Previous studies showed an increased intracellular calcium concentration during hypoxia in the newborn and fetal brain. The high-affinity Ca++-ATPase enzyme, located on the outer membrane of the nucleus, has been shown to be a primary mechanism of Ca++ uptake into the nucleus in normoxia. Recent studies also demonstrate that the IP3 receptor, located on the inner nuclear membrane, controls the IP3 dependent increase in calcium in the nucleoplasm and influences gene transcription during hypoxia. In order to elucidate the mechanism of regulation of nuclear Ca++ concentration during hypoxia, the present study examines the effect of brain tissue hypoxia on the activity of the high-affinity Ca++-ATPase and on the density and affinity of the IP3 receptor in cerebral cortical nuclear membranes of normoxic and hypoxic guinea pig fetuses. Nuclear membranes were prepared from normoxic (21% O2, n=4) and hypoxic (7% O2 for 1 hr, n=4) guinea pig fetuses.3 H-IP3 binding was performed in nuclear membranes and receptor characteristics Bmax (receptor number) and Kd (receptor affinity) determined,3 H-IP3 binding was performed in a medium containing 50 mM HEPES buffer, (pH 8.0), 2 mM EDTA, 3H-IP3 (7.5 to 100 mM) and 150 ug membrane protein. Results are as follows: In the hypoxic group the total, Mg++ dependent, and Mg++-independent Ca++-ATPase activity increased compared to the normoxic group: 969.7±79 vs 602.4±90.9, p<0.005; 774.7±147.4 vs 491.5±93.8, p<0.01; and 193.8±14.7 vs 132.8±26, p<0.01. In normoxic nuclear membranes the Bmax and the Kd values were 34±4 fmoles/mg protein and 18±2 nM, respectively. In hypoxic nuclear membranes the Bmax and the Kd values were 110±8fmoles/mg protein and 15±2 nM, respectively. The data show that while the affinity of the receptor for3 H-IP3 is not altered by hypoxia there is a 3-fold increase in the number of IP3 receptor sites in hypoxic nuclear membranes, indicating that hypoxia-induced alteration of nuclear membranes leads to an increase in accessible IP3 receptor sites. The increased Ca++-ATPase activity suggest that the structure of the outer nuclear membrane is altered during hypoxia. We speculate that an increase in the number of accessible IP3 receptor sites may lead to an increase in intranuclear Ca++, and result in altered gene expression in the hypoxic brain.