Intracellular Ph of Rat Atrial Muscle Fibers Measured by Glass Micropipette Electrodes.

• Caldwell* was the first to measure directly the pH of large cells (crab muscle fibres) with a glass electrode. Kostyuk and Sorokina reduced considerably the dimensions of the glass electrode and measured directly the intracellular pH of sartorius muscle (frog). Finally, Carter first measured the intracellular pH of a mammalian cell (rat voluntary muscle) by means of a double-barreled microelectrode. All these electrodes were based, however, on the same principle, i.e., interposing a pH sensitive glass membrane between the medium tested and a reference solution. So far, intracellular micro-electrodes have been made by reducing the volume of the sensitive head of conventional electrodes to dimensions which enabled them to be introduced into a cell. However, reducing this volume reduces also the surface of contact — the area of low resistance glass between the two solutions — thus increasing the electrical resistance of the electrode to very high values. It has been possible to compensate for the effect of the reduced size of the tip by decreasing, at the same time, the thickness of the glass wall. But the wall of the smallest pH sensitive micro-electrodes is now so thin (1000 A) that further reduction would impair considerably their mechanical strength. However, the uninsulated tip is still too large to be useful for intracellular