Stimulation-evoked changes in extracellular pH, calcium and potassium activity in the frog spinal cord

Double-barrel ion-sensitive microelectrodes were used to measure activity-related changes in extracellular pH (pHe), potassium and calcium concentration ([K+]e and [Ca2+]e) in the spinal dorsal horns of frogs. Repetitive stimulation (30-100 Hz) of the dorsal root evoked transient acidification in the lower dorsal horn by 0.25 pH units, which was accompanied by an increase in [K+]e by 4-5 mmol/l and a decrease in [Ca2+]e by 0.5 mmol/l. The pHe changes were found to have a typical depth profile and increased with the stimulation frequency, intensity and duration. The maximum of pHe changes was reached in 25-30 s of stimulation, and when stimulation continued further no greater pHe changes were achieved. Similarly as the K+ and Ca2+ transients, the pHe reached a ceiling level, which was 0.2-0.25 pH units more acid than the pH of the Ringer solution. The poststimulation K+ undershoot below the resting K+ level (3 mmol/l) was accompanied by an alkaline shift before the original pH base line. The rise time of the pHe changes was slower than that of [K+]e and [Ca2+]e changes. However, the redistribution of all the ionic changes had a similar time course. The clearance of changes in [K+]e and pHe was slowed by ouabain. The depression of the acid shift required higher concentrations of ouabain than the depression of the alkaline shifts. Acetazolamide, a carbonic anhydrase inhibitor, depressed the acid and enhanced the alkaline shift. Superfusion of the cord with elevated [K+]e was accompanied by a prompt and progressive acid shift, the lowering of [K+]e by an alkaline shift. The stimulus-evoked K+ increase and acid shift were depressed during the elevated [K+]e, while the alkaline shift was enhanced. Spontaneous elevations of [K+]e were accompanied by acid shifts of a similar time course. The results are discussed in terms of stimulus-evoked changes in extracellular strong ion differences [SID]e, and of their possible physiological significance.