SODIUM MODULATION OF H-3 AGONIST AND H-3 ANTAGONIST BINDING TO ALPHA(2)-ADRENOCEPTOR SUBTYPES

1 The alpha2-adrenoceptors on human platelets and neonatal rat lung we re characterized with the agonist and antagonist ligands [H-3]-adrenal ine and [H-3]-RS-15385-197 respectively. A correlation of affinities f or H-3-antagonist binding showed the receptors to be of the alpha2A-(p latelet) and alpha2B-(neonatal rat lung) adrenoceptor subtypes, wherea s a correlation of affinities for H-3-agonist binding showed the recep tors to have similar characteristics (r = 0.88).2 NaCl (100 mM) had no effect on [H-3]-RS-15385-197 binding in the human platelet, but incre ased the density of sites labelled with [H-3]-RS-15385-197 in neonatal rat lung by 52%. NaCl increased the density of sites labelled by [H-3 ]-adrenaline in neonatal rat lung, but there was a consequent 3.5 fold decrease in affinity. In the human platelet, no specific [H-3]-adrena line binding was observed in the presence of 100 mM NaCl. 3 In the neo natal rat lung, NaCl had no significant effect on the affinity of praz osin for [H-3]-RS-15385-197 binding; however, imiloxan affinity was in creased 13 fold. The affinity of the catecholamines, adrenaline and no radrenaline was significantly decreased, whereas the imidazolines, oxy metazoline and UK-14,304 were much less affected. The affinity of praz osin and imiloxan for [H-3]-adrenaline binding was significantly incre ased in the presence of 10 and 100 mM NaCl. Conversely, the affinity o f adrenaline and noradrenaline was decreased in the presence of NaCl, and there was no change in the affinity of the imidazoline agonists. 4 In the human platelet, NaCl had no effect on the affinity of prazosin for [H-3]-RS-15385-197 binding but the affinity of imiloxan was signi ficantly increased. NaCl significantly decreased the affinity of the c atecholamines adrenaline and noradrenaline, whereas the affinity of UK -14,304 and oxymatazoline was much less affected. Competition experime nts with [H-3]-adrenaline in the presence of NaCl in platelets were di fficult to characterize as there was no specific binding under these c onditions. 5 The results show that both the alpha2A- and alpha2B-adren oceptor subtypes are allosterically regulated by Na+, but only the alp ha2B-subtype showed a significant increase in density. Interestingly, there is a differential regulation of imidazoline (unchanged) and cate cholamine (decreased affinity) agonist interactions with these subtype s. Na+ may therefore critically affect receptor subtype selectivity of drugs. The implications for receptor subclassification are discussed.