Na+‐dependent regulation of extramitochondrial Ca2+ by rat‐liver mitochondria

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Abstract
The presence and significance of Na+-induced Ca2+ release from rat liver mitochondria was investigated by the arsenazo technique. Under the experimental conditions used, the mitochondria, as expected, avidly extracted Ca2+ from the medium. However, when the uptake pathway was blocked with ruthenium red, only a small rate of basal release of Ca2+ was seen (0.3 nmol Ca2+ .times. min-1 .times. mg-1), in marked contrast to earlier reports on a rapid loss of sequestered Ca2+ from rat liver mitochondria. The addition of Na+ in cytosolic levels (20 nM) led to an increase in the release rate by about 1 nmol Ca2+ .times. min-1 .times. mg-1. This effect was specific for Na+. The significance of this Na+-induced Ca2+ release, in relation to the Ca2+ uptake mechanism, was investigated (in the absence of uptake inhibitors) by following the change in the extramitochondrial Ca2+ steady-state level (set point) induced by Na+. A 5-fold increase in this level, from < 0.2 .mu.M to > 1 .mu.M, was induced by < 20 mM Na+. The presence of K+ increased the sensitivity of the Ca2+ homeostat to Na+. The effect of Na+ on the extramitochondrial level was equally well observed in an K+/organic-anion buffer as in a sucrose buffer. Liver mitochondria incubated under these circumstances actively counteracted a Ca2+ or EGTA [ethylene glycol-bis (.beta.-aminoethyl ether)-N,N,N'',N''-tetraacetic acid] challenge by taking up or releasing Ca2+, so that the initial level, as well as the Na+-controlled level, was regained. Liver mitochondria should be considered Na+-sensitive, the capacity of the Na+-induced efflux pathway was of sufficient magnitude to enable it to influence the extramitochondrial Ca2+ level biochemically and probably also physiologically, and the mitochondria have the potential to act as active, Na+-dependent regulators of extramitochondrial (cytosolic) Ca2+. Changes of cytosolic Na+ could be a mediator between certain hormonal signals (notably .alpha.1-adrenergic) and changes in this extramitochondrial (cytosolic) Ca2+ steady-state level.