Brown adipose tissue thermogenic responses of rats induced by central stimulation: effect of age and cold acclimation.

1. Urethane-anaesthetized, age-matched cold-(4 degrees C) and room (21 degrees C)-acclimated groups of Sprague-Dawley rats were given repeated (three) ventromedial hypothalamic electrical stimulations. Interscapular brown adipose tissue (IBAT), colonic, surface tail temperatures and blood pressure were monitored before and after each electrical stimulation. Propranolol HCl (2.5 mg kg-1 i.v.) was given 10 min prior to the last (third) ventromedial hypothalamic stimulation. 2. Repeated electrical stimulation of the ventromedial hypothalamic (VMH) nucleus to either small or large rats (kept at 21 degrees C) caused no significant change in interscapular brown fat, colonic or surface temperatures compared to respective pre-stimulation control values whereas mean arterial pressure was slightly but significantly increased during the 30 s stimulation period. 3. Electrical stimulation of the ventromedial hypothalamic nucleus of small or large rats, kept at 4 degrees C for 3 weeks prior to testing, caused significant (greater than 0.25 and greater than 0.40 degrees C, respectively) rises in interscapular BAT temperature from respective prestimulation control values. Colonic temperatures increased following ventromedial hypothalamic electrical stimulation only in the small, 4 degrees C-acclimated group whereas surface tail temperatures did not significantly change after stimulation of either cold-exposed group. Mean arterial pressures were significantly increased during ventromedial hypothalamic electrical stimulation in both 4 degrees C-acclimated groups, compared to pre-stimulation control levels and, in addition, were above those of age-matched rats kept at 21 degrees C. Intravenous propranolol, which decreased interscapular brown fat and colonic temperatures in all groups, blocked the rise in interscapular brown fat and colonic temperatures of the 4 degrees C-acclimated rats following the last electrical stimulation of the ventromedial hypothalamic nucleus. 4. In vitro biochemical analysis of the interscapular brown fat pads of another four groups of age-matched, small and large 21 and 4 degrees C-acclimated rats revealed that the thermogenic capacity of the 4 degrees C-acclimated groups was, in all cases, significantly increased from age-matched groups previously kept at 21 degrees C, as shown by significant increases in brown adipose tissue mass, BAT DNA and protein content, BAT mitochondrial protein and BAT mitochondrial GDP binding.(ABSTRACT TRUNCATED AT 400 WORDS)