Characterization of the energy-dependent calcium binding of a mitochondrial fraction isolated from bovine intrapulmonary vein

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Abstract
A mitochondrial-enriched fraction was isolated from bovine intrapulmonary vein; this fraction had a three- to four-fold enrichment factor for succinic dehydrogenase, a mitochondrial marker enzyme, and bound calcium in an azide-sensitive, energy-dependent fashion using either ATP or succinate as substrate. The calcium binding was sensitive to the concentrations of H+, ATP, Ca2+, and mitochondrial fraction. Binding and accumulation were time dependent, reaching at 10–12 min a plateau which was maintained over the time interval studied, i.e. 18 min. At the plateau, maximal binding was 221 ± 8.64 and maximal accumulation was 285 ± 20.2 nmol Ca2+/mg protein. The mitochondrial-enriched fraction contained an azide-sensitive, Mg2+-dependent, Ca2+-stimulated ATPase (Mg2+ + Ca2+ = 25.59 ± 1.03, Ca2+ = 3.96 ± 1.06 μmol P1/mg protein per hour). This study is the first to report the isolation from intrapulmonary vein of a subcellular organelle capable of binding calcium in an energy-dependent fashion; furthermore, it confirms and extends the observations of others that mitochondria may contribute to intracellular calcium homeostasis in vascular smooth muscle.