The Journal of Neurophysiology Vol. 83 No. 3 March 2000, pp. 1338-1345
Copyright ©2000 by the American Physiological Society

Extracellular pH Changes and Accompanying Cation Shifts During Ouabain-Induced Spreading Depression

Department of Physiology and Neuroscience and Department of Neurosurgery, New York University School of Medicine, New York, New York 10016

Menna, G., C. K. Tong, and M. Chesler. Extracellular pH Changes and Accompanying Cation Shifts During Ouabain-Induced Spreading Depression. J. Neurophysiol. 83: 1338-1345, 2000. Interstitial ionic shifts that accompany ouabain-induced spreading depression (SD) were studied in rat hippocampal and cortical slices in the presence and absence of extracellular Ca²⁺. A double-barreled ion-selective microelectrode specific for H⁺, K⁺, Na⁺, or Ca²⁺ was placed in the CA1 stratum radiatum or midcortical layer. Superfusion of 100 µM ouabain caused a rapid, negative, interstitial voltage shift (2-10 mV) after 3-5 min. The negativity was accompanied by a rapid alkaline transient followed by prolonged acidosis. In media containing 3 mM Ca²⁺, the alkalosis induced by ouabain averaged 0.07 ± 0.01 unit pH. In media with no added Ca²⁺ and 2 mM EGTA, the alkaline shift was not significantly different (0.09 ± 0.02 unit pH). The alkaline transient was unaffected by inhibiting Na⁺-H⁺ exchange with ethylisopropylamiloride (EIPA) or by blocking endoplasmic reticulum Ca²⁺ uptake with thapsigargin or cyclopiazonic acid. Alkaline transients were also observed in Ca²⁺-free media when SD was induced by microinjecting high K⁺. The late acidification accompanying ouabain-induced SD was significantly reduced in Ca²⁺-free media and in solutions containing EIPA. The ouabain-induced SD was associated with a rapid but relatively modest increase in [K⁺]_o. In the presence of 3 mM external Ca²⁺, the mean peak elevation of [K⁺]_o was 12 ± 0.62 mM. In Ca²⁺-free media, the elevation of [K⁺]_o had a more gradual onset and reached a significantly larger peak value, which averaged 22 ± 1.1 mM. The decrease in [Na⁺]_o that accompanied ouabain-induced SD was somewhat greater. The [Na⁺]_o decreased by averages of 40 ± 7 and 33 ± 3 mM in Ca²⁺ and Ca²⁺-free media, respectively. In media containing 1.2 mM Ca²⁺, ouabain-induced SD was associated with a substantial decrease in [Ca²⁺]_o that averaged 0.73 ± 0.07 mM. These data demonstrate that in comparison with conventional SD, ouabain-induced SD exhibits ion shifts that are qualitatively similar but quantitatively diminished. The presence of external Ca²⁺ can modulate the phenomenon but is irrelevant to the generation of the SD and its accompanying alkaline pH transient. Significance of these results is discussed in reference to the propagation of SD and the generation of interstitial pH changes.