Ca2+ influx into bovine retinal rod outer segments mediated by na
-ca+k exchange.
Schnetkamp, Paul P. M., Joseph E. Tucker, and Robert T. Szerencsei.
Department of Medical Biochemistry, University of Calgary, Health
Science Centre, 3330 Hospital Drive N.W., Calgary, Alberta, T2N 4N1,
Canada. TEL: 403-220 5448; FAX: 403-283-4740
APStracts 2:0202C, 1995.
Ca2+-depleted outer segments were purified from bovine retinal rod
photoreceptors (ROS) and factors influencing Ca2+ influx into ROS via
the plasma membrane Na-Ca+K exchanger were analyzed. Intracellular
alkali cation concentrations were manipulated by (1) previous loading
via the ionophore monensin followed by removal of monensin, and (2)
addition of the channel ionophore gramicidin during Ca2+ influx
measurements. Ca2+ influx was measured as a rise in cytosolic free
Ca2+ with the Ca2+-indicating dye fluo-3. An absolute requirement for
intracellular Na+ was observed with a Na+ dissociation constant of 30
- 40 mM, while intracellular K+ was a potent inhibitor of Ca2+
influx, apparently by competing with Na+ for a common site on the Na
-Ca+K exchanger. Half-maximal Ca2+ influx was observed at an external
free Ca2+ concentrations of 0.9 [mu]M when no external Na+ was
present and 3.5 [mu]M when 10 mM external Na+ was present. Our
observations are discussed in the context of (1) a three-site model
for the Na-Ca+K exchanger and in the context of (2) earlier
experiments on light adaptation in rods which depended on minimizing
Ca2+ fluxes across the ROS plasma membrane.
Received 9 March 1995; accepted in final form 11 May 1995.
APS Manuscript Number C135-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 26 May 1995.