Cytosolic free calcium concentrations in synaptosomes during histotoxic hypoxia.
Altered cytosolic free calcium concentrations ([Ca2+]i) accompany impaired brain metabolism and may mediate subsequent effects on brain function and cell death. The current experiments examined whether hypoxia-induced elevations in [Ca2+]i are from external or internal sources. In the absence of external calcium, neither KCl depolarization, histotoxic hypoxia (KCN), nor the combination changed [Ca2+]i. However, with external CaCl2 concentrations as small as 13 microM, KCl depolarization increased [Ca2+]i instantaneously while hypoxia gradually raised [Ca2+]i. The combination of KCN and KCl was additive. Increasing external calcium concentrations up to 2.6 mM exaggerated the effects of K+ and KCN on [Ca2+]i, but raising medium calcium to 5.2 mM did not further augment the rise. Diminishing the sodium in the media, which alters the activity and perhaps the direction of the Na/Ca exchanger, reduced the increase in [Ca2+]i due to hypoxia, but enhanced the KCl response. The changes in ATP following K+ depolarization, KCN or their combination in the presence of physiological calcium concentrations did not parallel alterations in [Ca2+]i, which suggests that diminished activity of the calcium dependent ATPase does not underlie the elevation in [Ca2+]i. Valinomycin, an ionophore which reduces the mitochondrial membrane potential, elevated [Ca2+]i and the effects were additive with K+ depolarization in a calcium dependent manner that paralleled the effects of hypoxia. Together these results suggest that hypoxia-induced elevations of synaptosomal [Ca2+]i are due to an inability of the synaptosome to buffer entering calcium.
Department of Neurology and Neuroscience, Cornell University Medical College, Burke Medical Research Institute, White Plains, NY 10605.
G
G
Gibson
L
L
Toral-Barza
H M
HM
Huang
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015687:Cell Hypoxia; D002540:Cerebral Cortex / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D007700:Kinetics; D008297:Male; D051379:Mice; D008815:Mice, Inbred Strains; D011189:Potassium Chloride / Q000494:pharmacology; D011190:Potassium Cyanide / Q000494:pharmacology; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism* / Q000502:physiology; D013997:Time Factors; D014634:Valinomycin / Q000494:pharmacology
D014634:Valinomycin; D011189:Potassium Chloride; D011190:Potassium Cyanide; D002118:Calcium