Bicarbonate and ammonia changes in brain during spreading depression.

An alkaline, followed by an acid-going transient, characterizes acid-base changes in the interstitial space during spreading depression in a variety of brain structures. In rat, such changes are associated with a significant rise in brain lactate content. How brain proton buffers behave during spreading depression is unknown. Techniques to significantly improve the response time of gas permeable membrane semimicroelectrodes for carbon dioxide and ammonia are reported. Measurements with such electrodes, when coupled to measurements of hydrogen ion concentration (from microelectrodes), permit rapid changes to be determined in bicarbonate concentration or ammonia and ammonium ion concentration, respectively. Bicarbonate concentration fell from 30 +/- 1 (n = 16) to 14 +/- 1 mM (n = 16) during spreading depression. On the other hand, ammonia concentration rose from 2.3 +/- 0.1 to 4.4 +/- 0.3 microM (n = 17) while ammonium ion concentration rose from 116 +/- 11 (n = 17) to 382 +/- 30 microM (n = 17) during spreading depression. Bicarbonate changes probably reflect titration of brain bicarbonate stores by accumulated lactic acid. Similar physicochemical changes do not explain the rise in ammonia and ammonium ion concentrations. Instead, elevation of the latter can only result from an increase in ammonia content of the interstitial space.