refludan and adenosine-3--phosphate-5--phosphate

Single-receptor pharmacology does not satisfactorily explain the physiology of the ADP-induced platelet aggregation response. It has been shown that, in addition to Gq-coupled receptor activation, one Gi-coupled receptor, either the ADP P2T or the alpha2-adrenoceptor, is required for elicitation of aggregation. The underlying mechanism of this action, however, has not been elucidated. By systematically assaying the entire time course of the aggregation and its fade using two methods of aggregometry, we have investigated the role of graded activation of these two Gi-coupled receptors. We demonstrate that constant activation of either of two Gq-coupled receptors, the ADP P2Y1 or the 5-HT2A, and incremental activation of either of the two Gi-coupled receptors, tightly regulates the aggregation response in vitro, through the apparent release of a tonic inhibition of platelet aggregation. This tightly regulated release of inhibition, which appears analogous to the phenomena of disinhibition observed in the central nervous system, may be instrumental for the continuous adaptation of the aggregation response to variable physiological conditions.

Topics: Adaptation, Physiological; Adenosine Diphosphate; Adenosine Monophosphate; Adrenergic alpha-2 Receptor Agonists; Adrenergic alpha-Agonists; Adult; Clopidogrel; Epinephrine; Female; GTP-Binding Protein alpha Subunits, Gi-Go; GTP-Binding Protein alpha Subunits, Gq-G11; Heterotrimeric GTP-Binding Proteins; Hirudins; Humans; Male; Membrane Proteins; Middle Aged; Platelet Aggregation; Platelet Aggregation Inhibitors; Purinergic P2 Receptor Agonists; Receptor, Serotonin, 5-HT2A; Receptors, Adrenergic, alpha-2; Receptors, Purinergic P2; Receptors, Purinergic P2Y1; Receptors, Purinergic P2Y12; Receptors, Serotonin; Recombinant Proteins; Serotonin; Serotonin Receptor Agonists; Ticlopidine