Current-evoked transcellular K+ flux in frog retina

1. Changes in extracellular K+ concentration (delta[K+]o) evoked by electrical current were measured with K+-selective microelectrodes (K-ISMs) in the retina of the frog eyecup. 2. In the superfusate at 20 microns above the inner limiting membrane (ILM), current-evoked delta[K+] was a function of current polarity and strength; its amplitude decreased as the K-ISM was moved higher above the ILM. Responses were similar whether measured with K-ISMs containing the Corning exchanger or a valinomycin-based liquid membrane. No current-evoked delta[Ca2+] could be detected with Ca-selective microelectrodes (Ca-ISMs). 3. Within the retina, a complex spatiotemporal profile of current-evoked delta[K+]o was observed. Strophanthidin abolished responses in the proximal retina, but had little effect on the response in the superfusate. A blocker of K+ channels (Ba2+) depressed responses in the superfusate, but not in the proximal retina. 4. Quantitative analysis of these responses indicates a transport number for K+ of 0.18 at onset of current, and that decreases over a few seconds. In contrast, a transport number of approximately 0.01 is predicted from the expected ionic concentrations within extracellular space. 5. These findings are compatible with the delta[K+] above the ILM being due to transcellular movement of K+ through Müller cells. The results suggest that K+ spatial buffering may be particularly potent in the retina. Furthermore, determinations of tissue characteristics by passage of electrical current must take into account that at least 17% of the current does not travel through extracellular space.