EFFECT OF IN-UTERO HYPOXIA ON THE OUABAIN STROPHANTHIDIN BINDING-SITEOF THE FETAL GUINEA-PIG BRAIN-CELL MEMBRANE NA-ATPASE(,K+)

This study investigates the effect of hypoxia on the high affinity str ophanthidin/ouabain binding site of brain cell membrane Na+,K+-ATPase in 30, 45 and 60 day (term) fetal guinea pigs. Studies were performed on 30 fetuses randomized to either normoxic or hypoxic conditions. The hypoxic fetuses were exposed to maternal hypoxia (FiO(2) = 7%) for 60 min. Brain cell membrane fractions were prepared, and the rate of ATP hydrolysis was determined at varying concentrations of strophanthidin . In every experiment with 45 and 60 day brain preparations hypoxia ca used a leftward shift in the IC50, but this did not reach the level of statistical significance (4.0 x 10(-5) normoxic, 9.0 x 10(-6) hypoxic , P = 0.069, at 45 days; 9.5 x 10(-6) M normoxic, 8.5 x 10(-6) M hypox ic, P = 0.23, at 60 days). If hypoxia does cause a true left shift thi s would indicate greater sensitivity of the hypoxic brain to inhibitor . In addition, [H-3]ouabain binding studies were performed. In the 30 day normoxic brain preparations, the K-d was 24.7 +/- 5.6 nM, and the B-max was 0.26 +/- 0.08 pmol/mg protein. At 45 days the ouabain bindin g sites showed no change in affinity following hypoxia (K-d = 14.6 +/- 1.7 nM normoxic, 13.0 +/- 0.8 nM hypoxic, P = NS); however, there was a significant decrease in receptor number following hypoxia (B-max = 22.1 +/- 2.2 pmol/mg protein normoxic, 16.9 +/- 0.3 pmol/mg protein hy poxic, P < 0.05). In 60 day hypoxic brain, ouabain binding sites showe d a significant decrease in K-d (K-d = 213.3 +/- 39.0 nM normoxic, 161 .1 +/- 33.6 nM hypoxic, P < 0.05) reflecting an increased affinity, an d a significant decrease in B-max (154.7 +/- 61.6 pmol/mg protein norm oxic, 76.0 +/- 28.5 pmol/mg protein hypoxic, P = 0.02). At 45 days onl y B-max is changed by hypoxia, indicating that following hypoxia, ther e are fewer enzyme molecules, but the remaining molecules are not affe cted. Term brain Na+,K+-ATPase molecules are even more sensitive to hy poxia, with hypoxia resulting in a decrease in both B-max and K-d. Thi s suggests that at term not only are there fewer enzyme molecules foll owing hypoxia, but that the remaining molecules are modified as well.