Binding, partial agonism, and potentiation of alpha(1)-adrenergic receptorfunction by benzodiazepines: A potential site of allosteric modulation

Benzodiazepines, a class of drugs commonly used to induce anesthesia and se dation, can attenuate intracellular calcium oscillations evoked by alpha(1) -adrenergic receptor (alpha(1)-AR) stimulation in pulmonary artery smooth m uscle cells. We postulated a direct action of benzodiazepines in modulating alpha(1)-AR function at the receptor level. Benzodiazepines bound to each of the cloned alpha(1)-AR subtypes (alpha(1a)-, alpha(1b)-, or alpha(1d)-AR ) on COS-1 cell membranes transiently transfected to express a single popul ation of alpha(1)-AR subtype. The ability of benzodiazepines to alter alpha (1)-AR signal transduction was investigated by measuring total inositol pho sphate generation in rat-1 fibroblast cells, stably transfected to express a single alpha(1)-AR subtype. By themselves, benzodiazepines displayed part ial agonism. At alpha(1b)-ARs and alpha(1d)-ARs, the maximal inositol phosp hate response to phenylephrine was potentiated almost 2-fold by either mida zolam or lorazepam (100 mu M). At alpha(1a)-ARs, diazepam, lorazepam, and m idazolam all increased the maximal response of the partial agonist clonidin e at these receptors, whereas the response to the full agonist phenylephrin e was unaltered or inhibited. The potentiating actions of midazolam and its partial agonism at alpha(1)-ARs was blocked by the addition of 1 mu M praz osin, an alpha(1)-AR antagonist, and not by a gamma-aminobutyric acid(A)-re ceptor antagonist. These studies show that benzodiazepines modulate the fun ction of alpha(1)-ARs in vitro, and this is the first report of a potential allosteric site on alpha(1)-ARs that may be therapeutically useful for dru g design.