Studies on α2-adrenoceptor subtypes, imidazoline binding sites and coupling to functional responses

• Main Author: Mackinnon, Alison Crawford
• Published: University of Edinburgh 1993
• Subjects: 572
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.654349

It was the aim of this thesis to characterise α2-adrenoceptor subtypes using radiolabelled agonist and antagonist ligands in a variety of tissue preparations. RS-15385-197 is a high affinity and selective α2-adrenoceptor antagonist, and the compound was shown, by the rank order of affinity of a number of competing ligands, to label α2A and α2B-adrenoceptor subtypes in human platelet and rat neonatal lung membranes, and a subtype in rat cortex which shows greatest similarity with the α2D-adrenoceptor subtype. Thus the receptor in rat brain was shown to form a distinct subtype. Differentiation of the α2-adrenoceptor into 2A and 2B subtypes could not be demonstrated however with the agonist ligand, [<SUP>3</SUP>H]-adrenaline, under normal assay conditions. The functional consquences of α2A-adrenoceptor activation were addressed in a model of α2-adrenoceptor mediated inhibition of cAMP accumulation. As a result of this work, [<SUP>3</SUP>H]-idazoxan an α2-adrenoceptor antagonist with an imidazoline structure, in addition to labelling α2-adrenoceptors, was also shown to label a population of imidazoline binding sites in rat kidney which were not adrenoceptors based on the low affinity of noradrenaline and RS-15385-197. Characterisation of these sites with [<SUP>3</SUP>H]-idazoxan and another imidazoline ligand [<SUP>3</SUP>H]-p-aminoclonidine suggested that the imidazoline sites labelled by these ligands were heterogeneous and were located over discrete areas of rat brain. As a direct consequence of this work a novel compound was identified which had greater than 10,000 fold selectivity for imidazoline sites over α2-adrenoceptors. In the hamster adipocyte, a tissue which I have shown previously to contain both α2-adrenoceptors and imidazoline binding sites, the inhibition of glycerol release by UK14304 was reversed by compounds showing selectivity for α2-adrenoceptors and not by imidazoline selective agents, suggesting that imidazoline sites are not involved in the UK14304 mediated inhibition of lipolysis in this tissue.