Circulatory effects of noradrenaline and adrenaline before and after labetalol

Properties of labetalol, a combined alpha- and beta-blocking agent, relevant to the treatment of myocardial ischemia

Labetalol, a combined alpha-beta-adrenergic antagonist, is one of the new group of beta-adrenergic blockers reduces peripheral and coronary vascular resistances while preserving cardiac output. Unlike alpha-adrenergic blockers, labetalol tends to reduce heart rate during rest and exercise. The drug is a potent antihypertensive agent which has been used by mouth and by vein to treat mild, moderate, and severe hypertension, including hypertensive emergencies. Labetalol has a hemodynamic profile which makes it an attractive agent for treating myocardial ischemia. The drug reduces blood pressure, left ventricular wall tension, heart rate, and contractility while preserving or even augmenting coronary blood flow. Studies with labetalol in hypertensive patients with angina have shown it to be more effective than placebo in reducing angina attacks and blood pressure while improving exercise tolerance. The drug appears to have antianginal and antihypertensive effects comparable to atenolol and propranolol. Side effects of treatment are observed and most are related to alpha- and beta-adrenergic blockade. Labetalol also appears to be effective for treatment of normotensive patients with angina and for silent myocardial ischemia. It has no apparent effects on serum lipids and lipoproteins. Labetalol appears to be a useful drug for treating the hypertensive heart and its many complications.

Effect of indoramin, labetalol and alinidine on sympathetic function in normal man

The effects of single oral doses of indoramin (mean dose 58 mg), abetalol (mean dose 150 mg), alinidine 80 mg and placebo on arterial pressure and heart rate in the supine and standing positions were studied in six normal volunteers. Doses were chosen to give equivalent reductions of arterial pressure in the standing position. Observations were made before and at 2 and 4 h after drug administration. Plasma noradrenaline (NA) was measured at each time interval in the supine position, and after 4 min of standing. Plasma renin activity (PRA) was measured at each time interval after 30 min in the standing position. In the supine position, alinidine produced a significant reduction of systolic arterial pressure from 124.0 +/- 3.0 mm Hg to 104.3 +/- 4.1 mm Hg at 2 h (P less than 0.01) and to 101.7 +/- 2.2 mm Hg at 4 h (P less than 0.01). Diastolic pressure was reduced from 74.7 +/- 2.6 mm Hg to 57.0 +/- 4.6 mm Hg at 4 h (P less than 0.01). Arterial pressure was unchanged after indoramin or labetalol administration. In the supine position, heart rate was unchanged after indoramin, and small reductions were observed after labetalol and alinidine. Indoramin produced a significant increase in plasma NA. A small increase of plasma NA was observed after labetalol, and a small decrease after alinidine. In the standing position, the three active drugs reduced systolic arterial pressure to a similar extent (indoramin, -26.7 mm Hg at 4 h after drug administration; labetalol, -21.3 mm Hg at 2 h; alinidine, -21.7 mm Hg at 4 h).(ABSTRACT TRUNCATED AT 250 WORDS)

Labetalol: a review of its pharmacology, pharmacokinetics, clinical uses and adverse effects

Labetalol is a combined alpha- and beta-adrenoceptor blocking agent for oral and intravenous use in the treatment of hypertension. It is a nonselective antagonist at beta-adrenoceptors and a competitive antagonist of postsynaptic alpha 1-adrenoceptors. Labetalol is more potent at beta that at alpha 1 adrenoceptors in man; the ratio of beta-alpha antagonism is 3:1 after oral and 6.9:1 after intravenous administration. Labetalol is readily absorbed in man after oral administration, but the drug, which is lipid soluble, undergoes considerable hepatic first-pass metabolism and has an absolute bioavailability of approximately 25%. There are no active metabolites, and the elimination half-life of the drug is approximately 6 hours. Unlike conventional beta-adrenoceptor blocking drugs without intrinsic sympathomimetic activity, labetalol, when given acutely, produces a decrease in peripheral vascular resistance and blood pressure with little alteration in heart rate or cardiac output. However, like conventional beta-blockers, labetalol may influence the renin-angiotensin-aldosterone system and respiratory function. Clinical studies have shown that the antihypertensive efficacy of labetalol is superior to placebo and to diuretic therapy and is at least comparable to that of conventional beta-blockers, methyldopa, clonidine and various adrenergic neuronal blockers. Labetalol administered alone or with a diuretic is often effective when other antihypertensive regimens have failed. Studies have shown that labetalol is effective in the treatment of essential hypertension, renal hypertension, pheochromocytoma, pregnancy hypertension and hypertensive emergencies. In addition, preliminary studies indicate that labetalol may be of value in the management of ischemic heart disease. The most troublesome side effect of labetalol therapy is posture-related dizziness. Other reported side effects of the drug include gastrointestinal disturbances, tiredness, headache, scalp tingling, skin rashes, urinary retention and impotence. Side effects related to the beta-adrenoceptor blocking effect of labetalol, including asthma, heart failure and Raynaud's phenomenon, have been reported in rare instances.

