Objective: 

Peripheral control of blood pressure involves a1- and a2-adrenoceptors, and the receptor subtypes involved can be studied in the pithed rat in the absence of central or reflex control. Confusion in this area occurs because yohimbine, which has been employed in a number of studies to identify a2-adrenoceptor components to pressor responses, has affinity for a1D-adrenoceptors. We have re-examined the actions of yohimbine in the pithed rat.

Methods: 

We have re-examined the actions of yohimbine in rats pithed under anaesthesia and ventilated with 100% O2 at 60 per min. The carotid artery and jugular vein were cannulated for pressure recording and drug injection.

Results: 

Pressor nerve responses to 1 Hz stimulation were markedly inhibited by the a1A-adrenoceptor antagonist RS 100329 (0.1 mg/kg) and by the a 1D-adrenoceptor antagonist BMY 7378 (0.1 mg/kg). Yohimbine (1 mg/kg) significantly reduced pressor nerve responses, but subsequent to BMY 7378 (0.1mg/kg), yohimbine failed to produce any further inhibition. The a2A-adrenoceptor antagonist BRL 44408 (1mg/kg) did not inhibit pressor nerve responses. Pressor responses to exogenous agonists involve a1A-and a1D-adrenoceptors, but again the role of a2-adrenoceptors was re-examined. Yohimbine or BRL 44408 (both 1mg/kg) significantly shifted, but BMY 7378 (5mg/kg) failed to affect, the pressor potency of the a2-adrenoceptor agonist xylazine. In addition, the potent a2A-adrenoceptor antagonist methoxyidazoxan produced great shifts in xylazine potency than the same dose of yohimbine or BMY 7378.

Conclusions: 

Pressor nerve responses in the pithed rat involve both a1A-and a1D-adrenoceptors, but responses to exogenous agonists involve additionally a2A-adrenoceptors but there is no clear evidence for the involvement of a2-adrenoceptors in pressor nerve responses.