bad no significant e ffect on these increases, while the alpha-blockade abolished the incre ase in blood pressure. It is concluded that the effect of stimulation of group I afferent fibers on the blood pressure is not dependent on a direct reflex effect on the heart, but can be better explained by a r eflex increase in the peripheral resistance.