Sympathetic nervous activity and the pressor effect of noradrenaline under chronic alpha-beta-adrenoceptor blockade with labetalol in hypertension

In 14 patients with essential hypertension, the influence of the alpha- and beta-adrenoceptor blocking drug labetalol on blood pressure, heart rate, plasma renin, plasma noradrenaline and pressor effect of exogenous noradrenaline was investigated during long-term treatment. During the initial four weeks of treatment, labetalol at a dose of 400 mg/day showed a slight effect only on supine blood pressure, whereas upright blood pressure was already lowered effectively after the second week of treatment (p less than 0.01). An increase in the mean dose to 850 mg/day had an additional blood pressure-lowering effect (p less than 0.001), whereby a preferential decrease of the orthostatic blood pressure was no longer apparent. Further increase in the mean dose to 1,000 mg/day at the end of the 12th week did not have an additional blood pressure-lowering effect. Body weight, plasma renin and plasma noradrenaline remained unchanged on labetalol treatment in the lowest and the highest dose. There was, however, an increased pressor effect of exogenous noradrenaline, i.e. an alpha-adrenoceptor antagonistic effect of labetalol was not detectable under these conditions. The cause of the increased pressor effect was a reduced elimination of noradrenaline from plasma, which is probably the consequence of an inhibition of the uptake 1 process by labetalol. During long-term treatment with the doses administered, the blood pressure-lowering effect of labetalol appears essentially to be the expression of the beta-adrenoceptor blocking properties of the drug.

Comparison of labetalol with other anti-hypertensive drugs

1 The anti-hypertensive effects of labetalol have been compared and contrasted with other groups of anti-hypertensive drugs in this review of the published literature. 2 The data show that the pharmacological and haemodynamic profile of labetalol in man is distinctly different from that of other specific anti-hypertensive agents; namely the properties of competitive alpha-and beta-adrenoceptor blockade leading to haemodynamic effects of reduced blood pressure and peripheral vascular resistance with little accompanying changes in resting heart rate or cardiac output. 3 The anti-hypertensive effects of labetalol are dose related. In fixed dose comparative studies equivalent anti-hypertensive effects to those of labetalol have been shown for individual drugs of the beta-adrenoceptor-blocking and diuretic groups. In dose titration studies, equivalent anti-hypertensive effects at given doses of labetalol have been demonstrated for drugs of the following types: beta-adrenoceptor blockers, beta-blockers plus diuretics, methyldopa, adrenergic neurone blockers and the combination of beta-blockers plus a peripheral vasodilator. 4 Comparing side-effect liabilities, it is clear that quantitatively labetalol produces no greater burden of side-effects than drugs of the beta-adrenoceptor-blocking group. Qualitative differences, however, do exist; in particular, symptomatic postural hypotension is dose related and is more likely to occur when excessive doses (greater than 2 g daily) are used.

The effect of labetalol and propranolol on the pressor response to sexual arousal in women

1 The effect of a single oral dose of labetalol (100 mg), propranolol (80 mg) and placebo on the pressor response to sexual autostimulation has been studied in six female volunteers. 2 Labetalol but not propranolol significantly reduced the increase in blood pressure that occurred at orgasm. 3 The subjective features of the sexual response were assessed by each subject using visual analogue scales. 4 Subjects reported a significant reduction in vaginal lubrication with labetalol compared to both placebo and propranolol. 5 No other effects were noted.