Cardiac energetics, cell volumes, sodium fluxes, and membrane permeability: NMR studies of cold ischemia

Intracellular sodium accumulation, cellular swelling, and energy deficiency are ischemia-associated processes that participate in the transition to irreversible ischemic injury. This study aims to determine the relationship among these parameters in intact rat hearts during global ischemia at 4°C. High-energy phosphates were determined by ³¹P nuclear magnetic resonance, intracellular sodium accumulation was measured by ²³Na spectroscopy with the shift reagent dysprosium triethyl tetraaminohexaacetic acid [Dy(TTHA)^3-], and cell volumes were measured by ⁵⁹Co and ¹H spectroscopy with use of the extracellular marker Co(CN)₆/^3-. Intracellular sodium flux rates were 1.53 ± 0.17, 0.17 ± 0.05, and 0.30 ± 0.06 μmol · g dry wt^-1 · min^-1 at 0-1.5, 2-7, and 9-12 h, respectively. Sodium influx resulted in accumulation of the ion: 10% after 4 h, 16% after 10 h, and 29% after 12 h. Water followed sodium into the cells at two constant molar ratios (Na⁺/H₂O): 3.80 ± 0.15 x 10^-3 during the first 8 h of ischemia and 7.8 x 10^-3 at 8-12 h. Relative to initial intracellular volume, cells swelled by 38% after 8 h and 46% after 12 h; reperfusion reduced cellular swelling to 25 and 36%, respectively. The last 4 h of storage were characterized by 1) absence of high-energy phosphates, 2) increased sodium influx rate and sodium accumulation, 3) increased Na⁺/H₂O ratio, indicating an enhanced efflux of intracellular components, 4) preserved membrane selectivity for the extracellular markers Dy(TTHA)^3- and Co(CN)₆/^3-, and 5) deterioration of hemodynamic function: left ventricular pressure of 75 and 40% and initiation of activity after 8 and 26 min for 8 and 12 h of ischemia, respectively. These data indicate that changes in membrane selectivity and permeability occur during hypothermic ischemia and are associated in time with depletion of energy stores and deterioration of functional recovery. It is suggested that these changes in permeability signal the onset of a gradual deterioration in membrane integrity.