Bimodal regulation of palsmalemmal calcium entry by mitochondria in rat brown adipocytes

Abstract

Endoplasmic reticulum (ER) stores and releases Ca²⁺ through IP₃ receptors in response to stimuli. Resultant Ca²⁺ depletion activates store-operated Ca²⁺ entry (STOC). Mitochondria also accumulates Ca²⁺ via a large membrane potential (V_mito), acting as local Ca²⁺ buffers for the close location to ER and plasma membranes. To examine how Ca²⁺ release from mitochondria affects STOC, intracellular Ca²⁺ ([Ca²⁺]_i) and V_mito were measured by fluorometry in brown adipocytes cultured from rats pre-exposed to the cold. Thapsigargin, a blocker of Ca²⁺ pump at ER, produced a sustained rise in [Ca²⁺]_i, which was abolished by a Ca²⁺ free, EGTA solution. FCCP, a protonophore, caused a biphasic rise in Ca²⁺]_i. The first phase was accompanied by a transient decrease in V_mito, while the second phase was blocked by a Ca²⁺ free, EGTA solution, but not by thapsigargin and enhanced at pH 9, but not in a Na⁺-free solution, indicating the activation of plasmalemmal Ca²⁺ entry. At a high basal [Ca²⁺]_i due to STOC activation by thapsigargin, FCCP reduced [Ca²⁺]_i for ten to tens of minutes. Likewise, STOC activated by noradrenaline was blocked by FCCP or thapsigargin. These results suggest that reduction of mitochondrial membrane potential activataes or inactivates plasmalemmal Ca²⁺ entry in brown adipocytes, when the basal level of [Ca²⁺]_i is low or high, respectively. [Jpn J Physiol 55 Suppl:S137 (2005)]