Comparative response of 2-week and 6-month old rat myocardium to hypoxia.

The effect of hypoxia on contractile performance of left ventricular papillary muscles from 2-week old and 6-month old rats was studied. Left ventricular glycogen concentration was not significantly different between 2-week and 6-month old hearts; however, the lactate dehydrogenase isoenzyme pattern of the left ventricular myocardium from the 2-week old rats demonstrated a shift toward M-type (anaerobic) distribution relative to 6-month-old animals. Papillary muscle performance was studied at 28 degrees C while contracting 12 times/min at the peak of the active tension curve in oxygenated (95% oxygen, 5% carbon dioxide) Krebs-Henseleit solution containing glucose concentrations from 5.5 to 22 mM. In a bath containing 5.5 mM glucose, the decline in developed tension during hypoxia (95% nitrogen, 5% carbon dioxide) of preparations from the 2-week old rats was significantly less (P less than 0.05) than the 6-month old group only at 15 min. Raising the glucose concentrations of the bath to 11 and 16.5 mM resulted in improved performance throughout 60 min of hypoxia in preparations from 2-week compared with 6-month old animals. Practolol (5 X 10(-5)M) did not prevent the increased tolerance to hypoxia observed in preparations from 2-week animals. Thus, increasing anaerobic substrate has differential effects on the performance of 2-week and 6-month myocardium during hypoxia. The data suggests that in contrast to 6-month old heart muscle, myocardium from 2-week old rats is capable of modulating its active mechanical activity during hypoxia in accordance with the availability of glycolyptic substrate.