Mechanism of Action

Labetalol produces equilibrium-competitive antagonism at p-receptors but does not exhibit selectivity for p1- or p2-receptors. Like certain other p-blockers (e.g., pindolol and timolol), labetalol possesses some degree of intrinsic activity. This intrinsic activity, or partial ago-nism, especially at p2-receptors in the vasculature, has been suggested to contribute to the vasodilator effect of the drug. The membrane-stabilizing effect, or local anesthetic action, of propranolol and several other p-blockers, is also possessed by labetalol, and in fact the drug is a reasonably potent local anesthetic.

The a-blockade produced by labetalol is also of the equilibrium-competitive type. In a manner similar to prazosin, labetalol exhibits selectivity for a1-receptors. Presynaptic a-receptors, which are of the a2 subclass, are not antagonized by labetalol. The drug also has some intrinsic activity at a-receptors, although this action is less than its intrinsic p-receptor-stimulating effects.

Labetalol appears to produce relaxation of vascular smooth muscle not only by a-blockade but also by a partial agonist effect at p2-receptors. In addition, la-betalol may produce vascular relaxation by a direct non-receptor-mediated effect.

Labetalol can block the neuronal uptake of norepi-nephrine and other catecholamines. This action, plus its slight intrinsic activity at a-receptors, may account for the seemingly paradoxical, although infrequent, increase in blood pressure seen on its initial administration.

Coping with Asthma

Coping with Asthma

If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.

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