Metabolic cold acclimation after repetitive intermittent cold exposure in rat

Repetitive intermittent cold exposure (5 degrees C, 6 h/day, 4 weeks) (ICE) resulted in the same cold adaptability as assessed by an enhanced cold tolerance (less drop of colonic temperature at -5 degrees C) and nonshivering thermogenesis (NST) (greater noradrenaline-induced heat production) as that elicited by continuous cold exposure (5 degrees C, 4 weeks) (CA) in rats. Although shorter intermittent (5 degrees C, 2 h/day, 4 weeks) (ICE-2 hr) as well as shorter continuous (5 degrees C, 1 week) (CA-1 wk) cold exposure effected an improved cold adaptability, the magnitude of cold tolerance and NST was smaller as compared with that in CA and ICE. The cold deacclimation process as reflected on the decreased NST did not differ between CA and ICE. Food intake was less in ICE than CA, while increase in body weight during the acclimation period was greater in the former. Increase in adrenal weight was greater in CA than ICE, but plasma corticosterone level did not differ among warm controls (WC), CA, and ICE in resting state (after 18-20 h at warm control temperature of 25 degrees C). Weights of interscapular and dorsocervical brown adipose tissue (BAT) increased to the same degree in CA and ICE. Plasma glucagon level in resting state did not differ among groups, while BAT glucagon levels significantly increased in CA and ICE, but they were higher in dorsocervical site than interscapular site in all acclimated states. Acute cold exposure (-5 degrees C, 15 min) caused increases in plasma corticosterone, glucagon levels, and in BAT glucagon levels in all acclimated groups. The extent of increase was significantly less for plasma glucagon in CA, while plasma corticosterone increased similarly in all groups. These results indicate that repetitive short-term cold exposure could elicit the same cold adaptability as that induced by continuous exposure, but requiring only one-fourth of the time of continuous cold exposure. Moreover, it is suggested that glucagon is involved in both CA and ICE, but the same extent of cold adaptability can be obtained in the less energy-requiring and less stressful state in ICE.