Isobolographic analysis of the interaction between fenoldopam and levodopa on arterial blood pressure of the rat

Fenoldopam (FD) and levodopa (LD) injected intravenously in rats in a noncumulative schedule induced dose-dependent reductions in mean arterial blood pressure. The doses that induced a 50% reduction in the initial control mean arterial pressure (referred as ED₅₀) were calculated by linear regression analysis of the corresponding parallel dose-response curves and were 0.88 and 0.068 mg/kg, respectively. The interaction between the effects of FD and LD on pressure reduction was evaluated by simultaneous administration of different fixed ratios of FD and LD (16:1) and obtaining a dose-response curve. An isobolographic analysis was then performed, which showed that the experimental point for the effect of the simultaneous administration of FD and LD was significantly different from the theoretically calculated additive point, denoting supradditivity. It was concluded that the effect of the combination of FD and LD on mean blood pressure reduction was synergic and was probably due to an activation of D₁ vascular receptors by both drugs, in conjunction with an activation of β₂ adrenoceptors by LD and a blocking action of FD on post-synaptic α₁-adrenoceptors.