Effect of divalent cations on AMPA-evoked extracellular alkaline shifts in rat hippocampal slices.
The generation of activity-evoked extracellular alkaline shifts has been linked to the presence of external Ca(2+) or Ba(2+). We further investigated this dependence using pH- and Ca(2+)-selective microelectrodes in the CA1 area of juvenile, rat hippocampal slices. In HEPES-buffered media, alkaline transients evoked by pressure ejection of RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) averaged approximately 0.07 unit pH and were calculated to arise from the equivalent net addition of approximately 1 mM strong base to the interstitial space. These alkaline responses were correlated with a mean decrease in [Ca(2+)](o) of approximately 300 microM. The alkalinizations were abolished reversibly in zero-Ca(2+) media, becoming indiscernible at a [Ca(2+)](o) of 117+/-29 microM. Addition of as little as 30-50 microM Ba(2+) caused the reappearance of an alkaline response. In approximately one-fourth of slices, a persistent alkaline shift of approximately 0.03 unit pH was observed in zero-Ca(2+) saline containing EGTA. In HEPES media, addition of 300 microM Cd(2+), 100 microM Ni(2+), or 100 microM nimodipine inhibited the alkaline shifts by roughly one-half, one-third, and one-third, respectively, whereas Cd(+) and Ni(2+) in combination fully blocked the response. In bicarbonate media, by contrast, Cd(+) and Ni(2+) blocked only two-thirds of the response. In the presence of bicarbonate, Ni(2+) caused an unexpected enhancement of the alkalinization by approximately 150%. However, when the extracellular carbonic anhydrase was blocked by benzolamide, addition of Ni(2+) reduced the alkaline shift. These results suggested that Ni(2+) partially inhibited the interstitial carbonic anhydrase and thereby increased the alkaline responses. These data indicate that an activity-dependent alkaline shift is largely dependent on the entry of Ca(2+) or Ba(2+) via voltage-gated calcium channels. However, sizable alkaline transients still can be generated with little or no external presence of these ions. Implications for the mechanism of the activity-dependent alkaline shift are discussed.
Smith
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Department of Physiology and Neuroscience and Department of Neurosurgery, New York University School of Medicine, New York, New York 10016, USA.
Chesler
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D000818:Animals; D001464:Barium / Q000494:pharmacology; D001639:Bicarbonates / Q000494:pharmacology; D002104:Cadmium / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D002413:Cations, Divalent / Q000494:pharmacology*; D004533:Egtazic Acid / Q000494:pharmacology; D004594:Electrophysiology; D018698:Glutamic Acid / Q000494:pharmacology; D006624:Hippocampus / Q000187:drug effects / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D009532:Nickel / Q000494:pharmacology; D051381:Rats; D020318:Rats, Long-Evans; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid / Q000494:pharmacology*
D001639:Bicarbonates; D002413:Cations, Divalent; D002104:Cadmium; D001464:Barium; D018698:Glutamic Acid; D004533:Egtazic Acid; D018350:alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid; D009532:Nickel; D002118:Calcium