Transport of K+ by Na(+)-Ca2+, K+ exchanger in isolated rods of lizard retina.
Transport of K+ by the photoreceptor Na(+)-Ca2+, K+ exchanger was investigated in isolated rod outer segments (OS) by recording membrane current under whole-cell voltage-clamp conditions. Known amounts of K+ were imported in the OS through the Ca(2+)-activated K+ channels while perfusing with high extracellular concentration of K+, [K+]o. These channels were detected in the recordings from the OS, which probably retained a small portion of the rest of the cell. The activation of forward exchange (Na+ imported per Ca2+ and K+ extruded) by intracellular K+, Ki+, was described by first-order kinetics with a Michaelis constant, Kapp(Ki+), of about 2 mM and a maximal current, Imax, of about -60 pA. [Na+]i larger than 100 mM had little effect on Kapp(Ki+) and Imax, indicating that Nai+ did not compete with Ki+ for exchange sites under physiological conditions, and that Na+ release at the exchanger intracellular side was not a rate-limiting step for the exchange process. Exchanger stoichiometry resulted in one K+ ion extruded per one positive charge imported. Exchange current was detected only if Ca2+ and K+ were present on the same membrane side, and Na+ was simultaneously present on the opposite side. Nonelectrogenic modes of ion exchange were tested taking advantage of the hindered diffusion found for Cai2+ and Ki+. Experiments were carried out so that the occurrence of a putative nonelectrogenic ion exchange, supposedly induced by the preapplication of certain extracellular ion(s), would have resulted in the transient presence of both Cai2+ and Ki+. The lack of electrogenic forward exchange in a subsequent switch to high Nao+, excluded the presence of previous nonelectrogenic transport.
Rispoli
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INFM, Dipartimento di Biologia dell'Università, Ferrara, Italy.
Navangione
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Vellani
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D000818:Animals; D002118:Calcium / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D004058:Diffusion; D006706:Homeostasis; D066298:In Vitro Techniques; D007700:Kinetics; D008116:Lizards; D008433:Mathematics; D008962:Models, Theoretical; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D015221:Potassium Channels / Q000187:drug effects / Q000502:physiology*; D012374:Rod Cell Outer Segment / Q000502:physiology*; D019831:Sodium-Calcium Exchanger; D013816:Thermodynamics
D002352:Carrier Proteins; D015221:Potassium Channels; D019831:Sodium-Calcium Exchanger; C074530:potassium-dependent sodium-calcium exchanger; D011188:Potassium; D002118:Calcium