Brain cell membrane Na,K-ATPase activity following severe hypoxic injury in the newborn piglet

This study tests the hypothesis that severe brain hypoxia causes decreased Na⁺,K⁺-ATPase activity, resulting in permanent alterations in the neuronal cell membranes. Seventeen anesthetized piglets (normoxic control (NC), no recovery after hypoxia (Group 1), 6 h normoxic recovery (Group 2), and 48 h normoxic recovery (Group 3) were studied. Hypoxia was induced by lowering the FiO₂ to maintain PCr/P_i ratio at 25% of baseline for 1 h as monitored by ³¹P-NMR spectroscopy. PCr/P_i returned to 57% of baseline by 6 h and was normal by 48 h. At termination, cortical tissue Na⁺,K⁺-ATPase activity was determined. Na⁺,K⁺-ATPase activity was measured in cortical membrane preparations by determining the rate of ATP hydrolysis. NC membranes had Na⁺,K⁺-ATPase activity of 58.3 ± 1.3 μM P_i/mg protein/h (mean ± S.E.M.). Na⁺,K⁺-ATPase activity was reduced in Groups 1, 2, and 3 (45.8 ± 1.3, 47.4 ± 3.6, 48.7 ± 2.9 μM P_i/mg protein/h) (P < 0.05 compared to NC). There was no differene in enzyme activity among Groups 1, 2, or 3. The data show that in spite of recovery of neuronal oxiditive phosphorylation (PCr/P_i) by 48 h, there is a permanent decrease in Na⁺,K⁺-ATPase activity in cells that have undergone severe hypoxic injury. The persistent decrease in Na⁺,K⁺-ATPase activity indicates ongoing cell injury following severe cerebral hypoxia, and that recovery of oxidative phosphorylation as indicated by PCr/P_i values cannot be used as an index of recovery of cell function.