Characterization of α-Adrenoceptor Activity in Term-newborn Piglet Mesentery

For further characterization of neonatal mesenteric α₁-adrenoceptor populations, an extracorporeal perfusion circuit was established to control intestinal blood flow in 0-2 day old piglets. Activation of α₁-adrenoceptors was first documented by observing dose-dependent increases in mesenteric perfusion pressure after intra-mesenteric arterial injection of methoxamine and noradrenaline. Peripheral intravenous injections of WB 4101 (a competitive α_1A-adrenoceptor antagonist), but not clorethylclonidine (CEC, an α_1B-adrenoceptor antagonist), significantly (P < 0.05, analysis of variance) blunted mesenteric vasoconstrictor responses to those agonists. That the mesenteric vasoconstrictor response to mesenteric plexus stimulation was unaltered by CEC, but was muted by both WB 4101 and SK&F 104856 (a post-junctional α₁- and α₂-adrenoceptor antagonist) suggests that pre- and post-junctional α_1A-adrenoceptors are present and functional at birth.