Reverse Mode Na⁺/Ca²⁺ Exchanger Mediates The Cardioprotection Of Ca²⁺ Preconditioning In Rat Heart

AIMS(1) To investigate the effect of calcium preconditioning (CPC) on rat heart.(2) To investigate the effect of KB-R7943, an inhibitor of reverse mode Na⁺/ Ca ²⁺ exchanger (NCX), on isolated rat heart.(3) To determine the role of reverse mode NCX on the cardioprotection of CPC.METHODSThe hearts of forty-two male SD rats were isolated, underwent Langendroff perfusion, and were randomized into 6 groups: (1) Control group (53 min perfusion with KH buffer, n=6), (2) Ca²⁺ PD group (5 min perfusion with KH buffer lacking Ca²⁺ followed by 30 min KH buffer containing Ca ²⁺, n=8), (3) CPC group (3 cycles perfusion of 1 min Ca²⁺ depletion/5 min Ca ²⁺ repletion+Ca²⁺ PD, n=8), (4) reverse mode NCX inhibitor KB-R7943 (3μmol/L)+CPC group (addition of KB-R7943 during calcium preconditioning, n=9), (5) KB-R7943 (3μmol/L)+Ca ²⁺ PD group (18 min perfusion with KH buffer containing KB-R7943 +Ca²⁺ PD, n=5), (6) KB-R7943 (3μmol/L)+Control group (18 min perfusion with KH buffer containing KB-R7943 followed by 35 min KH buffer perfusion, n=6). Left ventricular developed pressure (LVDP), left ventricular end diastolic pressure (LVEDP), rate of rise of left ventricular pressure (dp/dt), coronary flow (CF), and lactate dehydrogenase (LDH) release were measured. Sections with HE staining were analyzed to display the morphologic structures of the hearts.RESULTSMarked alterations in cardiac function and the release of LDH were seen in Ca²⁺-paradoxic hearts reperfused for 30 min. The levels of LVDP, dp/dt, and CF of the hearts in CPC group were significantly higher than those in Ca²⁺ PD group (all P < 0. 001), while the level of LVEDP and the release of LDH were reduced markedly in the CPC group as compared with the Ca²⁺ PD group (all P<0.001), indicating that CPC renders the heart more resistant to the injury from Ca²⁺ PD. However, CPC itself posed a mild injury to the myocardium, manifested by a moderate decay of the cardiac performance and a gentle increase in LDH release. Both of the improvement of the hemodynamics and the LDH release induced by CPC and the myocardial injury during CPC were significantly attenuated by the reverse mode NCX inhibitor KB-R7943 employed during the whole process of the CPC. Additionally, a negative effect of KB-R7943 on cardiac function was detected.CONCLUSION(1) The enhancement of reverse mode NCX is involved in the cardioprotection induced by CPC in the isolated rat heart model; (2) CPC poses a mild injury to the myocardium per se, which might be mediated by the actication of the reverse mode NCX during the process of CPC;(3) The reverse mode NCX inhibitor KB-R7943 has a negative but reversible effect on cardiac performance, which produces no injury to myocardium.