Effect of labetalol on the uptake of [3H]-(-)-noradrenaline into the isolated vas deferens of the rat

Labetalol facilitates GABAergic transmission to rat periaqueductal gray neurons via antagonizing beta1-adrenergic receptors--a possible mechanism underlying labetalol-induced analgesia

Labetalol, a combined alpha1, beta1, and beta2 adrenoceptor-blocking drug, has been shown to have analgesic properties in vivo. To determine the underlying mechanisms, we examined its effects on GABAA receptor-mediated spontaneous inhibitory postsynaptic currents (sIPSCs) and spontaneous firings of rat ventrolateral periaqueductal gray (PAG) neurons, either mechanically dissociated, or in acute brain slices. These PAG neurons mediate opioid-mediated analgesia and pain transmission and are under tonic control of GABAergic interneurons. An increase in GABAergic transmission to these neurons yields an inhibitory hyperpolarized state and may interrupt pain signal transmission. Using patch clamp techniques, we found that labetalol reversibly increases the frequency of sIPSCs without changing their mean amplitude. This indicates that labetalol enhances GABAergic synaptic transmission by a presynaptic mechanism. Metoprolol, a specific beta1-adrenoceptor antagonist, also reversibly enhanced sIPSC frequency. In the presence of metoprolol, labetalol-induced increase in sIPSC frequency was significantly attenuated or even abolished. These results suggest that labetalol shares the same pathway as metoprolol in enhancing GABAergic transmission via an inhibition of presynaptic beta1-adrenoceptors. We further showed that labetalol reversibly reduced the firing rate of PAG neurons. This reduction was significantly attenuated in the presence of bicuculline, a selective antagonist of GABAA receptors. These data indicate that labetalol-induced inhibition of PAG cell firing is attributable to its potentiation of GABAergic transmission. Based on these data, we postulate that labetalol-induced analgesia is at least in part ascribed to its antagonistic effects on presynaptic beta1-adrenoceptors.

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.

Non-specific, time-dependent desensitization of the vas deferens and anococcygeus preparations of the rat to alpha 1-adrenoceptor antagonists and atropine

Rat vas deferens preparations became desensitized to the alpha 1-adrenoceptor antagonist thymoxamine: after 6 h in vitro, the t 1/2 value (time to attain half the occupancy of receptors occupied at equilibrium) of the response to this drug was 1.50 fold greater in control strips (strips exposed to thymoxamine at 6 h) than in test strips (strips exposed to thymoxamine at 1 h). The rate of action of the alpha 1-adrenoceptor antagonist AR-C239 on the rat anococcygeus preparation was correlated with the rate of action of atropine. There was also a significant correlation between the t 1/2 ratios (1.37 and 1.30 for AR-C239 and atropine respectively) observed in the control muscles at 6 h. The in vitro slowing is thus due to some change in the longitudinal muscle and not to a change in the receptors. The in vitro slowing occurred when either phenylephrine or methoxamine was the alpha 1-adrenoceptor agonist used. The most likely mechanism of desensitization is a non-specific slowing of the access of drugs to receptors.

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.

A pharmacological method to estimate the pKi of competitive inhibitors of agonist uptake processes in isolated tissues

An equation is derived from a mathematical model (proposed by Furchgott) which, under certain circumstances, estimates the pKI (-log dissociation constant) of a competitive inhibitor of agonist uptake by utilizing the sensitization of isolated tissues, to the substrate-agonist, by uptake inhibition. The method is theoretically more sound and appears to be improved by the use of potency-ratios of the substrate-agonist and an agonist which is not a substrate for the uptake process since this allows for the detection and correction of receptor and toxic effects of uptake inhibitors. The pKI values of cocaine, desmethylimipramine and imipramine for the neuronal uptake of norepinephrine were estimated by this method in guinea-pig tracheae and left atria. Also, the pKI values for 17 beta-oestradiol, corticosterone, clonidine and metanephrine for the extraneuronal uptake of isoproterenol were estimated in guinea-pig tracheae (and cat left atria for 17 beta-oestradiol). All estimates were consistent with literature pKI values obtained biochemically with radiolabelled substrates.

