Effect of labetalol on the accumulation and release of noradrenaline in rat ventricle

Impact of medications on mIBG uptake, with specific attention to the heart: Comprehensive review of the literature

BACKGROUND

A critical review of the literature on drug interactions with mIBG uptake was performed to allow formulation of contemporary guidance regarding withholding medications prior to clinical imaging studies.

METHODS

Published information was extracted on the experimental system used, the quantitative characteristics of the measurements, and whether any data directly examining cardiac tissues were included. Level of evidence for each medication category was assessed on a qualitative scale of very low, low, medium, or high. Strength of medication effect for inhibition of mIBG uptake was judged as none, weak, moderate, or strong.

RESULTS

The only medications for which level of evidence was judged high were labetalol and reserpine. Level of evidence was judged medium for tricyclic antidepressants, calcium channel blockers, and antiarrhythmics (specifically amiodarone). Evidence was judged sufficient to recommend withholding labetalol and the tricyclic antidepressants prior to mIBG cardiac imaging. Mechanistic evidence was sufficient to suggest consideration of withdrawal of sympathomimetic amines and serotonin-norepinephrine reuptake inhibitors (SNRIs).

CONCLUSIONS

As there is strong evidence for inhibition of mIBG uptake in only a small number of compounds, clinical decisions regarding withdrawal of concomitant medications should be individualized by considering the potential consequences of a false-positive (artificially low cardiac uptake) imaging result.

Interaction of metaiodobenzylguanidine with cardioactive drugs: an in vitro study

Metaiodobenzylguanidine (MIBG), an analogue of noradrenaline, is used to explore the functional integrity of sympathetic nerve endings in the human heart. Various drugs inhibit noradrenaline transport systems and may block the uptake of MIBG. As in vivo studies of the effect of these drugs on myocardial [123I]MIBG uptake are often difficult to perform, we used an in vitro human blood platelet model for this purpose. A platelet preparation from healthy volunteers was incubated with [125I]MIBG alone or different concentrations of drugs currently used in cardiology. Labetalol and propranolol inhibited [125I]MIBG uptake, whereas all other drugs tested (other beta-blockers, calcium inhibitors, digoxin and amiodarone) had no effect even at doses exceeding 50 microM. The labetalol dose inhibiting 50% of [125I]MIBG uptake was lower than the plasma concentration of this drug in treated patients, whereas the propranolol dose was higher. This in vitro study of the effect of drugs on MIBG uptake by human blood platelets is predictive of their in vivo effect on myocardial uptake of [123I]MIBG in treated patients, provided that plasma concentration is taken into account.

Myocardial uptake of meta-[123I]-iodobenzylguanidine [( 123I]-MIBG) in patients with myocardial infarct

Meta-123I-iodobenzylguanidine (123I-MIBG), which is an analog of norepinephrine, can be used to evaluate the integrity and function of sympathetic nerve endings in the heart. Myocardial uptake of 123I-MIBG was studied in 30 myocardial infarction patients and compared with the distribution of blood flow assessed with 201Tl. It was found that when a cold defect appeared on the 201Tl scintigram, its localization was identical to the cold defect on the 123I-MIBG scintigram. On the other hand, in three cases, a defect was found on the 123I-MIBG scintigram, corresponding to the electrocardiographic localization of the infarct, whereas the 201Tl scintigram was normal. Most strikingly, the present study shows that drugs (antagonists of the adrenergic receptors, calcium antagonists, amiodarone) decrease or even abolish (as in the case of labetalol) myocardial uptake of 123I-MIBG. Consequently, any interpretation of the 123I-MIBG scintigram must take into account the treatment administered.

The effects of KF-4317, a novel combined alpha- and beta 1-adrenoreceptor antagonist, on the rat isolated right ventricle and aorta

The effects of KF-4317 on the accumulation of radioactivity from [3H]-noradrenaline, and on the subsequent spontaneous and noradrenergic nerve-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition the effects of KF-4317 on the contractions of the electrically-driven directly muscle stimulated rat right ventricle to isoprenaline and of the rat isolated aorta to phenylephrine and 5-hydroxytryptamine are reported. KF-4317 at 1 microM had no effect on the ability of the rat right ventricle to accumulate radioactivity from [3H]-noradrenaline. The spontaneous outflow of radioactivity, following loading of the ventricle with [3H]-noradrenaline, was increased by KF-4317 at 1 microM by a cocaine-insensitive mechanism. KF-4317 at 1 microM had no effect on the noradrenergic nerve-evoked outflow or radioactivity, and therefore is not an alpha 2-adrenoreceptor antagonist, but reduced the associated contractile response probably mainly by acting as an antagonist at postjunctional beta 1-adrenoreceptors. KF-4317 caused a parallel rightward shift of the concentration-response curve of the electrically-driven directly muscle stimulated rat right ventricle to isoprenaline. However the inhibitory effect, X9.0 and X237.2 in the presence of 0.1 and 1 microM KF-4317, was not closely concentration-related. At 1 microM, KF-4317 also depressed the maximum responses to isoprenaline. This suggests that in addition to beta 1-adrenoreceptor antagonism, KF-4317 probably exerts membrane stabilizing activity. The responses of the rat isolated aorta to phenylephrine were inhibited in a non-concentration related manner by KF-4317.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The effects of amosulalol, a novel combined alpha- and beta-adrenoreceptor antagonist, on the rat isolated right ventricle and aorta

