Release of noradrenaline by labetalol in the rat anococcygeus muscle

Comparison of the effects of labetalol and SCH 19927, the R1R isomer of labetalol, on the rat isolated right ventricle and aorta

The effects of labetalol and its R1R isomer, SCH 19927, on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and nerve-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition, the effect of these agents on the contractions of the electrically-driven rat right ventricle to isoprenaline and of the rat isolated aorta to phenylephrine and 5-hydroxytryptamine are reported. Labetalol and SCH 19927 (both at 10(-6)M) inhibited the accumulation of radioactivity from [3H]-noradrenaline by 26 and 37%, respectively. The spontaneous outflow of radioactivity, following loading of the ventricle with [3H]-noradrenaline, was increased by labetalol and SCH 19927 (both at 10(-6)M) by a cocaine and idazoxan-insensitive mechanism. The nerve-evoked outflow of radioactivity was increased by labetalol and SCH 19927 (both at 10(-6)M). The ability of labetalol and SCH 19927, to increase nerve-evoked outflow was maintained in the presence of cocaine (10(-5)M) or idazoxan (10(-7)M) but reversed in the presence of cocaine and idazoxan. It is suggested that labetalol and its R1R isomer act both to inhibit neuronal uptake of noradrenaline and as antagonists at prejunctional alpha 2-adrenoreceptors. Labetalol and SCH 19927 reduced the contractile responses associated with the nerve-evoked outflow of radioactivity probably mainly by acting as antagonists at postjunctional beta 1-adrenoreceptors. The contractile responses of the electrically-driven rat right ventricle to isoprenaline were inhibited by labetalol and SCH 19927. SCH 12297 (pA2 = 8.9) was 4 times more potent than labetalol (pA2 = 8.3) as a beta 1-adrenoreceptor antagonist. The ability of labetalol and SCH 19927 at greater than or equal to 10(-7)M to depress maximal responses to isoprenaline may represent membrane stabilizing activity. The contractile responses of rat aorta to phenylephrine were inhibited by labetalol and SCH 19927. Labetalol (pA2 = 7.5) was 4X more potent than SCH 19927 (pA2 = 6.9) as an alpha 1-adrenoreceptor antagonist. SCH 19927 and labetalol had no effect on contractile responses to 5-hydroxytryptamine.

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)

Pharmacology of labetalol in experimental animals

Labetalol represents the culmination of an effort to enhance the antihypertensive efficacy and to improve the hemodynamic profile of beta-adrenoceptor blockers by incorporating an additional anti-hypertensive action, that is, alpha blockade, into its pharmacologic mechanism. Reviewed here are the major aspects of the animal pharmacology of labetalol. The compound blocks beta1 and beta2-adrenoceptors nonselectively. Its blockade of alpha receptors is selective and directed at the alpha1 subset. Labetalol also dilates blood vessels independently of these mechanisms. This action is mediated by activation of vascular beta2 adrenoceptors. Thus, labetalol acts as a partial agonist on vascular smooth muscle. However, it differs markedly from other beta blockers with intrinsic sympathomimetic activity in that its agonism is directed specifically at beta2 receptors. Labetalol lowers blood pressure in a variety of animal models of hypertension. Unlike pure beta blockers, the compound reduces peripheral vascular resistance. On the basis of this profile, it is proposed that labetalol lowers blood pressure in human subjects by three independent mechanisms: (1) beta blockade, (2) alpha blockade, and (3) direct vasodilatation.

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.

Direct vasodilatation by labetalol in anaesthetized dogs

1 The effects of several doses of labetalol (0.03 to 1 mg/kg) given intravenously and into the vertebral artery were examined in anaesthetized dogs. Labetalol produced to immediate (5 min) change in blood pressure or heart rate when given by either route, with one exception. Heart rate increased after the first dose (0.03 mg/kg i.v.) of labetalol. By contrast, clonidine (1 micrograms/kg) elicited an immediate and prolonged fall in blood pressure and heart rate when given into the vertebral artery, but not intravenously. 2 In the isolated perfused gracilis muscle of the dog, following alpha- and beta-adrenoceptor blockade, intra-arterial injections of labetalol (0.3 to 10 mg) or diazoxide (0.3 to 1 mg) produced decreases in perfusion pressure that were dose-related in both magnitude and duration. The doses of labetalol and diazoxide required to produce a half-maximal vasodilatation were 1.5 mg and 0.7 mg respectively. 3 In adrenalectomized, vagotomized spinal dogs, both labetalol (0.1 to 1 mg/kg i.v.) and hydralazine (1 mg/kg i.v.) elicited a fall in blood pressure without changing heart rate or cardiac output. 4. These results suggest that the hypotension produced by systemically administered labetalol does not involve an action in the brain. It may involve instead a direct vasodilatation of resistance blood vessels, since labetalol in sufficient amounts, directly dilates resistance vessels and lowers blood pressure in dogs devoid of adrenergic tone. Direct vasodilatation may be a component of the hypotensive action of labetalol.

