Centrally induced impairment of the hypotensive effects of R 28935 and R 29814 by prazosin in anaesthetized cats.

Prazosin (3 micrograms/kg) infused prior to erythro-1-(1-[2-(1,4-benzodioxan-2-yl)-2-hydroxyethyl]-4-piperidyl)-2-benzimidazolinone (R 28935) into the left vertebral artery (v.a.) of chloralose-anaesthetized cats diminished the hypotensive response to R 28935 (3 micrograms/kg, v.a.). A similar i.v. treatment did not alter the central hypotensive effect of R 28935 (micrograms/kg, v.a.). After prazosin (3 micrograms/kg, v.a.) there was a parallel shift to the right of the dose-response curve of R 28935 (v.a.). The central hypotensive effect of the threo isomer (R 29814; 30 micrograms/kg) was also diminished by the previous administration of prazosin (3 micrograms/kg, v.a.). These results indicate the involvement of central alpha 1-adrenoceptors in the mechanism of action of R 28935 and R 29814.