The effect of amosulalol on the accumulation of radioactivity from [3H]-noradrenaline and on the subsequent spontaneous and nerve stimulation-evoked outflow of radioactivity have been investigated in the rat isolated right ventricle. In addition the effect of amosulalol on the contractions of the electrically-driven directly muscle stimulated rat right ventricle to isoprenaline and of the rat isolated aorta to phenylephrine and 5-hydroxytryptamine are reported. Amosulalol at 10(-6)M did not prevent the accumulation of radioactivity from a solution containing [3H]-noradrenaline. The spontaneous outflow of radioactivity, following loading of the ventricle with [3H]-noradrenaline, was increased by amosulalol at 10(-6)M by a cocaine- and idazoxan- insensitive mechanism. The nerve stimulation-evoked outflow of radioactivity was increased by amosulalol (10(-6)M), cocaine (10(-5)M) and idazoxan (10(-7)M). The ability of amosulalol to increase nerve-evoked outflow was maintained in the presence of cocaine but prevented by pretreatment with idazoxan. This suggests that amusulalol is an alpha 2-adrenoreceptor antagonist. The contractile response of the electrically-driven directly muscle stimulated right ventricle to isoprenaline were inhibited by amosulalol at 10(-7) and 10(-6)M with apparent pA2 values of 7.5 and 8.1, respectively. It is suggested that amosulalol at 10(-6)M may have an action additional to beta 1-adrenoreceptor antagonism on the right ventricle. The contractile responses of the rat aorta to phenylephrine were inhibited by amosulalol at 10(-7) and 10(-6)M with a pA2 of 8.6 which was independent of concentration. Amosulalol also reduced the magnitude of the maximal responses of the aorta 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)

Inhibition of synaptosomal [3H]noradrenaline uptake by beta-adrenoceptor blocking drugs: influence of lipophilicity

Ten beta-adrenoceptor blocking drugs varying in lipophilicity and beta-adrenoceptor blocking potency were examined for inhibitory effects on synaptosomal [3H]noradrenaline uptake. All compounds produced a concentration-dependent inhibition of noradrenaline uptake, but were at least one order of magnitude less potent than desmethylimipramine and cocaine. The order of potency was pronethalol greater than propranolol greater than betaxolol greater than alprenolol greater than oxprenolol greater than practolol greater than metoprolol greater than acebutolol greater than sotalol greater than atenolol, with IC50 values ranging from 4.0 X 10(-6) to 2.2 X 10(-3) M. Uptake inhibition was unrelated to beta-adrenoceptor blocking potency, but was highly correlated with drug lipophilicity. (+)-Propranolol was an effective uptake inhibitor, as was the local anaesthetic procaine. Kinetic analysis of uptake inhibition by propranolol, oxprenolol, metoprolol and procaine revealed a mixed inhibition for all four agents examined. It is suggested that this effect of beta-adrenoceptor blockers may be mediated, at least in part, by an action on membrane phospholipids associated with the noradrenaline carrier protein, and that noradrenaline uptake inhibition may underlie certain central side-effects observed with some drugs in this group.

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.

Effects of chronic alpha and beta adrenoceptor blockade with labetalol on plasma catecholamines and renal function in hypertension

Plasma catecholamines and renal function were evaluated in 18 patients with essential hypertension treated with the alpha and beta adrenoceptor blocking agent, labetalol. Following 6 weeks of labetalol therapy, blood levels of epinephrine and norepinephrine remained unaltered. Glomerular filtration rate and renal plasma flow were decreased similarly by about 20% (P less than 0.025). Tubular rejection fraction of sodium was increased by 36% (P less than 0.001) while sodium excretion was comparable to control conditions. Labetalol's potential to cause a mild reduction in kidney function should be considered, but may have no clinical consequences in most hypertensive patients receiving such treatment. The lack of increased plasma catecholamine levels during therapy supports the concept that labetalol's alpha-blocking potential is limited to post-junctional receptors, leaving the prejunctional feedback control of catecholamine release intact. Moreover, labetalol's blood pressure-lowering mechanism may be largely independent of changes in sympathetic nervous activity.