Hypoxia modifies nuclear calcium uptake pathways in the cerebral cortex of the guinea-pig fetus

Nuclear Ca2+ signals are thought to play a critical role in the initiation and progression of programmed cell death. The present study tests the hypothesis that hypoxia alters nuclear Ca2+ transport pathways and leads to an increase in nuclear Ca2+-influx in cerebral cortical neuronal nuclei. To test this hypothesis the effect of tissue hypoxia on high affinity Ca2+-ATPase activity and the binding characteristics of inositol 1,4,5-triphosphate (IP3) and inositol 1,3,4,5-tetrakisphosphate (IP4) receptors were studied in neuronal nuclei from the cerebral cortex of guinea-pig fetuses. Results show increased high-affinity Ca2+-ATPase activity (nmol/mg protein/h) in the hypoxic group 969.7±79 as compared with 602.4±90.9 in the normoxic group, P<0.05. The number of IP3 receptors (Bmax, fmol/mg protein) increased from 61±21 in the normoxic group to 164±49 in the hypoxic group, P<0.05. Kd values did not change following hypoxia. In contrast, IP4 receptor Bmax (fmol/mg protein) and Kd (nM) values increased from 360±32 in the normoxic group to 626±136 in the hypoxic group (P<0.001) and, from 26±1 in the normoxic group to 61±9 in the hypoxic group (P<0.001), respectively. 45Ca2+-influx (pmol/mg protein) significantly increased from 6.3±1.9 in the normoxic group to 10.9±1.1 the hypoxic group (P<0.001). The data show that hypoxia modifies nuclear Ca2+ transport pathways and results in increased nuclear Ca2+-influx. We speculate that hypoxia increases nuclear Ca2+ uptake from the cytoplasm to the nucleoplasm, resulting in increased transcription of proapoptotic genes and subsequent activation of programmed cell death pathways.