Inhibition of sodium influx and improved preservation of rat hearts during hypothermic ischemia by furosemide and bumetanide: A²³Na- and³¹P-NMR study

The importance of the Na⁺/K⁺/Cl^- co-transport system of the rat myocardial sarcolemma was studied under hypothermic ischemia by investigating the effect of the co-transport blockers furosemide and bumetanide on the sodium influx into the myocardium. The intracellular Na⁺ accumulation during hypothermic ischemia was followed by²³Na-NMR. For this purpose the shift reagent [Dy(TTHA)^3-] (SR) was added to the Krebs-Henseleit (KH) perfusion solution. The same solution was also present during the hypothermic preservation. A significant reduction in the intracellular Na⁺ accumulation after 12 h was found when 100 μM furosemide was present during the perfusion and preservation periods. The intracellular Na⁺ levels returned to the pre- ischemic values after 1 h of reperfusion with KH in both the treated and control groups. Dose-response studies have indicated that 1-100 μM furosemide or 0.1 μM bumetanide added to the KH-SR solution reduced the Na⁺ influx significantly over 4 h of hypothermic ischemia. No statistically significant effect was found with furosemide concentration of 0.1 μM or with bumetanide concentrations higher or lower than 0.1 μM.³¹P-NMR measurements showed no effect of the 100 μM furosemide on the intracellular ATP, the sum of inorganic phosphate and phosphomonoester, or pH levels over 4 h or after 12 h of hypothermic ischemia. Hearts treated with KH containing 100 μM furosemide showed, significantly higher functional recovery after 12 h of hypothermic ischemia than hearts treated only with KH. This study strongly indicates the existence of the Na⁺/K⁺/Cl^- co-transport system in the intact rat heart sarcolemma, and its major role in sodium influx during hypothermic ischemia. It is suggested that the improved hypothermic preservation by furosemide and bumetanide is related to the inhibition of the Na⁺ influx, and that these agents should be considered as additives for heart preservation solutions.