Alpha 1-adrenergic receptor subtypes. Molecular structure, function, and signaling.

Abstract

The α1ARs are important mediators of sympathetic nervous system responses, particularly those involved in cardiovascular homeostasis, such as arteriolar smooth muscle constriction and cardiac contraction.1 2 In addition, α1ARs have more recently been implicated in the pathogenesis of cardiac hypertrophy, in ischemia-induced cardiac arrhythmias, and in ischemic preconditioning.1 3 Like other ARs, α1ARs are activated by the catecholamines, norepinephrine and epinephrine. They are intrinsic membrane glycoproteins and are members of the GPCR superfamily. Over the past 10 to 15 years, data initially based on functional, radioligand, and biochemical studies have accumulated, indicating that the α1ARs are a heterogeneous group of distinct but related proteins. This conclusion has been confirmed with the molecular cloning of three distinct α1-receptor subtypes, although until recently discrepancies between the properties of the cloned expressed receptors and those characterized pharmacologically and biochemically have led to confusion in the classification of α1-receptor subtypes and their coupled effector responses. As detailed in the present review, much of this confusion has now been clarified for the three cloned α1ARs. These and other recent insights into the molecular structure, function, and signaling of α1ARs, the control of α1AR-gene expression, and pharmacological evidence for additional α1AR subtypes will be reviewed here. For additional information, the reader is also referred to several previous reviews of α1ARs.4 5 6 7 Functional studies of AR responses, particularly from the laboratories of McGrath8 and Ruffolo,9 provided the initial evidence that there may be subtypes of α1ARs. These studies indicated that postjunctional responses mediated by α1ARs could not be explained adequately on the basis of a single population of receptors. This concept was further advanced …