The glutamate-isolated mass receptor potential of the frog's retina. II. Sodium and potassium dependence.

The mass receptor potential of superfused frog retinas was isolated by means of sodium glutamate. The changes of the potential amplitudes due to the variation of sodium and potassium concentration were measured. Lithium and choline were tested as osmotic substitutes. In both lithium and choline containing solutions the mass receptor potential amplitude varied in direct linear proportion to the logarithm of the sodium concentration, and in inverse linear proportion to the logarithm of the potassium concentration. To obtain stable potentials, however, it was necessary to enhance the glucose content to 50 mM in lithium solutions. Choline is shown to be more suitable for an osmotic substitute than lithium. The effects of sodium and potassium on the mass receptor potential were strongly dependent on intensity and wavelength of the stimulus light. Both the responses to red light stimuli or strong white lights showed only a slight sodium and potassium dependence; the same was evident for the so-called rapid decay of the potentials. The responses to green light stimuli or dim white ones and the so-called remaining part, however, were strongly reduced in high potassium or low sodium concentrations. These results suggest that the retinal rod activity is more dependent on the ionic composition of superfusing solutions than that of the cones under the given experimental conditions. This might be caused by the faster reaction of the cone membrane pumps.