Activation and inactivation of plasmalemmal Ca²⁺ entry via mitochondria by the beta-action of noradrenaline in rat brown adipocytes

Abstract

The activation of β₃-adrenoreceptor in brown adipocytes hydrolyses triglycerides to free fatty acids, which increase the electrochemical potential for H⁺ via TCA cycle and activate an uncoupling protein, UCP-1, and thermogenesis. α₁-adrenoreceptor activation elicits a phasic rise in intracellular Ca²⁺ ([Ca²⁺]_i) via Ca²⁺ release through IP₃ receptors from endoplasmic reticulum (ER) and subsequent store-operated Ca²⁺ entry (STOC). To study how the β-action regulates [Ca²⁺]_i and how the α- and β-actions interact with each other, [Ca²⁺]_i and mitochondrial membrane potential (V_mito) were measured by fluorometry in cultured adipocytes. Isoprotelenol caused a biphasic rise in [Ca²⁺]_i . The first phase was blocked by FCCP, a protonophore, and accompanied by a transient decrease in V_mito, while the second was blocked by Ca²⁺ free, EGTA solution, but not by thapsigargin, a blocker of Ca²⁺ pump at ER, and enhanced at pH 9, but not in a Na⁺-free solution, indicating activation of plasmalemmal Ca²⁺ entry. At a high basal [Ca²⁺]_i via STOC activation by thapsigargin, isoprotelenol reduced [Ca²⁺]_i for ten to tens of minutes. Likewise, STOC activated by α-receptor activation was blocked by β-receptor activation, vice versa. Thus, the activation of β-receptor activates or suppresses plasmalemmal Ca²⁺ entry via changes in mitochondrial membrane potential, when the basal level of [Ca²⁺]_i is low or high, respcetively. [Jpn J Physiol 55 Suppl:S137 (2005)]