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.
D000328:Adult; D000641:Ammonia / Q000494:pharmacology; D001667:Binding, Competitive; D001792:Blood Platelets / Q000378:metabolism*; D002118:Calcium / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D002414:Cations, Monovalent / Q000494:pharmacology*; D004088:Dihydroergotoxine / Q000378:metabolism; D004492:Edetic Acid / Q000494:pharmacology; D004837:Epinephrine / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D011941:Receptors, Adrenergic / Q000187:drug effects*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects*; D012964:Sodium / Q000494:pharmacology
D002413:Cations, Divalent; D002414:Cations, Monovalent; D011941:Receptors, Adrenergic; D011942:Receptors, Adrenergic, alpha; D004088:Dihydroergotoxine; D000641:Ammonia; D004492:Edetic Acid; D012964:Sodium; D008274:Magnesium; D002118:Calcium; D004837:Epinephrine