Identification of a Ca2+-ATPase in brown adipose tissue mitochondria -: Regulation of thermogenesis by ATP and Ca2+

In brown adipose tissue ( BAT) adrenaline promotes a rise of the cytosolic Ca2+ concentration from 0.05 up to 0.70 mu M. It is not known how the rise of Ca2+ concentration activates BAT thermogenesis. In this report we compared the effects of Ca2+ in BAT and liver mitochondria. Using electron microscopy and immunolabeling we identified a sarco/endoplasmic reticulum ( ER) Ca2+-ATPase bound to the inner membrane of BAT mitochondria. A Ca2+-dependent ATPase activity was detected in BAT mitochondria when the respiratory substrates malate and pyruvate were included in the medium. ATP and Ca2+ enhanced the amount of heat produced by BAT mitochondria during respiration. The Ca2+ concentration needed for half-maximal activation of the ATPase activity and rate of heat production were the same and varied between 0.1 and 0.2 mu M. Heat production was partially inhibited by the proton ionophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone and abolished by thapsigargin, a specific ER Ca2+-ATPase inhibitor, and by both rotenone and KCN, two substances that inhibit the electron transfer trough the mitochondrial cytochrome chain. In liver mitochondria Ca2+ did not stimulate the ATPase activity nor increase the rate of heat production. Thapsigargin had no effect on liver mitochondria. In conclusion, this is the first report of a Ca2+-ATPase in mitochondria that is BAT-specific and can generate heat in the presence of Ca2+ concentrations similar to those noted in the cell during adrenergic stimulation.