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.
Karwoski
C J
CJ
Department of Psychology, University of Georgia, Athens 30602.
Coles
J A
JA
Lu
H K
HK
Huang
B
B
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D004553:Electric Conductivity; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects; D066298:In Vitro Techniques; D007700:Kinetics; D008433:Mathematics; D008839:Microelectrodes; D008954:Models, Biological; D011188:Potassium / Q000378:metabolism*; D011894:Rana pipiens; D012160:Retina / Q000502:physiology*; D013327:Strophanthidin / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D013779:Tetrodotoxin; D013327:Strophanthidin; D011188:Potassium