Evidence against a role for dopamine D₁ receptors in the myocardium of the pig

We investigated the presence of dopamine D₁ receptors in the myocardium of anaesthetized pigs using intravenous infusions of dopamine, alone and after α‐ and β‐adrenoceptor blockade and intracoronary infusions of the selective D₁ receptor agonist, fenoldopam. Intravenous infusion of dopamine (2.5, 5 and 10 μg kg⁻¹ min⁻¹ for 10 min, n = 6) caused dose‐dependent changes in heart rate (from 94 ± 6 to 132 ± 10 beats min⁻¹, P < 0.05), the maximal rate of rise of left ventricular pressure (LVdP/dt_max; from 2280 ± 170 to 4800 ± 410 mmHgs⁻¹, P < 0.05), mean arterial blood pressure (from 87 ± 5 to 62 ± 3 mmHg) and systemic vascular resistance (from 40 ± 4 to 28 ± 2 mmHg l⁻¹ min, P < 0.05). The increases in heart rate and LVdP/dt_max were abolished when dopamine was infused after α‐ and β‐adrenoceptor blockade. The vasodilator response was, however, only minimally affected. Intravenous infusions of dopamine decreased coronary vascular resistance from 0.90 ± 0.06 to 0.53 ± 0.07 mmHg ml⁻¹ min 100 g (P < 0.05). This action of dopamine was not observed when dopamine was infused after blockade of the α‐ and β‐adrenoceptors. Pretreatment with α‐ and β‐adrenoceptor blockade had no effect or only slightly attenuated the dopamine‐induced decrease in vascular resistance of the brain, kidneys, adrenals and small intestine. In 7 animals, intracoronary doses of 0.04, 0.1, 0.2 and 0.4 μg kg⁻¹ min⁻¹ of fenoldopam had no effect on coronary venous oxygen content, local myocardial oxygen consumption, coronary blood flow or coronary vascular resistance. However, systemic effects were observed at the highest two doses, as manifested by a drop in mean arterial blood pressure from 82 ± 4 to 72 ± 4 mmHg (P < 0.05) due to peripheral vasodilatation (e.g. cerebral vascular bed). Heart rate, LVdP/dt_max, regional myocardial segment length shortening and left ventricular end‐diastolic pressure were not affected at these doses. In 2 animals the infusion rate was increased to 4 μg kg⁻¹ min⁻¹, but again there was no evidence for coronary vasodilatation. We conclude that the intravenous infusion of dopamine after α‐ and β‐adrenoceptor blockade and the intracoronary infusion of fenoldopam provided no evidence for a major role of D₁ receptors in the coronary circulation of pigs. The absence of any effect of the employed doses of fenoldopam on LVdP/dt_max and on regional myocardial segment length shortening also indicates that fenoldopam does not exhibit any inotropic action in this species. 1991 British Pharmacological Society