DEVELOPMENTAL-CHANGES IN ATP UTILIZATION DURING GRADED HYPOXIA AND REOXYGENATION IN THE HEART IN-VIVO

Myocardial ATP utilization and resynthesis during hypoxia and reoxygen ation were studied in vivo as a function of maturation. Graded hypoxia was performed in newborn (NB; 4-10 days old, n = 6) and mature sheep (MAT; 30-60 days old, n = 6). Time-resolved P-31-nuclear magnetic reso nance was used to monitor myocardial phosphates throughout hypoxia and to monitor reoxygenation concomitant with rate of myocardial O-2 cons umption (MV over dot O-2) measurement. Oxygen delivery and MV over dot O-2, were constant in both groups throughout hypoxia, with substantia l and similar increases in both parameters during reoxygenation. Hypox ic myocardial lactate release was similar in NB and MAT. Phosphocreati ne (PCr), but not ATP, decreased in NB only during milder hypoxia. Rap id PCr and slower ATP depletion occurred with severe hypoxia, consiste nt with ATP utilization/synthesis imbalance. Depletion rates were high er in MAT. Creatine rephosphorylation rates, measures of mitochondrial function reported as percentage of predicted values, were similar: 34 +/- 12 in NB and 26 +/- 9% in mature lambs. In conclusion, 1) phospho rylation potential decreases in NB but not MAT in response to a decrea sing oxygen gradient; 2) ATP utilization during hypo;da increases more in mature lambs; 3) anaerobic ATP production is not greater in NB; an d 4) despite the greater energy imbalance imposed on MAT during hypoxi a, mitochondrial function is similar to NB during reoxygenation.