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 μg kg⁻¹, i.v.) induced platelet accumulation in the pulmonary vasculature whereas lower doses had no effect.
4. Dopamine (100 μg 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.