Na+‐dependent, α‐adrenergic mobilization of intracellular (mitochondrial) Ca2+ in brown adipocytes

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Abstract
The existence and significance of a hormone-sensitive, rapidly mobilizable intracellular pool of Ca2+ in hamster brown-fat cells was investigated with 45Ca2+-labeling techniques. Such a pool existed and was probably located within the abundant mitochondria. It was rapidly mobilized by norepinephrine (median effective concentration 50 nM) through .alpha.-adrenergic mechanisms. The mobilization of Ca2+ from the intracellular stores (mitochondria) required the presence of extracellular Na+, but not of Ca2+, K+ or Mg2+. Evidently, the experiments are in agreement with a hypothesis linking the mobilization of intracellular Ca2+ pools with an .alpha.-adrenergically-induced increase in plasma membrane Na+ permeability (observed as a membrane depolarization), and a subsequent activation of the mitochondrial Na+/Ca2+ exchange, leading to mobilization of mitochondrial Ca2+ and the mediation of .alpha.-adrenergic effects as a result of an elevated cytosolic Ca2+ level.

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