Leslie F. Nims

Tracers, Transfer through Membranes, and Coefficients of Transfer

The rate of flow of a tagged species of a material substance through a permeable membrane is proportional to the rate of flow of the substance itself when, and only when, the species mole fraction of the substance is the same on both sides of the barrier. The ratio of the osmotic transfer coefficient of a substance in a particular barrier to the exchange coefficient, determinable with a tracer, is greater than 1.

Changes of Hydrogen Ion Concentration of the Cerebral Cortex

Using a glass electrode (of the Maclnnes type) with an active area of less than 0.5 mm. 2 , in conjunction with the microvoltmeter recently described by Burr, Lane and Nims (1936), having a grid-leak of 100 megohms, it is possible to measure the hydrogen ion concentration in physico-chemical systems to ±.002 pH. The same apparatus is applicable to biological systems in vivo. In the present instance it was used for a study of the pH of the cerebral cortex. One glass electrode and 2 Ag-AgCl saline-wick electrodes were placed as close together as possible (circa 2 mm.) on a selected area of the cerebral cortex of the animal. The potential difference between the wick electrodes was measured with a microvoltmeter and a Leeds and Northrup galvanometer (No. 2420). The E.m.f. between the glass electrode and either wick electrode was measured with the modified microvoltmeter and a similar galvanometer, in conjunction with a portable Leeds and Northrup potentiometer. The apparatus was so adjusted that the variations in these voltage differences could be recorded photographically with a moving-paper camera through its F/1.25 anastigmatic lens (focal distance 5 cm.). The 2 wick electrodes were placed on the cortex to determine whether or not potential gradients were so large or unstable as to invalidate a pH-measurement. The difference of potential between the glass electrode and either wick electrode can be correlated with a pH by standardization in buffers of known pH. In this preliminary note we wish to confine ourselves to a few of the results thus far obtained. 1. The D.C. potential gradients are small enough to be neglected in estimating the pH to ±.05, and stable enough throughout an experiment to permit differential measurements of pH to ±.005, a precision more than sufficient for the measurements in question. 2. In the curarized animal (monkey, cat) under constant artificial respiration the indicated pH on the cortex is constant. Increase of ventilation produces a shift towards the alkaline side (see figure), decrease of ventilation one towards the acid side. In fact, it has been possible to maintain the pH on the cortex at any specified level compatible with life by proper adjustment of the ventilation. 3. Intravenous injection of sodium bicarbonate produces a shift towards the alkaline side, of hydrochloric acid towards the acid side, of Ringer-solution no comparable effect. 4. Thermocoagulation (at 80°C. for 5 seconds) of a small area of the cortex renders this area acid (e. g., pH = 6.6) with resgect to the adjacent normal cortex (e. g., pH = 7.3). This acidity slowly increases during both the initial local vasoconstriction and the subsequent local vasodilatation and oedema of the thermocoagulated area. From these findings we feel justified in concluding that with this method one measures pH, that, though the condition of the blood circulating through the cortex affects the indicated pH, this pH is that of the transudate on the surface of the cortex immediately subjacent to the glass electrode, and finally that the pH of this transudate is largely determined by the condition of that portion of the cortex, rather than merely reflecting its vascularity. 5. Changes in pH of the cortex produce changes in its “spontaneous” electrical activity, a low pH being associated with low electrical activity, a high pH with high activity.