EFFECT OF MATURATION ON ALPHA-ADRENOCEPTOR ACTIVITY IN NEWBORN PIGLETMESENTERY

To characterize the mesenteric alpha(1)- and alpha(2)-adrenoceptor pop ulations in newborn piglets, an extracorporeal circuit was established to control intestinal blood how in 0- to a-day old and 10- to 14-day old animals. In both groups, alpha-adrenoceptor activation was first d ocumented by observing dose-dependent increases in mesenteric perfusio n pressure after intramesenteric arterial injection of alpha-adrenocep tor agonists. In the 10-to 14-day old piglets, mesenteric vasoconstric tor responses to alpha(1)-adrenoceptor agonists (methoxamine and norep inephrine) and an alpha(2)-adrenoceptor agonist (BHT-933) were each bl unted (P < 0.05, analysis of variance) by peripheral intravenous injec tions of prazosin (an alpha(1)-adrenoceptor antagonist) and yohimbine (an alpha(2)-adrenoceptor antagonist), respectively. The mesenteric va soconstrictor responses to those agonists were not significantly atten uated by prazosin or yohimbine in 0- to 2-day old animals, nor were th ey blunted by YM-12617 (alpha(1)-adrenoceptor antagonist) or idazoxan (alpha(2)-adrenoceptor antagonist)-compounds that are structurally unr elated to prazosin and yohimbine, respectively. In addition, mesenteri c vasoconstrictor responses to other known vasoconstrictor agents-angi otensin II, neuropeptide Y, and a thromboxane A(2) mimic (U-46619)-wer e not effected in either age group by prazosin or yohimbine, implying these agents act independently of alpha-adrenoceptor mechanisms. These data suggest that (1) there exists functional mesenteric alpha(1)- an d alpha(2)-adrenoceptor-like activity in 10- to 14-day old piglets tha t, in 0- to 2-day old animals, is not specifically expressed; and (2) mesenteric alpha-adrenoceptor function becomes more selective as newbo rn piglets mature. (C) 1996 Academic Press, Inc.