Abstract
The arterial pressure and heart rate of cats anaesthetized with urethane and chloralose were measured following treatment with clonidine (5 μg/kg). Blood pressure and heart rate were consistently decreased, effects which were blocked by haloperidol and phenoxybenzamine in doses which block central noradrenaline (NA) receptor sites; antagonism did not seem to be related to their peripheral adrenolytic properties. Pimozide and spiroperidol, in doses which block central dopamine but not NA receptor sites, did not block the hypotensive effects of clonidine. Pimocide and spiroperidol alone caused slight hypotension and bradycardia which, at least with regard to pimozide were not related to peripheral α-adrenergic receptor blocking activity. Clonidine potentiated these effects of pimocide and spiroperidol. The present findings indicate that the hypotensive effect of clonidine is due to stimulation of central NA receptor sites. Furthermore, dopaminergic mechanism may also be involved in vasomotor regulation.
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