EFFECTS OF ALPHA(2)-ADRENOCEPTOR ANTAGONISTS ON METABOLIC PROCESSES OF SWINE - I - EFFECTS ON NONESTERIFIED FATTY-ACID AND PLASMA UREA NITROGEN CONCENTRATIONS IN JUGULARLY CATHETERIZED PIGS

The presence of alpha(2)-adrenoceptors in membranes from omental and s .c. adipose tissue from gilts and barrows was shown in saturation bind ing assays with [H-3]yohimbine. Four trials tested effects of alpha(2) -adrenoceptor antagonists (A2AA) on plasma concentrations of NEFA and urea nitrogen (PUN). In Trial 1, barrows were given i.v. injections of saline, 200 mu g/kg BW of one of three A2AA (efaroxan, idazoxan, or R X821002), or 25 mu g/kg BW of isoproterenol. Concentrations of NEFA we re measured in plasma harvested every 15 min from 1 h before to 2 h af ter treatment. Compared with results for saline-treated pigs, areas un der the curve (AUC) for NEFA were increased (P < .05) by efaroxan, RX8 21002, and isoproterenol. In Trial 2, barrows received i.v, doses of s aline, efaroxan (200 or 400 mu g/kg BW), or RX821002 (200 or 400 mu g/ kg BW). Levels of NEFA were quantified in plasma obtained at 15-min in tervals through 2 h after treatment. Among pigs treated with RX821002 at 400 mu g/kg BW, mean NEFA AUC was more than three times greater (P < .05) than that for saline-treated animals. Trial 3 tested whether NE FA responses to A2AA were due to direct effects on alpha(2)-receptors or involved beta-adrenoceptor mediation. Pigs were first treated i.v. with saline or propranolol (1 mg/kg BW). One hour later, pigs were tre ated i.v. with RX821002 (400 mu g/kg BW) or the beta-adrenoceptor agon ist cimaterol (25 mu g/kg BW). Compared to values for pigs treated wit h saline at both injections, NEFA AUC among pigs treated with saline a t the first injection and RX821002 at the second doubled (P > .05). Pl asma NEFA AUC among pigs treated with saline then cimaterol rose nearl y fourfold (P < .05) compared with saline-treated controls. Mean NEFA AUC among propranolol-treated pigs was similar to values for saline-tr eated pigs, suggesting beta-adrenoceptor involvement in the effect of A2AA on NEFA. In Trial 4, pigs were treated s.c. 10 times at 8-h inter vals with saline, RX821002 (400 mu g/[kg BW.injection]), cimaterol (20 mu g/[kg BW injection]) or recombinant porcine somatotropin (rpST; 1 mg/ [pig injection]). After the 10th treatment, only cimaterol increas ed NEFA AUC compared to saline-treated controls (P <.05). Mean PUN AUC was reduced by RX821002 and rpST compared to controls; PUN among rpST -treated pigs was lower than that among RX821002-treated pigs (P <.05) . In summary, A2AA increase lipolysis in swine by potentiating lipolyt ic effects of endogenous catecholamines on beta-adrenoceptors. Reduced PUN suggests improved nitrogen efficiency may result from treatment w ith A2AA.