Baroreceptors, alpha(1)-adrenergic receptors, and regulation of mesentericblood flow

In the mesenteric circulation of the rat a myogenic autoregulatory system o perates at 0.1-0.15 Hz. Negative admittance phase in the region above 0.2 H z suggested operation of an arterial baroreflex. The present study was desi gned to test this interpretation and to identify the neurotransmitter invol ved. In rats anesthetized with isoflurane, blood pressure and mesenteric bl ood flow (transit time ultrasound) were measured with central mechanisms in tact, after sinoaortic denervation, and after denervation of the mesenteric bed. Sinoaortic denervation abrogated the negative phase in the band from 0.3 to 0.6 Hz and increased admittance gain in this region. Subsequent mese nteric denervation had no further effect on the pressure-flow transfer func tion. In a separate experiment, alpha(1)-adrenergic blockade reduced, but d id not remove, the negative admittance phase in the 0.2- to 0.5-Hz band wit hout altering admittance gain. It is concluded that the baroreflex acting o n the mesenteric circulation can be identified by admittance phase, but tha t admittance gain is uninformative. Part of the response is mediated by alp ha(1)-adrenergic transmission.