In vitro effects of the selective dopamine 1-agonist fenoldopam on the coagulation system in native whole blood: comparison to dopamine and nitroprusside

Regulation of platelet function by catecholamines in the cerebral vasculature of the rabbit

1. 111In-labelled platelets were monitored continuously in the cerebral and pulmonary vascular beds of anaesthetized rabbits. Dopamine can, depending upon the concentration, either potentiate or inhibit thrombin-induced platelet accumulation in the cerebral vasculature of rabbits by unknown mechanisms. The effects of specific adrenergic and dopaminergic receptor antagonists were tested upon dopamine's actions on intracarotid (i.c.) thrombin-induced (80 u kg-1) platelet accumulation in the cerebral vasculature. The effect of adrenaline on the response to thrombin in this vascular bed was also investigated. 2. Thrombin-induced platelet accumulation was significantly (P<0.01) potentiated by dopamine (100 microgkg-1 min-1, i.c.) and this effect was significantly inhibited by infusion of the alpha-adrenoceptor antagonist, phentolamine. 3 A higher dose of dopamine (2 mg kg-1 min-1, i.c.) inhibited thrombin-induced platelet accumulation. The beta-adrenoceptor antagonist, propranolol, did not significantly alter this inhibitory effect whereas it was abolished by the dopamine D1 selective antagonist, SCH23390. 4 Adrenaline (when administered i.c. by bolus injection or infusion) had no significant effect on thrombin-induced accumulation at any of the doses tested. 5 Potentiation of in vivo platelet accumulation by dopamine therefore seems to occur via alpha-adrenergic receptors. However, the inhibitory effect of dopamine appears to be exerted via the activation of dopamine D1 receptors and not via beta-adrenergic receptors. Our findings confirm that dopamine, but not adrenaline, can modify platelet function in the cerebral vasculature and these observations may have implications for current and potential therapeutic uses of dopamine and selective dopaminergic compounds.

Effects of dopamine and selective dopamine agonists upon platelet accumulation in the cerebral and pulmonary vasculature of the rabbit

1. A selection of novel compounds were shown to exhibit dopaminergic activity in vitro. 2. 111Indium-labelled platelets were continuously monitored in the cerebral and pulmonary vasculature of anaesthetized rabbits. The effects of dopamine and selective dopamine receptor agonists on ADP and thrombin induced platelet accumulation were recorded. 3. Pretreatment with dopamine (2 mg kg(-1) min(-1), i.v.) significantly reduced ADP (20 microg kg(-1), i.v.) induced platelet accumulation in the pulmonary vasculature whereas lower doses had no effect. 4. Dopamine (100 microg kg(-1) min(-1) intra-carotid, i.c.) potentiated thrombin (90 u kg(-1), i.c.) induced platelet accumulation in the cerebral vasculature whereas higher doses (1-2 mg kg(-1) min[-1]) inhibited accumulation. 5. The selective dopamine receptor agonists tested did not significantly inhibit platelet accumulation induced by ADP or thrombin. Two of these selective agonists, at doses higher than the intended therapeutic doses, induced thrombocytopaenia and an associated increase in platelet accumulation in the lung in response to thrombin. 6. These results extend previous in vitro studies regarding the dual actions of dopamine upon platelets and show for the first time the effects of selective dopamine receptor agonists upon platelet aggregation in vivo.