Subtypes of functional α1- and α2-adrenoceptors

In this review, subtypes of functional alpha(1)- and alpha(2)-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven transmembrane spanning G-protein-linked family of receptors, which respond to the physiological agonists noradrenaline and adrenaline, alpha(1)-Adrenoceptors can be divided into alpha(1A)-, alpha(1B)- and alpha(1D)-adrenoceptors, all of which mediate contractile responses involving G(q/11) and inositol phosphate turnover. A 4th alpha(1)-adrenoceptor, the alpha(lL)-, has been postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. alpha(2)-adrenoceptors can be divided into alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors, all of which mediate contractile responses. Prejunctional inhibitory alpha(2)-adrenoceptors are predominantly of the alpha(2A)-adrenoceptor subtype (the alpha(2D)-adrenoceptor is a species orthologue), although alpha(2C)-adrenoceptors may also occur prejunctionally. Although alpha(2)-adrenoceptors are linked to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory actions probably involve restriction of Ca2+ entry or opening of K+ channels. Receptor knock-out mice are beginning to refine our knowledge of the functions of ol-adrenoceptor subtypes. (C) 1998 Elsevier Science B.V. All rights reserved.