Cardioregulatory properties of indoramin in the rat

The antihypertensive action of the competitive alpha-adrenoceptor antagonist indoramin is not accompanied by reflex tachycardia in animals or man. The possibility that the established local anaesthetic property of indoramin is involved in its cardioinhibitory action has been investigated. Indoramin evoked a dose-dependent bradycardia in anaesthetized/pithed rats. The decrease in heart rate was slightly greater than that evoked in anaesthetized intact animals suggesting that indoramin had a direct action on the heart. Reflex tachycardia was simulated in pithed rats be increasing the frequency of cardiac nerve stimulation from 0.3 to 1 Hz. Indoramin and the local anaesthetic agents, lignocaine and procaine, reduced the positive chronotropic response without markedly altering the basal rate. The response curves were parallel. In contrast, phentolamine decreased the positive chronotropic response, but only at high doses which were associated with a marked decrease in the basal rate. Thymoxamine and prazosin had so significant effects on the chronotropic response. These experiments suggest that the cardioregulatory action of indoramin is attributable to its local anaesthetic property had this action further distinguishes it from the other alpha-adrenoceptor antagonists tested.

One the importance of agonist concentration-gradients within isolated tissues. Increased maximal responses of rat vasa deferentia to (--)-noradrenaline after blockade of neuronal uptake

It is assumed often that blockade of agonist uptake processes in isolated tissues results only in shifts to the left of the concentration-response curves to the agonist with no concomitant increase in the maximal response. This may not be true in tissues where diffusion is not fast enough to permit penetration of the agonist to a sufficient number of muscle cells for production of tissue maximal response. Under these circumstances an agonist-concentration gradient is created within the tissue which, when altered, could lead to an increase in the maximal response to the agonist. The increased maximal responses of rat visa deferentia to (--)-noradrenaline after blockade of neuronal uptake by either cocaine or desmethylimipramine have been analysed in terms of a concentration-gradient hypothesis. The data are compared with a theoretical calculation based on a model of restricted diffusion of enzyme-substrates into structured tissues. Both the experimental data and theoretical calculations suggest that an altered concentration-gradient of (--)-noradrenaline within the muscle layers of rat vasa deferentia is responsible for the increased maximal response. The effects of such gradients are discussed in terms of quantitation of drug-receptor phenomena and as a caveat to ascribing increases in maximal responses to post-synaptic effects of uptake inhibitors.

Circulatory effects of noradrenaline and adrenaline before and after labetalol

1 Two studies were carried out in the same six normal healthy males to compare the circulatory effects of exogenously infused noradrenaline and adrenaline before and after intravenous labetalol.

2 Noradrenaline before and after labetalol produced a similar pattern of circulatory change, namely dose related systolic and diastolic pressor responses accompanied by profound bradycardia and reduced cardiac output.

3 Labetalol competitively antagonized both the systolic and diastolic pressor effects of noradrenaline.

4 Adrenaline at low doses produced a diastolic depressor response accompanied by increases in heart rate and cardiac output. The highest dose also produced smaller increases in heart rate and cardiac output and diastolic pressure increased slightly. All doses provoked increases in systolic pressure.

5 After labetalol all doses of adrenaline provoked a diastolic pressor response which was marked at high doses. The systolic response was attenuated compared with that before labetalol. At all doses, heart rate and cardiac output were reduced.

6 Since in the presence of labetalol both noradrenaline and adrenaline profoundly reduce heart rate and cardiac output whilst increasing blood pressure, it may be unwise to use either of these catecholamines alone if attempts are being made to reverse excessive hypotension from labetalol overdose.

7 Preliminary observations indicate that labetalol does not increase endogenous plasma catecholamines immediately after intravenous injection. Plasma concentrations of noradrenaline and adrenaline after exogenous infusion were increased after labetalol compared with those levels before labetalol. It appears that labetalol inhibits the clearance of catecholamines from the plasma possibly by inhibiting neuronal uptake mechanisms.