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, young 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 agonist (clonidine) in each case. The heat production (HP) was continuously measured by indirect calorimetry using climatized respiratory chambers. Post-clonidine, the preprandial HP fell in all variants but the strongest decrease occurred at 4 degrees C, 1.6 times the MEm. The postprandial HP rose 1.3-fold the HP of animals received the carrier (saline) 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 after clonidine administration, suggesting different sympathetic outflow corresponded to differing resting metabolic rate, occurring in the environmental variants. Circulating fuels (glucose, non-esterified fatty acids) responded to alpha 2-adrenergic reduction of the sympathetic outflow but did not parallel the HP changes. Studies on monocytes revealed a linear correlation (r2 > 0.9) between resting metabolic rate and expression of sulfonylurea receptors, the constitutive component of ATP-sensitive K+ channels (KATP) suggesting a function of KATP in coupling the systemic HP with cellular metabolism.