Sympathetic outflow to muscles during treatment of hypertension with metoprolol.

Abstract

SUMMARY Microelectrode recordings of multiunit sympathetic vasoconstrictor activity were made in muscle branches of the peroneal nerve in patients with essential hypertension before and during long-term treatment with the cardioselective beta-adrenergic receptor antagonist metoprolol. Nerve activity was quantified by counting the number of sympathetic bursts in the mean voltage neurogram. Metoprolol treatment lowered blood pressure and heart rate in all subjects. During longterm treatment, nerve activity was reduced both when compared to the level of activity after the first dose of the drug (p < 0.01) and when compared to the control level before treatment (p < 0.05). It is suggested that the reduction of sympathetic vasoconstrictor outflow to muscles contributed to the blood pressure reduction. (Hypertension 6: 557-562, 1984) KEY WORDS • microelectrode recording • blood pressure • beta-adrenergic receptor antagonists D ESPITE widespread use of beta-adrenergic receptor antagonists in the treatment of arterial hypertension, we do not fully understand how they exert their antihypertensive effect. Acute intravenous administration of beta-adrenergic receptor antagonists devoid of intrinsic sympathomimetic activity reduces heart rate and cardiac output, while the blood pressure remains unchanged. Initially the peripheral vascular resistance increases, but with prolonged therapy it decreases and causes the blood pressure to fall, since the cardiac output remains reduced.1 It is believed that the initial increase and the subsequent decrease in peripheral resistance is due to corresponding changes in sympathetic outflow, but no direct experimental evidence is available in humans because of the lack of adequate methods for measuring sympathetic nerve activity.The development of the microneurographic technique 2 now makes possible the direct recording of sympathetic activity in human peripheral nerves. With this technique it has been found that sympathetic outflow to muscles is involved in blood pressure control. The sympathetic impulses are discharged in bursts in synchrony with cardiac rhythm, and their outflow is influenced by both arterial baroreceptors and intrathoracic low-pressure receptors. The activity can be quantified by counting the bursts and, although there are wide interindividual differences, in a given individual the level of sympathetic activity is remarkably constant over many months. 3 In the present study, we used this technique to investigate whether sympathetic outflow to muscles in patients with essential hypertension is influenced by long-term treatment with metoprolol, a cardioselective beta-adrenergic receptor antagonist without intrinsic sympathomimetic activity. The effect of acute intravenous administration of the drug has been presented in a previous report. 4 Methods PatientsEight patients who had been hospitalized for hypertension were recruited for the study, seven men and one woman aged 26 to 51 (mean 40) years. Hypertension was defined according to the World Health Organization (WH...

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