alpha-adrenoreceptor-mediated phasic and tonic activity in rat portal vein in vitro.

The effects of alpha 1- and alpha 2-adrenoreceptor agonists on phasic or tonic activity have been examined in rat portal vein in vitro. Noradrenaline (NA) and Phenylephrine (PE) increased phasic activity at low concentrations, these effects were superimposed on a sustained contracture at higher concentrations. The alpha 2-adrenoreceptor agonists UK 14304 or TL 99 increased extracellular electrical activity and associated mechanical phasic activity without inducing sustained contracture. These alpha 2-adrenoreceptor mediated effects were particularly sensitive to changes in Ca2+ concentration. Prazosin was a potent, competitive antagonist of contracture to NA, or PE, but did not antagonise the phasic responses to NA, UK 14304 or TL 99, except at high concentrations. Prazosin was significantly less potent against PE-induced phasic responses than PE-induced contracture. The results suggest that alpha 1-adrenoreceptors are predominantly involved in contracture. Yohimbine was significantly more potent against UK 14304 or TL 99-induced phasic response than against PE-induced responses, however, phasic responses to NA were competitively antagonised only by combination of yohimbine and prazosin. It is concluded that postsynaptic alpha 2-adrenoreceptor mechanisms are involved in the phasic responses to selective alpha 2-adrenoreceptor agonists, and to NA in rat portal vein. The response of contracture is mediated through alpha 1-adrenoreceptors.