Effect of graded adrenaline infusion on arterial adrenaline clearance in normotensive and hypertensive man

Effects of hypertension on cardiovascular responses to epinephrine in humans

Cardiac beta-receptor responsiveness is diminished by both aging and hypertension. However, concomitant decreases in the activity of counterregulatory mechanisms, such as the arterial baroreflex and neuronal catecholamine uptake, influence the ultimate cardiac responses to adrenergic agents in vivo. In the present study, we evaluated by echocardiography cardiac responses to intravenous infusion of epinephrine in 14 young and 18 older normotensive men and women and in 10 young and 17 older hypertensive men and women. To assess the relative contribution of intrinsic cardiac and counterregulatory components to the overall response, infusions were repeated combined with a ganglionic blocker in the young groups. Epinephrine-induced increases in heart rate were similar in the four groups. Increases in stroke volume, ejection fraction, and cardiac index were similar in the two hypertensive and two young normotensive groups. In contrast, they were attenuated in the older normotensive group, resulting in higher left ventricular responses in older hypertensive than in normotensive subjects. Heart rate and left ventricular responses to epinephrine in the presence of ganglionic blockade did not differ between the two young groups. Increases in plasma norepinephrine due to epinephrine infusion were larger in hypertensive than in normotensive subjects. One may conclude that compared with young normotensive subjects, in hypertensive subjects mechanisms increasing versus decreasing cardiac responses to epinephrine may remain in balance, and, compared with older normotensive subjects, older hypertensive subjects exhibit enhanced cardiac responses to sympathetic stimulation.

Epinephrine and the genesis of hypertension

Several lines of evidence suggest a psychophysiological link between stress, adrenomedullary activation, and the genesis of hypertension. Experimental data support four important concepts: 1) epinephrine stimulates prejunctional beta 2-adrenergic receptors that facilitate norepinephrine release from sympathetic nerve endings; 2) epinephrine can be converted into a cotransmitter by neuronal uptake and on subsequent release augment the simultaneous discharge of norepinephrine; 3) exogenous epinephrine can induce sustained hypertension in rats; and 4) there is a period of critical sensitivity to endogenous epinephrine in a genetic model of rat hypertension. Plasma epinephrine concentrations are elevated in many young subjects with borderline or mild hypertension. The hypothesis that intermittent surges in epinephrine could initiate or promote the development of primary hypertension by amplifying peripheral neurotransmission, both directly (facilitative effect) and indirectly (cotransmitter action), is supported by reports that hemodynamic and noradrenergic responses to sympathetic activation can be augmented by increases in endogenous epinephrine or by its local or systemic (up to 30 ng/kg/min) infusion. Such responses have been documented in both normotensive and hypertensive subjects and can be blocked by propranolol. Although the weight of evidence (mostly indirect) indicates that epinephrine can augment norepinephrine release in humans, the epinephrine hypothesis, itself, remains unproven. Expression of hypertension by this mechanism may be restricted to a specific epinephrine-sensitive subset of individuals with a genetic predisposition to high blood pressure.

Plasma norepinephrine and dihydroxyphenylglycol in essential hypertension

The aim of the present study was to examine whether essential hypertension is associated with altered plasma concentrations of dihydroxyphenylglycol, the principal presynaptic metabolite of norepinephrine. Forearm venous plasma dihydroxyphenylglycol and norepinephrine were determined at rest and during graded orthostasis in 47 normotensive control subjects and 58 outpatients with essential hypertension. There was no group difference in age. At supine rest as well as during sitting and standing, hypertensive subjects had plasma norepinephrine concentrations similar to those in normotensive control subjects, but plasma dihydroxyphenylglycol concentrations were higher than those in normotensive control subjects. Both groups showed a linear relation between plasma dihydroxyphenylglycol (ordinate) and plasma norepinephrine (abscissa). The resulting regression line was steeper (p less than 0.02) and its ordinate intercept higher (p less than 0.01) in hypertensive than in control subjects. Eleven normotensive and 14 hypertensive subjects were also tested 3 hours after desipramine (1.5 mg/kg orally) was administered to inhibit neuronal norepinephrine reuptake. The drug did not alter plasma norepinephrine, but did reduce plasma dihydroxyphenylglycol and did abolish plasma dihydroxyphenylglycol responses to upright posture in both groups of subjects. The mean plasma dihydroxyphenylglycol concentration observed in the presence of desipramine again was higher in the hypertensive than in the control group (p less than 0.01) and closely agreed, in both groups, with the dihydroxyphenylglycol concentration given by the ordinate intercept of the dihydroxyphenylglycol versus norepinephrine regression line in the absence of desipramine.(ABSTRACT TRUNCATED AT 250 WORDS)