Inositol tetrakisphosphate (IP4)- and inositol triphosphate (IP3)-dependent Ca2+ influx in cortical neuronal nuclei of newborn piglets following graded hypoxia.
Previous studies have shown that hypoxia results in a modification of the binding characteristics of the neuronal nuclear membrane inositol tetrakisphosphate (IP4) and inositol triphosphate (IP3) receptors. The present study tests the hypothesis that hypoxia-induced modification of the IP4 and IP3 receptors results in increased IP4 and IP3 dependent Ca2+ influx in neuronal nuclei as a function of the degree of cerebral tissue hypoxia in newborn piglets. Studies were performed in piglets, 3-5 days old, divided into normoxic (N = 5) and hypoxic (N = 6) groups. The hypoxic group was exposed to decreased FiO2 ranging from 0.15 to 0.05 for 1 h. Brain tissue hypoxia was documented biochemically by determining ATP and phosphocreatine (PCr) levels. Neuronal nuclei were isolated and 45Ca2+ influx was determined in a medium containing 50 mM Tris buffer (pH 7.4), neuronal nuclei (150 microg protein), 1 microM 45Ca2+, with or without 10 microM IP4 or IP3. In normoxic and hypoxic groups, ATP levels were 4.27 +/- 0.80 and 1.40 +/- 0.69 micromoles/g brain, respectively, P < .001 (ranging from 4.78 to 0.82). PCr levels were 3.40 +/- 0.99 and 0.91 +/- 0.57 micromoles/g brain, respectively, P < .001 (raning from 4.07 to 0.60). During hypoxia, IP4-dependent intranuclear 45Ca2+ influx increased from 3.39 +/- 0.64 in normoxic nuclei to 13.30 +/- 2.18 pM/mg protein in hypoxic nuclei (P < .01). There was an inverse correlation between the 45Ca2+ influx in neuronal nuclei and the levels of cerebral tissue ATP (r = 0.83) and PCr (r = 0.85). Similarly, IP3-dependent intranuclear 45Ca2+ influx increased from 2.26 +/- 0.38 pmoles/mg protein in normoxic nuclei to 11.12 +/- 1.65 pmoles/mg protein in hypoxic nuclei and showed an inverse correlation between 45Ca2+ influx in neuronal nuclei and the levels of cerebral tissue ATP (r = 0.86) and PCr (r = 0.71). The data demonstrate that there is an IP4- as well as IP3-dependent increase in nuclear Ca2+ influx with increasing cerebral tissue hypoxia, suggesting a hypoxia-induced modification of the nuclear membrane IP4 and IP3 receptors. We propose that there is a specific level of tissue hypoxia that results in a critical increase of intranuclear Ca2+ that leads to altered transcription of apoptotic genes and activation of nuclear endonucleases resulting in hypoxia-induced programmed neuronal death.
Department of Pediatrics, Drexel University College of Medicine and St. Christopher's Hospital for Children, Philadelphia, Pennsylvania 19129, USA. Mishra@drexel.edu
Om Prakash
OP
Mishra
Maria
M
Delivoria-Papadopoulos
D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D002467:Cell Nucleus / Q000378:metabolism; D002540:Cerebral Cortex / Q000187:drug effects / Q000378:metabolism*; D000860:Hypoxia / Q000378:metabolism*; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000494:pharmacology*; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007295:Inositol Phosphates / Q000494:pharmacology*; D009474:Neurons / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D013552:Swine
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007295:Inositol Phosphates; D018160:Receptors, Cytoplasmic and Nuclear; C116606:inositol-1,2,4,5-tetrakisphosphate; C061107:inositol-1,3,4,5-tetrakisphosphate receptor; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium