Mitochondrial Ca2+ uptake at submicromolar [Ca2+]i in permeabilised pancreatic acinar cells

We studied the uptake of Ca2+ in different intracellular Ca2+ pools by use of steady-state measurement of 45Ca2+ in a preparation of digitonin-permeabilized pancreatic acinar cells. The Ca2+ content of all intracellular pools was assessed at different free Ca2+ concentrations between 0.1 and 1 microM. At low Ca2+ concentrations, almost 50% of Ca2+ was taken up, ATP-dependent, in a non-mitochondrial pool. Even at 0.1 microM free Ca2+ the mitochondrial Ca2+ pool contained 26% (3.4 +/- 0.8 nmol Ca2+/mg prot) of the total Ca2+ and this percentage was increased to 49% at 1 microM Ca2+ (16.7 +/- 1.1 nmol Ca2+/mg prot). Our data also show that the uptake into the non-mitochondrial pool can be further differentiated on the basis of ATP-dependence, and that the ATP-independent uptake has a biphasic dependency on the free Ca2+ concentration.