Abnormalities in the regulation of sympathetic activity in human hypertension

Various biochemical, pharmacologic, and physiologic techniques were used to evaluate the sympathetic tone and reactivity in labile and sustained hypertension in humans. The results of these studies suggest the existence of an important subgroup of hypertensive patients characterized by increased basal sympathetic tone and reactivity to standing. Such abnormalities could be the result of various dysfunctions, involving the activity of central and peripheral cardiovascular sympathetic fibers, the presynaptic modulation of sympathetic fibers (including the interaction with the parasympathetic system), the inactivation of circulating norepinephrine, and the sensitivity of the efferent cells. The increase in circulating norepinephrine in a group of hypertensive patients seems to reflect a functional increase in the sympathetic tone as shown by the presence of hyperkinetic cardiac functions in hyperadrenergic patients (elevated catecholamine levels), while cardiac functions are normal in normoadrenergic patients (catecholamine levels within normal range). Moreover, the better hypotensive response, combined with normalization of the basal and reactive circulating norepinephrine levels following beta-blockade in hyperadrenergic hypertensive patients, strongly supports the participation of the sympathetic system in the maintenance of hypertension in those patients. The identification and characterization of this subpopulation of patients may be helpful in the development of more rational therapeutic approaches and could eventually permit us to devise better predictors of outcome in hypertension.