Mitochondrial and Extramitochondrial Ca2+ Pools in the Perfused Rat Liver. Mitochondria Are Not the Origin of Calcium Mobilized by Vasopressin

A nondisruptive technique developed by Bellomo et al. (Bellomo, G., Jewell, S. A., Thor, H., and Orrenius, S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 6842-6846) has been used to examine the distribution of calcium ions between mitochondrial and extramitochondrial compartments in the perfused rat liver. The amount of calcium released by the uncoupler 2,4-dinitrophenol from the mitochondrial compartment was 19 +/- 2 nmol X g-1, wet weight, which is equivalent to a total calcium concentration of 3.5 X 10(-4) M in the mitochondria and is by several orders of magnitude smaller than the concentration thought to be present in these organelles. The amount of calcium released from the liver in the presence of the divalent cation ionophore A 23187 was 96 +/- 7 nmol X g-1, wet weight, which is of the same order of magnitude as the amount released by the calcium-dependent hormone vasopressin (97 +/- 11 nmol X g-1, wet weight). Experiments with different sequential combinations of hormone with uncoupler or ionophore reveal that in the perfused liver, in contrast to isolated hepatocytes or isolated mitochondria, the amount of calcium attributable to the mitochondria is too small to account for the calcium released during hormonal stimulation. Consequently extramitochondrial calcium stores are the main source of cellular calcium mobilized under this condition. In addition these findings imply that in the liver several mitochondrial enzymes, e.g. alpha-oxoglutarate dehydrogenase, can be effectively regulated by calcium and that the role of mitochondria in buffering the cytosolic free calcium in vivo has to be reconsidered.