Skeletal muscle resting metabolism in cold-acclimated rats: effect of age, noradrenaline and hyperosmolarity.

1. A myothermic technique has been used to measure the resting metabolism of small bundles of a fast twitch muscle, extensor digitorum longus (EDL), and a slow twitch muscle, soleus (SOL), in 7-week-old rats. At 27 degrees C, mean (+/-SEM) resting heat rates were 2.33 +/- 0.41 and 2.09 +/- 0.37 mW/g in EDL and SOL, respectively (n = 16). 2. Seven-week-old rats were cold acclimatized at 4 degrees C for 1-4 weeks and the metabolic rates of the fast and slow twitch muscles were monitored and compared with 7- and 11-week-old controls. There was a 160% increase in metabolic rate from week 7 to week 11, but the increase also occurred in the control group. 3. In accordance with several literature reports, noradrenaline at concentrations of 10(-7) and 10(-6) mol/L had no effect on either the control or cold-acclimatized resting heat rate. 4. The osmolarity of the physiological solution bathing the muscle bundles was increased by 100 mosmol using sodium sulphate. Basal metabolism increased by similar amounts (approximately 250%) in both the fast and slow muscle bundles. Periods of cold exposure had no significant effect on the magnitude of the increment. 5. Bumetanide, a potent inhibitor of Na(+)-Cl- co-transport, produced only a slight reduction in the heat increments caused by hyperosmolar challenge.