β3-Adrenergic activation of sequential Ca2+ release from mitochondria and the endoplasmic reticulum and the subsequent Ca2+ entry in rodent brown adipocytes

Abstract

We studied how mitochondrial uncoupling by β3-adrenergic stimulation elicits Ca2+ signals in rodent brown adipocytes by fluorometry of Ca2+ concentrations ([Ca2+]i, [Ca2+]m and [Ca2+]ER) in the cytoplasm, mitochondria and the endoplasmic reticulum (ER), respectively, and mitochondrial membrane potential, using fura-2, rhod-5N, cameleon and rhodamine 123. Immunoblotting demonstrated α1A- and β3-adrenergic receptor and UCP1 in adipocytes, while RT-PCR revealed the mRNA of type 3, 7 and 9 adenylate cyclase, UCP1, UCP2, UCP3 and type 1 and 2 inositoltrisphosphate receptors. Isoproterenol and BRL37344, β-agonist, caused triphasic rises in [Ca2+]i (β-responses) with mitochondrial depolarization in adipocytes. BRL37344 transiently decreased [Ca2+]m. β-Responses were blocked by propranolol, β-antagonist, H-89, protein kinase A blocker, and knockout of UCP1 gene. The late phase of β-responses was depressed by a Ca2+ free, EGTA solution, U73122, a phospholipase C blocker, and thapsigargin, ER-Ca2+ pump blocker, and by transfecting siRNA for type 2 IP3R. Intracellular loading of BAPTA/AM depressed the late phase more strongly than the initial phase. β-Agonists, phenylephrine, α-agonist, and cyclopiazonic acid, ER-Ca2+ pump blocker, decreased [Ca2+]ER. Thus, the mitochondrial uncoupling by β3-adrenergic activation causes Ca2+ release from mitochondria and subsequently from the ER and further evokes plasmalemmal Ca2+ entries, including the store-operated Ca2+ entry.