Cr. Rose et Jw. Deitmer, STIMULUS-EVOKED CHANGES OF EXTRACELLULAR AND INTRACELLULAR PH IN THE LEECH CENTRAL-NERVOUS-SYSTEM .1. BICARBONATE DEPENDENCE, Journal of neurophysiology, 73(1), 1995, pp. 125-131
1. We have measured the effect of repetitive electrical nerve root sti
mulation on the extracellular potassium activity (aK(e)) and the extra
cellular pH (pH(e)) and intracellular pH (pH(i)) in segmental ganglia
of the leech Hirudo medicinalis with double-barreled K+- and pH-sensit
ive microelectrodes. To investigate the influence of CO2/HCO3-, we com
pared the stimulus-evoked changes in aK(e), pH(e), and pH(i) in the pr
esence and absence of 5% CO2-24 mM HCO3- in the saline. 2. An electric
al nerve root stimulation at 20-30 Hz for 1 min caused a rapid increas
e of 1.11 +/- 0.79 (SD) mM in aK(e), followed by an aK(e) undershoot o
f 0.17 +/- 0.15 mM when the stimulation was discontinued (n = 6). aK(e
) 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 pH(e) transi
ent. This alkalinization was followed by a larger and longer-lasting e
xtracellular acidification when the stimulation was intensified and pr
olonged. A stimulation at 20 Hz, 5 V for 1 min caused an average alkal
ine shift of 0.083 +/- 0.055 pH units, followed by an acidosis of 0.07
9 +/- 0.038 pH units (n = 63). A change from 5 mM HEPES-buffered salin
e to 20 mM HEPES-buffered saline attenuated the stimulus-evoked pH(e)
transients by 50-60%. 4. pH(e) transients were strongly dependent on t
he presence of CO2/HCO3-. In a saline containing 5% CO2-24 mM HCO3-, a
n extracellular alkaline shift could only be resolved in about half of
the experiments. The subsequent acidification amounted to 0.035 +/- 0
.012 pH units (n = 21), which is a reduction by 60% in comparison with
that in 5 mM HEPES-buffered, CO2/HCO3--free solution. 5. In sensory p
ressure neurons, the electrical stimulation induced an intracellular a
cidification of 0.071 +/- 0.025 pH units in 5 mM HEPES-buffered, CO2/H
CO3--free saline (n = 21). This intraneuronal acidification was attenu
ated by 45% in CO2/HCO3-, buffered saline (n = 12). 6. Neuropile glial
cells were depolarized by 8.4 +/- 2.3 mV, on average, during the stim
ulation (n = 49). In 5 mM HEPES-buffered, CO2/HCO3--free saline this d
epolarization was accompanied by an acidification of 0.025 +/- 0.009 p
H units, which was preceded by a small and brief alkalinization in 10
experiments (n = 25). In contrast, in the presence of CO2/HCO3- all gl
ial cells showed a distinct alkalinization of 0.025 +/- 0.009 pH units
(n = 24), which lasted during the entire stimulation. This alkaliniza
tion was followed by a weak acidification in nine experiments. 7. Both
the reduction of the stimulus-evoked extracellular alkalinization and
the appearance of a distinct intraglial stimulus evoked alkalinizatio
n in CO2/HCO3--buffered saline compared with CO2/HCO3--free saline wer
e a result of the increase in physicochemical buffering power due to C
O2/HCO3-, as well as of active transport of acid/base equivalents acro
ss the,glial membrane, which may contribute to the H+-muffling of the
stimulus-induced pH(e) changes.