CHARACTERIZATION OF ALPHA-ADRENOCEPTOR ACTIVITY IN TERM-NEWBORN PIGLET MESENTERY

For further characterization of neonatal mesenteric alpha(1)-adrenocep tor populations, an extracorporeal perfusion circuit was established t o control intestinal blood dow in 0-2 day old piglets. Activation of a lpha(1)-adrenoceptors was first documented by observing dose-dependent increases in mesenteric perfusion pressure after intra-mesenteric art erial injection of methoxamine and noradrenaline. Peripheral intraveno us injections of WB 4101 (a competitive alpha(1A)-adrenoceptor antagon ist), but not clorethylclonidine (CEC, an alpha(1B)-adrenoceptor antag onist), significantly (P < 0.05, analysis of variance) blunted mesente ric vasoconstrictor responses to those agonists. That the mesenteric v asoconstrictor response to mesenteric plexus stimulation was unaltered by CEC, but was muted by both WB 4101 and SK&F 104856 (a post-junctio nal alpha(1)- and alpha(2)-adrenoceptor antagonist) suggests that pre- and post-junctional alpha(1A)-adrenoceptors are present and functiona l at birth.