Alkaline and acid transients in cerebellar microenvironment

Abstract

1. Extracellular pH (pHo) was measured in the cerebellar cortex of the rat using a recently developed liquid membrane ion-selective micropipette (ISM). pHo was determined during stimulus-evoked neuronal activity, elevated extracellular potassium concentration, [K+]o, spreading depression (SD), and complete ischemia. In many experiments [K+]o was simultaneously determined. 2. A train of local surface stimuli (LOC) produced an initial alkaline shift in pHo from a base line of 7.20-7.30 to 7.25-7.35. This was followed by a long-lasting acid phase that reached a plateau of 7.05-7.15 after 64 s of stimulation. pHo decrease was related to stimulus frequency, intensity, and duration. 3. Superfusion with Ringer solution containing manganese ions rapidly abolished parallel fiber-induced Purkinje cell synaptic depolarization together with the alkaline shifts while enhancing the acid shifts. 4. Superfusion of the cerebellar cortex with Ringer solution containing increasingly elevated [K+] progressively lowered pHo to a plateau of 6.95-7.05. The acidification occurred in the presence of ouabain but was reversed on return to the normal [K+]o or with the addition of the glycolytic blocker, fluoride. Stimulus-evoked alkaline shifts were enhanced by K+-Ringer superfusion. These experiments suggested that the acid shift was due to the metabolic production of an anion, possibly lactate. 5. Elevation of [K+]o above 8-12 mM often produced oscillation in pHo and [K+]o with a period of about 40 s. Sometimes these oscillations ended in a spontaneous SD or SD could be evoked by stimulation. Under these conditions of raised [K+]o, the SD consisted of a very pronounced alkaline transient followed by a small, long-lasting acid shift. When SD was induced by conditioning the cerebellum with proprionate or lowered NaCl, the alkaline phase was reduced and the acid enhanced. 6. Complete ischemia began with a progressive decrease of pHo and rise in [K+]o. When [K+]o reached 12 mM, a second more rapid rise in [K+]o to 40 mM or more occurred. This was correlated with 0.1-0.2 pHo transient increase similar to that seen during SD. pHo eventually reached a plateau of 6.60-6.80, close to neutrality. 7. Superfusion with Ringer solution containing acetazolamide immediately altered pHo homeostasis by increasing base-line pHo by about 0.10 and enhanced the induced pHo changes. These results suggest that carbonic anhydrase (CA) is important for acute buffering of the brain extracellular microenvironment. 8. The above results were interpreted in terms of changes in extracellular strong ion concentration differences ( [SID]o), extracellular concentration of total weak acid ( [Atot]o) and partial pressure of CO2 (Pco2) in the brain microenvironment. The results indicate that neuronal activity produces changes in many of the constituents of the microenvironment.

Extracellular pH Changes during Spreading Depression and Cerebral Ischemia: Mechanisms of Brain pH Regulation

“…Kraig et al (1983) have reported similar findings in rat cerebellar cortex, also using the R+ -LIX microelectrode technique.…”

“…The cortex was su perfused with mock CSF containing 7.5 mM Mn 2 + to assess the importance of voltage-dependent Ca 2 + channels on the pHe changes during spreading depression since it has previously been stated that the alkaline shift in spreading depression and isch emia was abolished by Mn 2 + (Kraig et al, 1983). Following application of Mn 2 + , we found no influ ence on the pHe change in spreading depression.…”

“…Kraig et al (1983) have analyzed their pH data using the formalism of Stewart (1978Stewart ( , 1981. In the case of brain interstitial fluid devoid of weak acids or bases, pH is determined only by Peo2 and the strong ion difference, which is the sum of all strong cations minus all strong anions, this value being equal to 'buffer base' or base excess (Siesjo, 1972).…”

Extracellular pH, Potassium, and Calcium Activities in Progressive Ischaemia of Rat Cortex

“…It is in teresting to note that the types of changes in Ke in terminal ischaemia of the rat (i.e., after intravenous administration of KCl or MgCI2) are more similar to those in partial ischaemia in the primate than to those in partial ischaemia in the rat (Hansen, 1977(Hansen, ,1978Kraig et a!., 1983).…”

“…If pHj has remained close to its normal level (for the purpose of this calculation taken to be a decrease from pH 7.05 to pH 7.00), then equili bration of hydrogen ions when Ke rises (and pHe is 6.87) will cause a 0.10 pH rise in comparison to the 0.14 pH rise measured. Kraig et al (1983) have sug gested that the alkaline transients are Ca 2 + -me diated because they are abolished by Mn 2 + , a cal cium channel blocker. Our observation that the Ke at which the alkalotic transient begins is very sim ilar to the Ke at which Cae falls supports this sug gestion.…”