391
Inositol Tetrakisphosphate (IP4)- and Inositol Triphosphate
(IP3)-Dependent Ca
2
Influx in Cortical Neuronal Nuclei
of Newborn Piglets Following Graded Hypoxia
Om Prakash Mishra
1,2
and Maria Delivoria-Papadopoulos
1
(Accepted July 25, 2003)
Previous studies have shown that hypoxia results in a modification of the binding characteristics
of the neuronal nuclear membrane inositol tetrakisphosphate (IP
4
) and inositol triphosphate (IP
3
)
receptors. The present study tests the hypothesis that hypoxia-induced modification of the IP
4
and
IP
3
receptors results in increased IP
4
and IP
3
dependent Ca
2+
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 FiO
2
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
45
Ca
2+
influx was determined in a medium containing 50 mM Tris buffer
(pH 7.4), neuronal nuclei (150 g protein), 1 M
45
Ca
2+
, with or without 10 M IP
4
or IP
3
. In
normoxic and hypoxic groups, ATP levels were 4.27 0.80 and 1.40 0.69 moles/g brain,
respectively, P .001 (ranging from 4.78 to 0.82). PCr levels were 3.40 0.99 and 0.91
0.57 moles/g brain, respectively, P .001 (raning from 4.07 to 0.60). During hypoxia, IP
4
-
dependent intranuclear
45
Ca
2+
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
45
Ca
2+
influx in neuronal nuclei and the levels of cerebral tissue ATP (r = 0.83) and PCr (r =
0.85). Similarly, IP
3
-dependent intranuclear
45
Ca
2+
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
45
Ca
2+
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 IP
4
- as well as IP
3
-dependent
increase in nuclear Ca
2+
influx with increasing cerebral tissue hypoxia, suggesting a hypoxia-
induced modification of the nuclear membrane IP
4
and IP
3
receptors. We propose that there is a
specific level of tissue hypoxia that results in a critical increase of intranuclear Ca
2+
that leads
to altered transcription of apoptotic genes and activation of nuclear endonucleases resulting in
hypoxia-induced programmed neuronal death.
KEY WORDS: Ca
2+
-Influx; neuronal nuclei; IP
3
-receptor; IP
4
-receptor; hypoxia; brain.
INTRODUCTION
Intracellular increase in Ca
2+
is a primary mediator
of activity-dependent gene transcription under a number
of experimental conditions (1–4). The neuronal impulse
patterns and the specific properties of the stimulus-induced
0364-3190/04/0200–0391/0 © 2004 Plenum Publishing Corporation
Neurochemical Research, Vol. 29, No. 2, February 2004 (© 2004), pp. 391–396
1
Department of Pediatrics, Drexel University College of Medicine and
St. Christopher’s Hospital for Children, Philadelphia, Pennsylvania.
2
Address reprint requests to: Om Prakash. Mishra, Department of
Pediatrics, Room 701, 7th Floor Heritage Building, Medical College of
Pennsylvania Hospital, 3300 Henry Avenue, Philadelphia, Pennsylvania
19129. Tel: 215-842-4960; Fax: 215-843-3505; E-mail: Mishra@
drexel.edu