Calcium-binding protein regucalcin stimulates the uptake of Ca2+ by rat liver mitochondria

The effect of the calcium-binding protein regucalcin on the Ca2+ transport system in rat liver mitochondria was investigated. Ca2+ transport was assayed by the method of Millipore filtration to estimate mitochondrial 45Ca2+ accumulation. 45Ca2+ uptake was stimulated by the presence of regucalcin (1.0 and 2.0 microM). This stimulation was remarkable during 1.0 min after 45Ca2+ addition, while appreciable stimulation was no longer seen at 3 min. Regucalcin (2.0 microM)-induced stimulation of 45Ca2+ uptake was prevented by the presence of ruthenium red (1.0 microM) and lanthanum chloride (0.1 mM). Regucalcin (2.0 microM) did not increase the mitochondrial adenosine triphosphatase (ATPase) activity during 3.0 min after Ca2+ addition. Meanwhile, 45Ca2+, which accumulated in the mitochondria during 5.0 min after 45Ca2+ addition, was not released by the addition of regucalcin. Regucalcin may stimulate Ca2+ uptake in rat liver mitochondria independently of the energy.