Calcium overload and mechanical function in posthypoxic myocardium: biphasic effect of pH during hypoxia

Mechanical function and calcium accumulation in the myocardium during and after hypoxia were examined in the isolated but arterially perfused interventricular rabbit septum. The pH of the perfusate during hypoxia was varied from 7.4 to 6.6 by increase of the pCO₂. All septa were reoxygenated for 30 min at pH 7.4. In the posthypoxic period the recovery of developed tension was greatest and the magnitude of contracture least in those septa perfused at pH 6.8 during hypoxia; calcium overload did not occur. By contrast, marked calcium overload (3.5 μmol/g wet wt) occurred in septa perfused at pH 7.4 during hypoxia. Reduction of pH to 6.6 during hypoxia did not result in a greater degree of recovery of developed tension or complete reversal of contracture in the posthypoxic period, and marked calcium overload was not prevented. These results indicate that: (1) partial recovery of mechanical function in the posthypoxic period can occur concurrent with a net gain of calcium; (2) the beneficial effects on recovery in the posthypoxic period in septa perfused at pH 6.8 during hypoxia may be in be related related to prevention of calcium overload; (3) the beneficial effects of acidosis are lost when the perfusate pH is reduced to 6.6 during hypoxia.