Subtypes of functional alpha(1)- and alpha(2)-adrenoceptors

In this review, subtypes of functional alpha(1)- and alpha(2)-adrenoceptors are discussed. These are cell membrane receptors, belonging to the seven t ransmembrane spanning G-protein-linked family of receptors, which respond t o the physiological agonists noradrenaline and adrenaline, alpha(1)-Adrenoc eptors can be divided into alpha(1A)-, alpha(1B)- and alpha(1D)-adrenocepto rs, all of which mediate contractile responses involving G(q/11) and inosit ol phosphate turnover. A 4th alpha(1)-adrenoceptor, the alpha(lL)-, has bee n postulated to mediate contractions in some tissues, but its relationship to cloned receptors remains to be established. alpha(2)-adrenoceptors can b e divided into alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors, all of w hich mediate contractile responses. Prejunctional inhibitory alpha(2)-adren oceptors are predominantly of the alpha(2A)-adrenoceptor subtype (the alpha (2D)-adrenoceptor is a species orthologue), although alpha(2C)-adrenoceptor s may also occur prejunctionally. Although alpha(2)-adrenoceptors are linke d to inhibition of adenylate cyclase, this may not be the primary signal in causing smooth muscle contraction; likewise, prejunctional inhibitory acti ons probably involve restriction of Ca2+ entry or opening of K+ channels. R eceptor knock-out mice are beginning to refine our knowledge of the functio ns of ol-adrenoceptor subtypes. (C) 1998 Elsevier Science B.V. All rights r eserved.