Intracellular pH transients of mammalian astrocytes

Intracellular pH (pHi) is an important physiologic variable that both reflects and influences cell function. Glial cells are known to alter their functional state in response to a variety of stimuli and accordingly may be expected to display corresponding shifts in pHi. We used fine-tipped, double-barreled, pH-sensitive microelectrodes to continuously monitor pHi in glial cells in vivo from rat frontal cortex. Cells were identified as glia by a high membrane potential and lack of injury discharge or synaptic potentials. Continuous, stable recordings of pHi from astrocytes were obtained for up to 80 min but typically lasted for approximately 10 min. Resting pHi was 7.04 +/- 0.02 with a membrane potential of 73 +/- 0.9 mV (mean +/- SEM; n = 51). With cortical stimulation, glia depolarized and became more alkaline by 0.05-0.40 pH (n = 50). During spreading depression (SD), glia shifted more alkaline by 0.11-0.78 pH (n = 26). After stimulation or SD, glia repolarized and pHi became more acidic than at resting levels. Superfusion of the cortical surface with 0.5-2 mM Ba2+ caused glia to hyperpolarize during stimulation and completely abolished the intracellular alkaline response. The predominant pH response of the interstitial space during stimulation or SD was a slow acidification. With superfusion of Ba2+ an early stimulus-evoked interstitial alkaline shift was revealed. The mechanism of the intracellular alkaline shift is likely to involve active extrusion of acid. However, internal consumption of protons cannot be excluded. The sensitivity of the response to Ba2+ suggests that it is triggered by membrane depolarization. These results suggest that glial pHi is normally modulated by the level of local neuronal activity.