Inhibitory effects of apomorphine and pergolide on neurogenic vasoconstriction in the hindquarters of the rat.

The effects of locally administered apomorphine and pergolide were studied in the isolated autoperfused hindquarters of the rat, in an attempt to assess the possible role of presynaptic dopamine receptors at that level in the hypotensive effect of these dopamine agonists. Local infusion of apomorphine (1 micrograms . kg-1 . min-1 for 5 min) or pergolide (1 micrograms . kg-1 . min-1 for 5 min) [into the hindquarters] did not alter perfusion pressure per se, but reduced the pressor response to electrical stimulation of the lumbar sympathetic chains for the whole frequency range used during a cumulative frequency-response curve (0.25-16 Hz, 1 ms, supramaximal voltage). Apomorphine and pergolide reduced the pressor response elicited by 4 Hz electrical stimulation (applied until maximum response was reached) to 54.8 +/- 7.1% and 53.9 +/- 1.7% respectively, but they did not modify similar increases of perfusion pressure produced by locally administered noradrenaline. The inhibition by apomorphine and pergolide of the 4 Hz stimulation-evoked pressor response was completely antagonized by local administration of the dopamine antagonist haloperidol (1 microgram . kg-1), but was not influenced by the alpha 2-antagonist rauwolscine (100 micrograms . kg-1). This dose of rauwolscine antagonized the inhibitory effect of the alpha 2-agonist UK-14,304, which was not influenced by haloperidol. Local administration of rauwolscine increased the pressor response to stimulation at 4 Hz by 37.4-46.2%. In contrast, local administration of haloperidol did not influence the 4 Hz stimulation-evoked pressor response.(ABSTRACT TRUNCATED AT 250 WORDS)