Vasopressor nerve responses in the pithed rat, previously identified as α2-adrenoceptor mediated, may be α1D-adrenoceptor mediated

Responses to pressor nerve stimulation in the pithed rat have been variously described as mediated, at least in part, by α2-adrenoceptors and by α1A and α1D-adrenoceptors. We have examined the subtypes of α-adrenoceptor involved in rises in diastolic blood pressure in the pithed rat preparation produced by vasopressor nerve stimulation with 10 pulses at 1 Hz or 20 pulses at 5 Hz. Vasopressor nerve responses to 1 Hz stimulation were markedly inhibited by the α1A-adrenoceptor antagonist RS 100329 (0.1 mg/kg) and by the α1D-adrenoceptor antagonist BMY 7378 (0.1 mg/kg). The α2-adrenoceptor antagonist yohimbine (0.1 mg/kg) significantly increased pressor nerve responses to 1 Hz stimulation, but yohimbine (1 mg/kg) significantly reduced pressor nerve responses. However, following BMY 7378 (0.1 mg/kg), yohimbine (1 mg/kg) did not produce any further inhibition of pressor nerve responses to 1 Hz stimulation. The α2A-adrenoceptor antagonist BRL 44408 (1 mg/kg) did not reduce pressor responses to 1 Hz stimulation. BMY 7378 produced much less inhibition of pressor nerve responses to 5 Hz stimulation, whereas RS 100329 produced similar inhibition of 1 Hz and 5 Hz responses. Yohimbine (0.1 and 1 mg/kg) did not significantly affect pressor nerve responses to 5 Hz stimulation. In conclusion, pressor nerve responses in the pithed rat involve both α1A and α1D-adrenoceptor, but there is no clear evidence for the involvement of α2-adrenoceptors.