The slow P III response of the isolated frog retina.

The slow PIII response of the electroretinogram (ERG) was studied in the isolated, aspartate-treated bullfrog retina. The slow PIII response reflected mainly rod activity in its time course and spectral response curve, and the ratio of the peak amplitude of the fast PIII response to the slow one remained almost constant within a scotopic range when rhodopsin was kept at a constant concentration. Reducing Na+ or K+ concentration in the extracellular fluid or decreasing the rhodopsin content in the retina, however, caused different influences on the fast and slow PIII responses, and, in particular, the reduction in amplitude of the slow PIII response was more marked than that of the fast PIII response. Under these circumstances, the log of the amplitude of the slow PIII response was in linear proportion to the amplitude of the fast PIII response. In addition, the amplitude reduction of the fast PIII response was not accompanied by a decrease in peak latency but that of the slow PIII response was. An explanation on the production of the slow PIII response was attempted with reference to other results related to the slow PIII response, ionic mechanism of the electrical activity of photoreceptor cells and their Na-K pump activity.