Stimulus-evoked changes of extra- and intracellular pH in the leech central nervous system. I. Bicarbonate dependence.

1. We have measured the effect of repetitive electrical nerve root stimulation on the extracellular potassium activity (aKe) and the extracellular pH (pHe) and intracellular pH (pHi) in segmental ganglia of the leech Hirudo medicinalis with double-barreled K(+)- and pH-sensitive microelectrodes. To investigate the influence of CO2/HCO3-, we compared the stimulus-evoked changes in aKe, pHe, and pHi in the presence and absence of 5% CO2-24 mM HCO3- in the saline. 2. An electrical nerve root stimulation at 20-30 Hz for 1 min caused a rapid increase of 1.11 +/- 0.79 (SD) mM in aKe, followed by an aKe undershoot of 0.17 +/- 0.15 mM when the stimulation was discontinued (n = 6). aKe transients were not significantly affected by CO2/HCO3-. 3. In 5 mM N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES)-buffered, nominally CO2/HCO3(-)-free saline, low stimulus intensities or stimulus durations up to a few seconds resulted in a fast alkaline pHe transient. This alkalinization was followed by a larger and longer-lasting extracellular acidification when the stimulation was intensified and prolonged. A stimulation at 20 Hz, 5 V for 1 min caused an average alkaline shift of 0.083 +/- 0.055 pH units, followed by an acidosis of 0.079 +/- 0.038 pH units (n = 63). A change from 5 mM HEPES-buffered saline to 20 mM HEPES-buffered saline attenuated the stimulus-evoked pHe transients by 50-60%.(ABSTRACT TRUNCATED AT 250 WORDS)