Pharmacological characterization of epinephrine-stimulated GTPase activity in human platelet membranes.
The alpha 2-adrenergic receptor-mediated stimulation of GTPase activity was investigated in human platelet membranes. The stimulatory effect of (-)-epinephrine was strictly dependent on Mg2+ and derived from a high-affinity GTPase activation. (-)-Epinephrine and (-)-norepinephrine stimulated GTPase activity in a concentration-dependent manner with EC50 values of 200 and 600 nM, respectively. These effects were stereospecific, since (+/-)-epinephrine, (+/-)-norepinephrine, and (+)-epinephrine were less potent in stimulating the enzyme activity with EC50 values of 4, 1 and 3 microM, respectively. Thrombin also stimulated GTPase activity concentration dependently with an EC50 value of 0.02 U/mL. The maximal effects of (-)-epinephrine, (-)-norepinephrine, and thrombin were not additive in any combination. Clonidine did not stimulate GTPase activity, whereas another synthetic alpha 2-adrenergic agonist, p-aminoclonidine, had the characteristics of a partial agonist. The rank order of potency for antagonists to inhibit the activation of GTPase by 1 microM (-)-epinephrine was yohimbine = rauwolscine > idazoxan = oxymetazoline = phentolamine = WB4101 = (+)-mianserin > (-)-mianserin > prazosin > (-)-propranolol. Negative logarithms of the IC50 values of these antagonists corresponded well with the negative logarithmic values of Ki(pKi) for the alpha 2A-adrenergic receptors determined by a receptor-binding technique in human platelets. These results indicate that epinephrine stimulates high-affinity GTPase activity of G proteins (putatively Gi2), which are also coupled with thrombin receptors, in a Mg(2+)-dependent and stereospecific manner, via alpha 2A-adrenergic receptor activation in human platelet membrane preparations.
Odagaki
Y
Y
Department of Psychiatry and Neurology, Hokkaido University School of Medicine, Sapporo, Japan.
Koyama
T
T
Yamashita
I
I
D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D001792:Blood Platelets / Q000187:drug effects* / Q000201:enzymology; D002462:Cell Membrane / Q000187:drug effects; D004789:Enzyme Activation / Q000187:drug effects; D004837:Epinephrine / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D020558:GTP Phosphohydrolases / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000494:pharmacology; D006801:Humans; D015636:Magnesium Chloride / Q000494:pharmacology; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects
D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D011942:Receptors, Adrenergic, alpha; D015636:Magnesium Chloride; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases; D004837:Epinephrine