Increased calcium uptake by muscle mitochondria of cold-acclimated rats.

Skeletal muscle mitochondria of cold-acclimated rats have an altered morphology that is related to the occurrence of nonshivering thermogenesis. The transport of calcium by these mitochondria was studied in a search for an alteration in an energy-dissipating mechanism which might be related to the altered morphology and to the altered mode of thermogenesis in the cold-acclimated animal. The rates of calcium uptake, of calcium-stimulated respiration, and of state 4 respiration after calcium uptake were increased in the altered mitochondria. The capacity to accumulate calcium without phosphate was increased, whereas with phosphate all the calcium was removed from the medium and no difference in total uptake was seen. Spontaneous release of calcium was greater but sodium-induced release was unchanged. No effect of cyclic AMP or prostaglandin E1 on release of calcium was seen. The increase in rate of calcium uptake occurred gradually during the first 3-5 wk of acclimation to cold. The results are considered to give some support to the hypothesis that adaptive changes in the mitochondrial calcium transport cycle in skeletal muscle occur during acclimation to cold.