Effect of mianserin on noradrenergic transmission in the rat anococcygeus muscle

1 The effects of mianserin on the accumulation of (-)-[(3)H]-noradrenaline and on contractile responses to field stimulation, exogenously applied (-)-noradrenaline, and to tyramine were studied in the rat anococcygeus muscle.2 Mianserin (10(-9) to 10(-5) M) and nortriptyline (10(-9) to 10(-5) M) inhibited the accumulation of (-)-[(3)H]-noradrenaline. In this aspect mianserin had a similar potency to nortriptyline, the most potent tricyclic antidepressant in inhibiting noradrenaline accumulation in this tissue.3 Mianserin 10(-9) or 10(-8) M alone had no effect on contractile responses to field stimulation and to (-)-noradrenaline but inhibited the responses to tyramine. The responses to field stimulation at low frequencies and to (-)-noradrenaline were potentiated by 10(-7) and 10(-6) M mianserin. It is suggested that the inhibitory effect mianserin has on neuronal accumulation is primarily responsible for these effects. Mianserin 10(-5) M inhibited responses to field stimulation and to (-)-noradrenaline.4 In the presence of nortriptyline (10(-6) M), the contractile responses to field stimulations were potentiated by mianserin (10(-8), 10(-7) and 10(-6) M), 10(-8) M being the most potent in this aspect. Mianserin 10(-8), 10(-7), 10(-6) and 10(-5) M had a similar inhibitory effect on responses to (-)-noradrenaline. In the absence of neuronal uptake, the potentiating effect of mianserin on responses to field stimulation may be due to antagonism at presynaptic alpha-adrenoceptors. In the presence of 10(-6) M nortriptyline, 10(-5) M mianserin abolished responses to field stimulation.5 Following incubation of the tissue in the presence of 6-hydroxydopamine (10(-3) M for 3 h), mianserin (10(-7), 10(-6) and 10(-5) M) nortriptyline (10(-7) and 10(-6) M) and phentolamine (5 x 10(-8) and 5 x 10(-7) M) inhibited contractile responses to (-)-noradrenaline. This illustrates the ability of these agents to inhibit the responses to noradrenaline at a postsynaptic site. The inhibitory effect was dose-related with nortriptyline and phentolamine; this illustrates the ability of these agents to antagonize postsynaptic alpha-adrenoceptors. The inhibitory effect observed with mianserin was not dose-related. This suggests that in addition to its reported ability to antagonize postsynaptic alpha-adrenoceptors, mianserin may have another post-synaptic action at the level of, or distal to, the alpha-adrenoceptor.6 These results illustrate that, in the rat anococcygeus muscle, mianserin is a potent inhibitor of noradrenaline accumulation and may be an antagonist at presynaptic alpha-adrenoceptors. Mianserin also inhibits the responses to exogenously applied noradrenaline in this tissue by an action or actions at the level of, or distal to, the postsynaptic alpha-adrenoceptor.

Effects of pyroxamidine and guanethidine on contractile responses to field stimulation and to noradrenaline in the anococcygeus muscle and vas deferens of the rat

The effects of pyroxamidine (EMD 21192) and guanethidine on contractile responses were studied in the anococcygeus muscle and vas deferens of the rat. Pyroxamidine (10−6 and 10−5 M) and guanethidine (6 times 10−6 and 10−5 M) potentiated the responses to low concentrations of acetylcholine in the rat anococcygeus muscle. Following incubation of the muscle with 6-hydroxydopamine (10−3M for 3 h), pyroxamidine (10−5 M) and guanethidine (10−5 M) had no effect on responses to acetylcholine. This suggests that the potentiating effect of pyroxamidine and guanethidine on responses to acetylcholine is due to the release of subthreshold concentrations of noradrenaline.

In the anococcygeus, pyroxamidine (10−6 and 10−5 M) and guanethidine (10−6 and 10−5 M) inhibited responses to field stimulation and potentiated responses to exogenously applied (−)-noradrenaline. The responses to field stimulation in the vas deferens were also inhibited by 10−5M pyroxamidine and by 10−5 M guanethidine. 10−6 M guanethidine, but not 10−5 M pyroxamidine, potentiated responses to (−)-noradrenaline in the vas deferens. In the presence of nortriptyline (10−6 M), a potent inhibitor of neuronal uptake, the inhibitory effects of pyroxamidine and guanethidine on responses to field stimulation were reduced or reversed and these drugs had no effect on responses to (−)-noradrenaline. This suggests that pyroxamidine is a noradrenergic neuron blocker and that its action is dependent on continued neuronal uptake. Following 6-hydroxydopamine incubation, 10−5 M pyroxamidine and 10−5 M guanethidine inhibited the responses to (−)-noradrenaline in the rat anococcygeus muscle. Thus it seems likely, at high concentrations, that these compounds have postsynaptic blocking activity.