INVOLVEMENT OF ALPHA(2)-ADRENERGIC ACTIVITIES IN THERMOGENIC RESPONSES TO FEEDING, FEEDING LEVEL, AND AMBIENT-TEMPERATURE

To investigate the alpha(2)-adrenergic effect on the metabolic rate, y oung bulls were exposed to environmental variants (feeding levels of 1 .0 and 1.6 times the MEm and ambient temperatures of 18 degrees C and 4 degrees C) and treated preprandially with a alpha(2)-adrenergic agon ist (clonidine) in each case. The heat production (HP) was continuousl y measured by indirect calorimetry using climatized respiratory chambe rs. Post-clonidine, the preprandial HP fell in all variants but the st rongest decrease occurred at 4 degrees C, 1.6 times the MEm. The postp randial HP rose 1.3-fold the HP of animals received the carrier (salin e) at 4 degrees C, 1.6 times the MEm. Animals exposed to 18 degrees C, 1.6-fold the MEm did not significantly increase the postprandial HP a fter clonidine administration, suggesting different sympathetic outflo w corresponded to differing resting metabolic rate, occurring in the e nvironmental variants. Circulating fuels (glucose, non-esterified fatt y acids) responded to alpha(2)-adrenergic reduction of the sympathetic outflow but did not parallel the HP changes. Studies on monocytes rev ealed a linear correlation (r(2) > 0.9) between resting metabolic rate and expression of sulfonylurea receptors, the constitutive component of ATP-sensitive K+ channels (K-ATP) suggesting a function of K-ATP in coupling the systemic HP with cellular metabolism.