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KMID : 0360419820180020089
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Korean Journal of Pharmacology
1982 Volume.18 No. 2 p.89 ~ p.102
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The Study on Na+-Ca4+ Exchange in Heart Mitochondria
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Abstract
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The Na+-and K+-induced Call release was measured isotopically by miliipore filter technique in pig heart mitochondria. With EGTA-quenching technique, the characteristics of mitochondrial Cal´-pool and the sources of Call released from mitochondria by Na+ or K+ were: analyzed.
The mitochondrial Ca++-pool could be distinctly divided into two components; internal and external ones which were represented either by uptake through inner membrane, or by energy independent passive binding to external surface of mitochondria, respectively. In energized mitochondria, a large portion of Call was transported into internal pool with little external binding, while in de-enerigzed state, a large portion of transported Call existed in the external pool with limited amount of Ca++ in the internal pool which was possibly transported through the Ca++-carrier present in the inner membrane.
Na induced the Ca++ release from both internal pool and external pool and external. binding pool of mitochondria. In contrast, K} did not affect Call of the internal pool, but,. displaced Ca++ bound to external surface of the mitochondria.
When the Ca++-reuptake was blocked by EGTA, the Ca++ release from the internal pool by Na+ was rapid; the rate of Ca++-efflux appeared to be a function of Na+J2 and about 8 mM Na+ was required to elicit half-maximal velocity of Ca++-efflux. So it was revealed that Ca++-efflux velocity was particularly sensitive to small changes of the Na+ concentration. in physiological range.
Energy independent Ca++-binding sites of mitochondrial external surface showed unique characteristics. The total number of external Ca++-binding sites of pig heart mitochondria was 29 nmoles per mg protein and the dissociation constant(Kd) was 34pM. The Ca++binding to the external sites seemed to be competitively inhibited by Na+ and K+; the. inhibition constant (Ki) were 9.7 mM and 7. 1 mM respectively. Considering the intracellular ion concentrations and large proportion of Call uptake in energized mitochondria, the external Ca++-binding pool of the mitochondria did not seem to play a significant role on the regulation of intracellular free Ca- concentration.
From this experiment, it was suggested that a small change of intracellular free Na+ concentration might play a role on regulation of free Ca++ concentration in cardiac cell by influencing Ca++-efflux from the internal pool of mitochondria.
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