Paradoxical Relationship between the Superior Cervical Ganglia and the Antihypertensive Action of Propranolol in the

The effect of d/-propranolol (1.0 mg/kg/day i.p.) on heart rate and blood pressure in young and old normo- and hypertensive rats was studied before and after bilateral sympathectomy of the superior cervical ganglia. Denervation alone produced no significant changes in blood pressure or heart rate in the normo- or hypertensive rat of either age. Thirty-five consecutive days of propranolol treatment significantly lowered blood pres-sure in the older established spontaneous hypertensive rat and prevented the increases from occurring in the younger, devel-oping hypertensive rat. Blood pressure was not modified in the normotensive Wistar-Kyoto rat by propranolol. Denervation of the superior cervical ganglia at the beginning of the propranolol treatment or midway through the protocol abolished the anti-hypertensive effects in the young and old spontaneous hyper-tensive rat. A close temporal association of denervation with the loss of the antihypertensive effect was demonstrated. Al-though propranolol administration continued after denervation, the antihypertensive effects of the drug did not reappear. The carotid sinus is supplied with adrenergic postganglionic fibers from the SCG. These fibers may play a role in the control of sinus muscle tone and, indirectly, blood pressure (Floyd and Neil, 1952; Kezdi, 1954). A number of studies suggests a collaborative relationship between the carotid sinus, its adrenergic innervation, and the depressor reflex. Thus, stimulation of the sympathetic trunk supplying the SCG was shown to enhance the depressor reflex or sinus nerve activity, resulting from mechanical or hemody-namic distension of the sinus (Kezdi, 1954; Sampson and Mills, 1970; Endy and Tuttle, 1980). Propranolol, in one of the studie