MOLECULAR MECHANISM OF RESISTANCE OF EQUINE (EQUUS-CABALLUS) PLATELETS TO ALPHA(2)-ADRENERGIC REGULATION

Adrenaline is a weak aggregating agonist for human platelets acting th rough G-protein-coupled alpha(2)-adrenoceptors to inhibit adenylate cy clase and thus reduce cyclic AMP levels. Studies of equine platelets h ave shown that adrenaline is unable to promote their aggregation. We n ow confirm that adrenaline is without effect on equine platelet aggreg ation and demonstrate that it is also without effect on equine platele t membrane adenylate cyclase activity. We have previously shown that e quine platelet membranes contain conventionally regulated adenylate cy clase activity, with both stimulatory ligands (forskolin and PGE(1)) a nd inhibitory ligands (collagen and PAF) each showing substantial and dose-dependent effects. We now show, in Western blots, that equine pla telet membranes contain G proteins, including G(i)2 (which mediates in hibition of adenylate cyclase by adrenaline in human platelets), G(i)3 , G(5), and G(q). Hence, all the necessary components and responses ar e in place in equine platelets to provide for a conventional role for cyclic AMP and adenylate cyclase in modulating platelet aggregation. T he basis for the failure of adrenaline, unlike other ligands, to deliv er such a signal, appears to be a marked lack of alpha(2)-adrenoceptor s. This is supported by the low receptor density we found in idazoxan binding studies. (C) 1997 Elsevier Science Inc.