Effects of hypoxia on respiratory muscle metabolism during exercise in rat

Twenty male Wistar rats were studied in normoxia or hypoxia (F(I)0₂ = 0.12) at rest and during exercise to exhaustion (24 ± 10 min.). Diaphragm, intercostal, and plantaris muscles were analyzed for metabolite levels. At rest, hypoxia (PaO₂ = 47 mmHg) had no effect on muscle metabolites. During hypoxic exercise (HEX) PaCO₂ averaged 21.6 mmHg, PaO₂ 61 mmHg and pHa 7.17. Arterial lactate rose from 2.9 to 16mM and liver glycogen fell from 308-155 μmoles/g. In all 3 muscles the increase in lactate was less than in blood, i.e., from 1.0 to 8.5, from 1.0 to 7.5 and from 1.6 to 6.0 μmoles/g in plantaris, diaphragm and intercostal respectively. ATP and CP were unchanged in all three muscles. Glycogen fell to 44, 58 and 20% of control for diaphragm, intercostal and plantaris respectively. G-6-P increased 166 and 113% in diaphragm and intercostal, but was unchanged in plantaris. We previously showed that normoxic exercise of any severity or duration (i.e., 3 min at 106% VO₂ max/38 min at 84% max) did not deplete diaphragm glycogen; we attributed this to an allosteric inhibition of phosphorylase by the exercise-induced increase in G-6-P. In HEX glycogen depletion in diaphragm is less than in plantaris but is not completely prevented, suggesting that inhibition of phosphorylase by G-6-P is overridden in HEX, possibly by catecholamines.