The effects of monovalent and divalent cations on the alpha-adrenoceptor of intact human platelets.

1 We have examined the effects of monovalent and divalent cations and purine nucleotides on the binding of agonists and antagonists to the alpha-adrenoceptor of intact human platelets. 2 Replacement of Na+ (150mM) by NH4+ (150mM) in the incubation medium significantly reduced the binding affinity of [3H]-dihydroergocryptine (P less than 0.05) but did not alter the binding capacity. The competitive binding affinity of adrenaline and noradrenaline was unaltered. 3 The addition of Ca2+ (1mM) or Mg2+ (1mM) to the platelet suspension significantly reduced the platelet alpha-adrenoceptor capacity as indicated by either [3H]-dihydroergocryptine (P less than 0.05) or [3H]-yohimbine (P less than 0.01; Ca2+ only). 4 The addition of Ca2+ (1mM) or Mg2+ (1mM) had no effect on the binding affinity of [3H]-dihydroergocryptine but significantly reduced that of [3H]-yohimbine (P less than 0.05). The competitive affinity of adrenaline and noradrenaline determined by inhibition of [3H]-dihydroergocryptine binding, was unchanged in the presence of either cation. 5 Addition of the purine nucleotides ADP, ATP, GDP or GTP (final concentration 10 microM), either alone or in the presence of 1 mM Ca2+ or 1 mM Mg2+, had no effect on the binding of [3H]-dihydroergocryptine or on the competitive affinity of adrenaline or noradrenaline. 6 We conclude that the alpha-adrenoceptor of intact human platelet displays the binding characteristics of the alpha 2L form of the receptor previously identified in the platelet lysate preparation.