The pH of brain extracellular fluid in the cat

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Abstract
The blood supply to the medulla was determined by the injection of indian ink via the vertebral arteries. Virtually the whole medulla was supplied by penetrating vessels from the ventral surface. The highest density of small arterioles and venules was found close to the roots of XII and on the ventrolateral surface. The pH of extracellular fluid (pHecf) was measured with pH microelectrodes of tip size 1-3 .mu.m in cortex and medulla in 17 cats anesthetized with pentobarbitone or a chloralose-urethane mixture. Parallel measurements were made of the pH of CSF and plasma, the DC potential between plasma and brain and ventilation or phrenic nerve discharge. In the majority of tests under steady conditions, the pH of ECF was lower than that of CSF by between 0.03-0.08 units. No systematic pH gradients could be found to a depth of 5 mm beneath the surface of either medulla or cortex. When plasma PCO2 [partial CO2 pressure] was altered, pH ecf changed with a latent period and speed of response related to the density of blood vessels. In vascular areas of the medulla and in the cortex, the latent period of 4 s and the change of pHecf coincided with changes in ventilation. Changes in pHcsf over the same areas were invariably slower. CO2 buffering capacities were in the order plasma > ECF > CSF. Typical values were, respectively, -2.2, -2.1 and -1.6. The pH of ECF was unaffected by the i.v. injection of H+ and only slowly by the injection of HCO3-. Only up to a depth of 1 mm beneath the surface was pHecf affected by superfusion of mock CSF in the range 6.8-8.0 units. This response had a latent period of 2-3 min and was complete in 15 min. The pH of ECF fell with hypoxia after a latent period of > 1 min and if all vasosensory nerves were cut, pHecf was markedly affected by changes of blood pressure. Even under steady conditions, the pH of ECF and CSF are not identical. pHecf is more obviously affected by changes in PaCO2 [partial arterial CO2 pressure] than pHcsf. Putative H+ sensors which drive respiratory neurons are likely to be similarly affected.