Sequential activation of Ca²⁺ release at mitochondria and ER and plasmalemmal Ca²⁺ entry by the β-action of noradrenaline in rat brown adipocytes

Abstract

In brown adipocytes, α1 adrenoceptor activation elicits IP3-induced Ca²⁺ release and store-operated Ca²⁺ entry (SOC) from the ER, while β3-receptor activity activates lipolysis and uncoupling proteins in mitochondria, producing heat. Uncoupling of mitochondrial respiration by FCCP activates mitochondrial Ca²⁺ release, non-SOC Ca²⁺ entry, Ca²⁺ release from the ER and SOC. How this mitochondria-ER-plasmalemma Ca²⁺ coupling is regulated by an endogenous activator, we have measured Ca²⁺ in the ER ([Ca²⁺]ER) using cameleon expressed in the ER as well as intracellular Ca²⁺ ([Ca²⁺]i) and mitochondrial potential. The first phase of isoprotelenol-induced rises in [Ca²⁺]i was accompanied by mitochondrial depolarization. The second phase was paralleled by mitochondrial repolarization, accompanied by a rise in intracellular Mg2+ and partially blocked by removing external Ca²⁺ and fully by oligomycin. The second and third phases were blocked by removing external Ca²⁺ and U73122. FCCP as well as cyclopiazonic acid (CPA) transiently reduced [Ca²⁺]ER. Likewise, isoprotelenol decreased [Ca²⁺]ER for a period much longer than those by FCCP and CPA. Thus, β3-receptor activation causes uncoupling of mitochondrial respiration that depolarizes the mitochondrial membrane, releases Ca²⁺, activates uncoupling-associated non-SOC Ca²⁺ entry and activates Ca²⁺ release from the ER via activation of phospholipase C and subsequently SOC. [J Physiol Sci. 2007;57 Suppl:S131]