Maturation in liver mitochondria of Ruthenium Red-sensitive calcium-ion-transport activity and the influence of glucagon administration in vivo and in utero

The maturation of Ca2+ transport in mitochondria isolated from rat liver was examined, from 5 days before birth. The mitochondria used were isolated from liver homogenates by centrifugation at 22000g-min. Ca2+ transport by mitochondria isolated from foetal liver is energy-dependent and Ruthenium Red-sensitive. The transmembrane pH gradient in these mitochondria is higher by about 7mV and the membrane potential lower by about 20mV than in adult mitochondria. The inclusion of 2mm-Pi in the incubation medium enhances the protonmotive force by approx. 30mV. The rate of Ca2+ influx in foetal mitochondria measured in buffered KCl plus succinate is low until about 2–3h after birth, when it increases to about 60% of adult values; approx. 24h later it has reached near-adult values. Higher rates of Ca2+ influx are observed in the presence of 2mm-Pi; 3–5 days before birth the rates are about one-third of adult values and decline slightly as birth approaches. By 2–3h post partum they have reached adult values. The inclusion of 12.5μm-MgATP with the Pi stimulates further the initial rate of Ca2+ influx in foetal mitochondria. The rates observed are constant over the prenatal period examined and are 50–60% of those observed in adult mitochondria. Mitochondria isolated from foetal livers 4–5 days before birth retain the accumulated Ca2+ for about 50min in the presence of 2mm-Pi. In the period 2 days before birth to birth, this ability is largely lost, but by 2–3h after birth Ca2+ retention is similar to that of adult mitochondria. The presence of 12.5μm-MgATP progressively enhances the Ca2+ retention time as development proceeds until 2–3h after birth, when it becomes less sensitive to added MgATP. Glucagon administration to older foetuses in utero enhances both the rate of mitochondrial Ca2+ influx assayed in the presence of 2mm-Pi and the time for which mitochondria retain accumulated Ca2+ in the presence of 12.5μm-MgATP and 2mm-Pi. Its administration to neonatal animals leads to an increase in mitochondrial Ca2+ retention similar to that seen in adult mitochondria. The data provide evidence that the Ruthenium Red-sensitive Ca2+ transporter is potentially as active in foetal mitochondria 5 days before birth as it is in adult mitochondria. They also show that foetal mitochondria have an ability to retain accumulated Ca2+ reminiscent of mitochondria from tumour cells and from hormone-challenged rat liver.