Localization of vasodilator dopamine receptors in the canine hindlimb.

1 Vascular responses to local injection of dopamine and isoprenaline have been compared in the blood-perfused gracilis muscle and hind paw pads of dogs anaesthetized with chloralose. 2 In the paw pads, dopamine (0.5 to 5.0 microgram) caused a transient vasoconstriction followed by dose-dependent vasodilatation. alpha-Adrenoceptor blockade converted this response to pure vasodilatation, which was attenuated or abolished by the dopamine-receptor antagonist, haloperidol (1 to 2 mg i.a.). In the gracilis, dopamine produced only vasoconstriction. Following alpha-adrenoceptor blockade this was abolished, but only a very small dilator response was revealed. 3 Isoprenaline (0.05 to 0.5 microgram) caused dose-dependent dilatation in both beds, which was attenuated by propranolol (0.1 mg/kg i.v.). 4 Glyceryl trinitrate (0.2 to 5.0 microgram) was used to assess vascular reactivity. When responses to isoprenaline and dopamine were compared with those to glyceryl trinitrate, both beds had approximately equal reactivity to isoprenaline. In contrast the paw pads were 10 fold more responsive to dopamine than was the gracilis. 5 We conclude that the vessels of the paw pads play an important part in the femoral dilator response to dopamine.