Role of intracellular na⁺ kinetics in preconditioned rat heart

To elucidate the role of intracellular Na⁺ kinetics in the mechanism for ischemic preconditioning (IPC), we measured intracellular Na⁺ concentration ([Na⁺]_i) using ²³Na-magnenc resonance spectroscopy in isolated rat hearts. IPC significantly delayed the initial [Na⁺]_i increase (d[Na⁺]_i/dt) compared with non-IPC control, resulting in attenuation of Na⁺ accumulation (Δ[Na⁺]_i) during 27 minutes of ischemia with better functional recovery. [Na⁺]_i in IPC, but not in control, recovered to preischemic level during a 6-minute reperfusion. The Na⁺-H⁺ exchange inhibitor further suppressed d[Na⁺]_i/dt in both control and IPC hearts with concomitant improvement of functional recovery, suggesting little contribution to the mechanism of IPC. The mitochondrial ATP-sensitive K⁺ (mito K_ATP) channel activator diazoxide (30 μmol/L) completely mimicked both [Na⁺]_i kinetics and functional recovery in IPC without any additive effects to IPC. The mito K_ATP channel blocker 5-hydroxydecanoic acid (100 μmol/L) lost protective effect as well as the attenuation of d[Na⁺]_i/dt and [Na^-]_i recovery induced by diazoxide. However, 5-hydroxydecanoic acid also lost IPC-induced protection, but incompletely abolished the alteration of d[Na⁺]_i/dt and the [Na⁺]_i recovery. The Na⁺/K⁺-ATPase inhibitor ouabain (200 μmol/L) did not change d[Na⁺]_i/dt in non-IPC hearts, but it abolished the IPCor diazoxide-induced reduction of d[Na⁺]_i/dt and the [Na⁺]_i recovery, whereas IPC followed by ouabain treatment showed partial functional recovery with smaller Δ[Na⁺]_i than other ouabain groups. In conclusion, alteration of Na⁺ kinetics by preserving Na⁺ efflux via Na⁺/K⁺-ATPase mediated by mito K_ATP channel activation mainly contributes to functional protection in IPC hearts. The contribution of mito K_ATP channel-independent pathway relating to Na⁺ kinetics including reduced Na⁺ influx is limited in functional protection of IPC.