K+ and pH homeostasis in the developing rat spinal cord is impaired by
early postnatal X-irradiation.
Sykova E, Jendelova P, Simonova Z, Chvatal A
Laboratory of Cellular Neurophysiology, Czechoslovak Academy of Sciences,
Bulovka, Prague.
Activity-related transient changes in extracellular K+ concentration
([K+]e) and pH (pHe) were studied by means of ion-selective microelectrodes
in neonatal rat spinal cords isolated from pups 2-14 days of age. Pups
1 to 2 days old were X-irradiated to impair gliogenesis and spinal cords
were isolated 2-13 days postirradiation (PI). In 2- to 14-day-old pups
PI stimulation produced ionic changes that were the same as those in 3-
to 6-day-old control (non-irradiated) pups; e.g. the [K+]e increased by
4.03 +/- 0.24 mM (mean +/- S.E.M., n = 30) at a stimulation frequency of
10 Hz and this was accompanied by an alkaline shift of 0.048 +/- 0.004
pH units (mean +/- S.E.M., n = 32) pH units. By contrast, stimulation in
non-irradiated 10- to 14-day-old pups produced smaller [K+]e changes, of
1.95 +/- 0.12 mM (mean +/- S.E.M., n = 30), and an acid shift of 0.035
+/- 0.003 pH units which was usually preceded by a scarcely discernible
initial alkaline shift, as is also the case in adult rats. Our results
show that the decrease in [K+]e ceiling level and the development of the
acid shift in pHe are blocked by X-irradiation. Concomitantly, typical
continuous development of GFAP-positive reaction was disrupted and densely
stained astrocytes in gray matter of 10- to 14-day-old pups PI revealed
astrogliosis. In control 3- to 6-day-old pups and in pups PI the stimulation-evoked
alkaline, but not the acid, shift was blocked by Mg2+ and picrotoxin (10(-6)
M). The acid shift was blocked, and the alkaline shift enhanced, by acetazolamide,
Ba2+, amiloride and SITS. Application of GABA evoked an alkaline shift
in the pHe baseline which was blocked by picrotoxin and in HEPES-buffered
solution. By contrast, the stimulus-evoked alkaline shifts were enhanced
in HEPES-buffered solutions. The results suggest a dual mechanism of the
stimulus-evoked alkaline shifts. Firstly, the activation of GABA-gated
anion (Cl-) channels induces a passive net efflux of bicarbonate, which
may lead to a fall in neuronal intracellular pH and to a rise in the pHe.
Secondly, bicarbonate independent alkaline shifts may arise from synaptic
activity resulting in a flux of acid equivalents.