The use of microelectrodes for measurement of local H+ activity in the cortical subarachnoidal space of cats.

pH microelectrodes with pointed tip (Hinke-type) were constructed for the continuous measurement of the local pH in the perivascular space of pial arteries in the feline cerebral cortex. The sensitive tip had a length of 20-60 mu and a base diameter of 10-25 mu. As reference electrode, a micropipette (tip diameter 2 mu), filled with 150 mM KCl was used. Calibration curves were linear and showed a sensitivity of 54.5-57.5 mV/pH unit at 38 degrees C. Advantages of such electrodes are the easy penetration of the subarachnoid membrane, the long life span, the quick response, and a minimal drift. The electrodes were tested in vivo during hyper- and hypoventilation and during local perivascular injection of mock spinal fluid at varying pH. A close correlation was observed between the change in perivascular pH and the corresponding change in pial arterial diameter.