Unidirectional Na+, Ca2+, and K+ fluxes through the bovine rod outer segment Na-Ca-K exchanger.
The properties of the Na-Ca exchanger in the plasma membrane of rod outer segments isolated from bovine retinas (ROS) were studied. Unidirectional Ca2+, Na+, and K+ fluxes were measured with radioisotopes and atomic absorption spectroscopy. We measured K+ fluxes associated with the Ca-Ca self-exchange mode of the Na-Ca exchanger to corroborate our previous conclusion that the ROS Na-Ca exchanger differs from Na-Ca exchangers in other tissues by its ability to transport K+ (Schnetkamp, P. P. M., Basu, D. K. & Szerencsei, R. T. (1989) Am. J. Physiol. 257, C153-C157). The Na-Ca-K exchanger was the only functional cation transporter in the plasma membrane of bovine ROS with an upper limit of a flux of 10(5) cations/ROS/s or a current of 0.01 pA contributed by other cation channels, pumps, or carriers; cation fluxes via the Na-Ca-K exchanger amounted to 5 x 10(6) cations/ROS/s or a current of 1 pA. Ca2+ efflux via the forward mode of the Na-Ca-K exchanger did not operate with a fixed single stoichiometry. 1) The Na/Ca coupling ratio was increased from three to four when ionophores were added that could provide electrical compensation for the inward Na-Ca exchange current. 2) The K/Ca coupling ratio could vary by at least 2-fold as a function of the external Na+ and K+ concentration. The results are interpreted in terms of a model that can account for the variable Ca/K coupling ratio: we conclude that the Ca2+ site of the exchanger can translocate independent of translocation of the K+ site, whereas translocation of the K+ site requires occupation of the Ca2+ site, but not its translocation. The results are discussed with respect to the physiological role of Na-Ca-K exchange in rod photoreceptors.
Schnetkamp
P P
PP
Department of Medical Biochemistry, University of Calgary, Alberta, Canada.
Szerencsei
R T
RT
Basu
D K
DK
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D007700:Kinetics; D008954:Models, Biological; D011188:Potassium / Q000378:metabolism*; D012374:Rod Cell Outer Segment / Q000378:metabolism*; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology
D002352:Carrier Proteins; D012964:Sodium; D012413:Rubidium; D011188:Potassium; D002118:Calcium