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.
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