K + and pH homeostasis in the developing rat spinal cord is impaired by early postnatal X-irradiation

Abstract

Activity-related transient changes in extracellular K + concentration ([K +] c) and pH (pH c) 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 +] c 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 = 30) pH units. By contrast, stimulation in non-irradiated 10- to 14-day-old pups produced smaller [K +] c 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 +] c ceiling level and the development of the acid shift in pH c 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 Mg 2+ and picrotoxin (10 −6 M). The acid shift was blocked, and the alkaline shift enhanced, by acetazolamide, Ba 2+, amiloride and SITS. Application of GABA evoked an alkaline shift in the pH c 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 pH c. Secondly, bicarbonate independent alkaline shifts may arise from synaptic activity resulting in a flux of acid equivalents.