Influence of the dopamine receptor agonists fenoldopam and quinpirole in the rat superior mesenteric vascular bed

Abstract

The effect of local administration of the dopamine2 (DA2)‐receptor agonist quinpirole and of the DA1‐receptor agonist fenoldopam was studied in the in situ, constant flow autoperfused, superior mesenteric vascular bed of the rat. Local infusion of quinpirole (30 μg kg−1 min−1 for 5 min) had no effect on baseline perfusion pressure; it reduced the pressor responses to electrical stimulation (4 Hz, 1 ms, supramaximal voltage) of the periarterial sympathetic nerves to 45.6 ± 2.1% of its original value but did not modify similar pressor responses produced by locally administered noradrenaline. The inhibitory effect of quinpirole was antagonized by the selective DA2‐receptor antagonist domperidone (10 μg kg−1) but not by the selective DA1‐receptor antagonist SCH 23390 (50 μg kg−1). Local infusion of fenoldopam (30 μg kg−1 min−1 for 5 min) reduced baseline perfusion pressure to 89.9 ± 1.9%, increased the pressor response to electrical stimulation (4 Hz, 1 ms, supramaximal voltage) of the periarterial nerves to 134.7 ± 14.0%, but reduced the pressor response to locally administered noradrenaline to 37.2 ± 8.2%. Similar pressor responses induced by the selective α1‐adrenoceptor agonist phenylephrine were also reduced by fenoldopam (to 38.4 ± 6.4%), but responses to locally administered angiotensin II were not modified. Pretreatment with SCH 23390 (50 μg kg−1) antagonized the effect of fenoldopam on baseline perfusion pressure, but had no influence on the effect of fenoldopam on responses to electrical stimulation or to noradrenaline. Pretreatment with the selective α2‐adrenoceptor antagonist rauwolscine (100 μg kg−1) had no effect on the reduction in baseline perfusion pressure induced by fenoldopam nor on its inhibitory effect on the response to noradrenaline, but it antagonized the stimulatory effect of fenoldopam on the response to electrical stimulation. The results show that quinpirole inhibits neurogenic vasoconstriction in the rat superior mesenteric vascular bed through stimulation of presynaptic DA2‐receptors while fenoldopam stimulates postsynaptic vasodilatory DA1‐receptors. In addition, our results suggest that the inhibitory effect of fenoldopam on the vasoconstrictor response to noradrenaline may be due to an antagonistic action at postsynaptic α1‐adrenoceptors, while its potentiating effect on neurogenic vasoconstriction is due to blockade of presynaptic α2‐adrenoceptors. 1987 British Pharmacological Society