Blood pressure, plasma catecholamines, and sympathetic outflow in pithed SHR and WKY rats.

After destruction, by pithing, of the central nervous system, the blood pressure (BP) of the spontaneously hypertensive (SHR) rat falls, but remains higher than in the pithed normotensive (WKY) rat. At all ages examined, stimulation of the sympathetic outflow evoked greater increases in BP of pithed SHR rats, but the increases in plasma catecholamines were similar in the two strains. The BP increase evoked by administered norepinephrine was greater in old SHR than in old WKY rats, but this increased response was not found in young animals. Both young and old SHR rats, however, were more sensitive than WKY rats to the pressor effects of epinephrine. Results of selective pharmacologic blockade of cardiac or vascular beta-adrenoceptors suggest that a smaller vasodilator response in young SHR rats to the beta-adrenergic effects of epinephrine is responsible for the greater pressor response than in WKY rats. This factor diminishes in older SHR rats in which, as suggested by Folkow, structural changes in the resistance blood vessel diameter may account for the enhanced pressor responsivity of SHR rats.