Fluxes and distribution of calcium in rat liver cells: kinetic analysis and identification of pools

Abstract

1. Fluxes and distribution of Ca were studied in the perfused rat liver. Kinetic analysis of (45)Ca exchange revealed three compartments with time constants of 4, 14 and 223 min, pool sizes of 250, 385 and 670 mumole.kg(-1) wet wt. respectively, and one non-exchangeable compartment of 400 mumole.kg(-1).2. (45)Ca uptake by in situ mitochondria followed, as a function of cell loading time, a mono-exponential function with a time constant of 16 min. This suggests that the second compartment may be identified as the intracellular pool of Ca. The calculated cell transmembrane flux of Ca was 28 mumole.kg(-1) min(-1) or 0.17 p-mole.cm(-2).sec(-1).3. The maximum (45)Ca uptake by in situ mitochondria was 2.3 n-mole.mg(-1) of protein which represents, on the basis of 50 mg of mitochondrial protein per g of fresh liver, 115 mumole.kg(-1) or 30% of the cytoplasmic pool. A pool of 10.8 n-mole.mg(-1) protein (or 540 mumole.kg(-1)) of non-exchangeable Ca (at steady state) was probably in the form of Ca phosphate precipitated in the mitochondrial matrix.4. Extracellular Ca pools were studied using competitor of Ca binding (La) or Ca chelators (EGTA). La displaced specifically a homogeneous pool of Ca (tau = 5.1 min) which represented a fraction (55 mumole.kg(-1)) of the rapidly exchangeable Ca (first compartment) without perturbing other external pools. On the other hand, EGTA displaced completely that compartment, and about 85% of the Ca of the third compartment. These results suggest that the first and the major fraction of the third compartments are extracellular. They account for 63% of total exchangeable Ca.5. A model of distribution and exchange of Ca in hepatocytes is proposed.