The Relationship between Extramitochondrial Ca2+ Concentration, Respiratory Rate, and Membrane Potential in Mitochondria from Brown Adipose Tissue of the Rate

Abstract

The ability of isolated mitochondria from rat brown‐adipose tissue to regulate extramitochondrial Ca²⁺ (measured by arsenazo) was studied in relation to their ability to produce heat (measured polarographically). The energetic state of the mitochondria was expressed as a membrane potential, Δψ (estimated with safranine), and was varied semi‐physiologically by the use of different GDP concentrations. In these mitochondria GDP binds to the 32‐kDa polypeptide, thermogenin, which regulates coupling. Ca²⁺ uptake (at 5 μM extramitochondrial Ca²⁺) was maximal at Δψ > 150mV. Basal Ca²⁺ release increased from 1 to 2 nmol × min⁻¹× mg ⁻¹ below 150 mV. Na⁺‐stimulated rate of Ca²⁺ release was stable within the investigated Δψ span (100–160 mV). Initial Ca²⁺ levels were maintained below 0.2 μM for 100mV < Δψ < 160 mV. Ca²⁺ levels maintained after Ca²⁺ challenge (20 nmol Ca²⁺× mg⁻¹ were below 0.4μM for Δψ > 135 mM. Respiration was unstimulated for Δψ > 150 mV and was maximal at Δψ≤ 135mV. In the presence of well‐oxidised substrates, the respiration at maximally activated thermogenin was markedly below fully uncoupled respiration and was probably limited by thermogenin activity–i.e. by a limited H⁺ reentry (OH⁻ exit) and therefore by a membrane potential maintained at about 135 mV. It is concluded that at membrane potentials of 135 mV and above the mitochondria exhibit full Ca²⁺ control and are able to regulate thermogenic output up to maximum without interfering with this Ca²⁺ control. Membrane potential probably does decrease below 135 mV in vivo. Therefore, Ca²⁺ homeostasis and thermogenesis are non‐interfering and can be hormonally independently regulated, e.g. by α‐adrenergic and β‐adrenergic stimuli, respectively.