The Sodium‐Induced Efflux of Calcium from Heart Mitochondria

Abstract

The interrelations between the transport of Na+ and Ca2+ in heart mitochondria have been investigated. Na' induces a partial release of Ca2' from heart mitochondria; in the presence of ruthenium red the release of Ca2+ by Na' is complete. The rate of Caz+ release appears to be a function of [Nail3 and is stimulated by energy-linked respiration. Ca2' efflux is also induced by Li' although Na' is the preferred cation. The data are consistent with the existence of a system able to catalyze an exchange between Na' (or Lit) and Ca2+ across the inner membrane of heart mitochondria.The ability of mitochondria to accumulate Ca2+ by an energy-dependent process and, in so doing, to reduce the extramitochondrial Ca2+ concentration to the pM range is extensively documented (for reviews, see [1,2]). The influx of Ca2+ into heart mitochondria is strongly inhibited by physiological concentrations of Mg2+ [ 2 , 3 ] . However, an evaluation of the true of mitochondria in regulating the intracellular Ca2+ requires knowledge not only of how influx may be limited, but also of the process(es) by which Ca2+ is released from mitochondria. The observation [4] that Na' causes a release of the endogenous Ca2+ of isolated heart mitochondria is clearly relevant to this problem since heart is known to undergo small, but definite, variations in intracellular Na+ . This report is concerned with the interrelations between the transport of Ca2' and Na' in heart mitochondria, and presents evidence that these mitochondria contain a system able to catalyse an exchange between N a + and Ca2' across the inner membrane. MATERIALS AND METHODS Mitochondria1 PrepurationHeart mitochondria were isolated from female Sprague-Dawley rats using a Polytron homogenizer by the procedure described by Scarpa and Graziotti [5]. The medium used for homogenisation contained 210 mM mannitol, 70 mM sucrose, 10 mM Tris-HC1 pH 7.4, and 0.1 mM EDTA. The mitochondria were washed subsequently twice in the medium without EDTA. The mitochondria were suspended finally in medium without EDTA (approx. 20 mg of mitochondrial protein/ml).Rat liver mitochondria were prepared according to the procedure of Klingenberg and Slenczka [6] using media identical to those described for the preparation of heart mitochondria.The protein content of the mitochondria was determined by a modification of the biuret procedure [7] with bovine plasma albumen (Sigma Chemical Co.) as standard.Heart mitochondria were depleted of their endogenous Ca2+ by washing them in the presence of an inhibitor of respiration. Mitochondria containing about 50 mg of protein, were incubated at 25 "C in 10 ml of medium containing 120 mM KCl, 10 mM potassium (N-2-hydroxyethylpiperazine-N '-2-ethanesulphonic acid pH 7.2 and 30 pg of rotenone for 7 min. The suspension was diluted with 90 ml of ice-cold medium containing 210 mM mannitol and 70 mM sucrose and the mitochondria sedimented by centrifugation. The mitochondria were washed once more in the same manner. Mitochondria depleted in this way and incubated in the mediu...

The uptake and release of calcium by heart mitochondria

“…Fig. 8 NaCi media compared with that in KCl media (Crompton et al, 1976b) is consistent with the presence of a Na+-activated ATPase.…”

“…The most interesting and potentially important way to initiate a discharge of Ca2+ is to add a Na+ salt solution (Crompton et al, 1976b;Carafoli et al, 1974). This discharge can be of a limited quantity of Ca2+, or can proceed to complete discharge, depending on the conditions.…”

“…Vol. 168 Evidence is presented that an oligomycin-sensitive energy load imposed by a low concentration of Na+ added to the incubation mixture is the mechanism for the Ca2+ discharge described by Crompton et al (1976b). The discharging action of palmitoyl-CoA appears to stem from its surfactant property of stimulating an ATPase,* since the effect is prevented by either provision of external ATP or adsorbing the compound on albumin.…”

“…Some of these were described by Drahota et al (1965) in relation to the similar phenomena occurring with liver mitochondria. In the context of the heart, attention to the Ca2+ discharge has been intensified (Crompton et al, 1976b;Carafoli et al, 1974), because only by a cycle of uptake and non-traumatic release could the intracellular mitochondrion continue to regulate the free [Ca2+]. Vol.…”

Amarcord: I Remember

“…It was to take four decades and great conceptual and technological advances before the uniporter was eventually tracked down (18,19). However, we also kept working on the properties of sodium-promoted release of calcium from mitochondria, and we characterized it as a process mediated by a novel exchanger, thanks to the work of a series of students led by Martin Crompton, a British co-worker that had joined the laboratory as an assistant (22). We described the properties of the exchange process in great detail, and our papers on this topic still contain all of the essential information on this process.…”