Dynamics of interstitial and intracellular pH in evolving brain infarct

Abstract

We examined the relationships between intracellular pH (pHi) and interstitial pH (pHe) in a rat model of focal ischemia. Interstitial pH was measured with pH-sensitive microelectrodes, and the average tissue pH was measured with the [14C]dimethadione method in rats subjected to occlusion of the right middle cerebral and common carotid arteries (MCA-CCAO). In normal cortex, pHe and pHi were 7.24 +/- 0.97 and 7.01 +/- 0.13 (means +/- SD, n = 6), respectively. In the ischemic cortex, pHe fell to 6.43 +/- 0.13, whereas pHi decreased only to 6.86 +/- 0.11 (n = 5) 1 h after MCA-CCAO. After 4 h of ischemia, the pHe was 6.61 +/- 0.09 and pHi was 6.62 +/- 0.20 (n = 4). Treatment with glucose before ischemia markedly lowered the pHe (5.88 +/- 0.17) but not pHi (6.83 +/- 0.03, n = 4) measured 1 h after ischemia. In the ischemic cortex of animals made hypoglycemic by pretreatment with insulin, neither pHe (7.25 +/- 0.06) nor pHi (6.99 +/- 0.13, n = 4) decreased. The demonstrated difference in pHi and pHe indicates that some cells remained sufficiently functional to maintain a plasma membrane gradient of protons within the evolving infarct. If the calculated pHi values accurately reflect the true pHi of cells within zones of severe focal ischemia, then cerebral infarction can proceed at pHi levels not greatly altered from normal.

FIG. 1

Coronal section Of frozen rat brain stained with hematoxylin-eosin. Right middle cerebral and common carotid arteries were occluded 24 h before death. Neocortical infarct is demarcated by pale staining area. Loci of interstitial pH (pH_e) measurements are indicated by arrows, 1–5 mm from midline in ischemic hemisphere and 4 mm from midline in nonischemic hemisphere.

FIG. 2

Direct current (DC) potential (bottom trace) and interstitial concentration of tetramethylammonium chloride (TMA) (top trace) in cortex after middle cerebral artery (MCA) occlusion (arrow). A: before MCA occlusion, brain was exposed to cerebrospinal fluid (CSF) in which 50 mM NaCl was replaced by 50 raM TMAC1. Concentration of TMA was measured at a depth of 400 μm by an ion-sensitive microelectrode with liquid exchanger (Corning 477317). Note rapid rise in TMA concentration at time of ischemic depolarization, indicating ~50% reduction in interstitial space. B: brain was exposed to CSF without TMA. No change in TMA concentration was detected at time of ischemic depolarization, indicating that rise in A in caused by an increase in TMA concentration ([TMA]) only.

FIG. 3

Interstitial pH (top trace) and DC potential (bottom trace) in a hypoglycemic animal (54 mg/dl plasma glucose). Recording was made from 7 to 20 min after MCA-CCA occlusion 3 mm from midline. Repeated spontaneous depolarizations indicative of spreading depression-like waves were observed in DC potential recording. Initial alkalization, characteristic of spreading depression, occurred but following acidification usually seen with spreading depression was absent.

FIG. 4

Dimethadkme (DMO) autoradiograms from 1 h normogly-cemia (A), 4 h normogiycemia (B), 1 h hypoglycemia (C), and 1 h hyperglycemia (D).

FIG. 5

Interstitial (pH_e) vs. intracellular (pH_i) pH in rats subjected to occlusion of right middle and common carotid arteries. Normal pH_e and pH_i, values from nonischemic left cortex are indicated by arrows. Loci values are given millimeters lateral to midline. Most severely ischemic tissue lies between loci 3 and 5.