Total Matrix Ca2+ Modulates Ca2+ Efflux via the Ca2+/H+ Exchanger in Cardiac Mitochondria

Published on 2020-09-16T14:05:31Z (GMT) by
<p>Mitochondrial Ca<sup>2+</sup> handling is accomplished by balancing Ca<sup>2+</sup> uptake, primarily via the Ru360-sensitive mitochondrial calcium uniporter (MCU), Ca<sup>2+</sup> buffering in the matrix and Ca<sup>2+</sup> efflux mainly via Ca<sup>2+</sup> ion exchangers, such as the Na<sup>+</sup>/Ca<sup>2+</sup> exchanger (NCLX) and the Ca<sup>2+</sup>/H<sup>+</sup> exchanger (CHE). The mechanism of CHE in cardiac mitochondria is not well-understood and its contribution to matrix Ca<sup>2+</sup> regulation is thought to be negligible, despite higher expression of the putative CHE protein, LETM1, compared to hepatic mitochondria. In this study, Ca<sup>2+</sup> efflux via the CHE was investigated in isolated rat cardiac mitochondria and permeabilized H9c2 cells. Mitochondria were exposed to (a) increasing matrix Ca<sup>2+</sup> load via repetitive application of a finite CaCl<sub>2</sub> bolus to the external medium and (b) change in the pH gradient across the inner mitochondrial membrane (IMM). Ca<sup>2+</sup> efflux at different matrix Ca<sup>2+</sup> loads was revealed by inhibiting Ca<sup>2+</sup> uptake or reuptake with Ru360 after increasing number of CaCl<sub>2</sub> boluses. In Na<sup>+</sup>-free experimental buffer and with Ca<sup>2+</sup> uptake inhibited, the rate of Ca<sup>2+</sup> efflux and steady-state free matrix Ca<sup>2+</sup> [mCa<sup>2+</sup>]<sub>ss</sub> increased as the number of administered CaCl<sub>2</sub> boluses increased. ADP and cyclosporine A (CsA), which are known to increase Ca<sup>2+</sup> buffering while maintaining a constant [mCa<sup>2+</sup>]<sub>ss</sub>, decreased the rate of Ca<sup>2+</sup> efflux via the CHE, with a significantly greater decrease in the presence of ADP. ADP also increased Ca<sup>2+</sup> buffering rate and decreased [mCa<sup>2+</sup>]<sub>ss.</sub> A change in the pH of the external medium to a more acidic value from 7.15 to 6.8∼6.9 caused a twofold increase in the Ca<sup>2+</sup> efflux rate, while an alkaline change in pH from 7.15 to 7.4∼7.5 did not change the Ca<sup>2+</sup> efflux rate. In addition, CHE activation was associated with membrane depolarization. Targeted transient knockdown of LETM1 in permeabilized H9c2 cells modulated Ca<sup>2+</sup> efflux. The results indicate that Ca<sup>2+</sup> efflux via the CHE in cardiac mitochondria is modulated by acidic buffer pH and by total matrix Ca<sup>2+</sup>. A mechanism is proposed whereby activation of CHE is sensitive to changes in both the matrix Ca<sup>2+</sup> buffering system and the matrix free Ca<sup>2+</sup> concentration.</p>

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Natarajan, Gayathri K.; Glait, Lyall; Mishra, Jyotsna; Stowe, David F.; Camara, Amadou K. S.; Kwok, Wai-Meng (2020): Total Matrix Ca2+ Modulates Ca2+ Efflux via the Ca2+/H+ Exchanger in Cardiac Mitochondria. Frontiers. Collection.