Inhibition of intravascular platelet aggregation in the rat by adrenaline

Intravascular platelet aggregation to adenosine diphosphate (ADP) was measured in anesthetized rats using 111indium-labelled platelets. Acute adrenalectomy increased the aggregatory effect of ADP in vivo, whereas infusions of low concentrations of adrenaline into adrenalectomized rats suppressed ADP-induced platelet aggregation. Similar antiaggregatory effects were seen with the alpha 2 agonist B-HT 933 and the alpha 1 agonist methoxamine, but not with isoprenaline. The effect of adrenaline was inhibited by phentolamine, yohimbine, the selective alpha 2 adrenoceptor antagonist WY 26392 and by indomethacin, but not by propranolol or prazosin. Adrenaline thus inhibits ADP-induced aggregation in vivo by a mechanism that may involve stimulation of an alpha 2 adrenoreceptor and may be dependent on activation of cyclooxygenase enzyme.