Effect of bromocriptine on neurogenic vasoconstriction in the isolated autoperfused hindquarters of the rat

The effects of local administration of bromocriptine were studied in the isolated autoperfused hindquarters of the rat, and compared to the actions of apomorphine and pergolide. Local injection of bromocriptine (1 microgram kg-1) (into the hindquarters) did not alter perfusion pressure, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains at all frequencies used (0.5-10 Hz; 5 ms; 35 V). Bromocriptine (1 microgram kg-1) did not alter the increases in perfusion pressure induced by local administration of noradrenaline. The effects of local administration of apomorphine (1 microgram kg-1) and pergolide (1 microgram kg-1) were similar to that of bromocriptine. The inhibitory effect by bromocriptine, apomorphine and pergolide of the stimulation-evoked pressor responses was completely antagonized by intravenous administration of the dopamine receptor antagonist sulpiride (0.3 mg kg-1) but was not by the alpha 2-adrenoceptor antagonist yohimbine (1 mg kg-1). In this dose, yohimbine antagonized the inhibitory effect of the alpha-adrenoceptor agonist clonidine (1 microgram kg-1). The inhibitory effect of clonidine was not altered by sulpiride but was antagonized by yohimbine. The results indicate that bromocriptine like apomorphine and pergolide inhibit neurally-induced pressor responses in the autoperfused hindquarters of the rat by stimulation of presynaptic dopamine receptors. Stimulation of these receptors leading to a fall in noradrenaline release and consequently of vasomotor tone, might at least in part explain the vasodilatator effects of bromocriptine in the rat.