Na+-Ca2+ exchange in bovine rod outer segments requires and transports K+.
Intact outer segments isolated from bovine retinas (bovine ROS) display a high activity of Na+-Ca2+ exchange, and Na+-Ca2+ exchange appears to be the only functional ion transporter present. Here we demonstrate for the first time that Na+-Ca2+ exchange requires and transports K+ from the following observations. 1) Na+-Ca2+ exchange in bovine ROS required the simultaneous presence of K+ and Ca2+ on one side of the membrane and the presence of Na+ on the other side. 2) Na+-stimulated Ca2+ release from bovine ROS was accompanied by an equally large release of K+. We used the electrogenic protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) as an added electrical shunt; in the intact rod cell, electrogenic Na+-Ca2+ exchange is shunted by K+ channels present in the rod inner segment. In the presence of FCCP, an inward Na+-Ca2+ exchange current was accompanied by an outward current of protons with a stoichiometry of 1 H+/Ca2+; in the absence of FCCP, no Na+-induced proton current was observed. Addition of FCCP did not uncouple Na+-induced K+ release from Na+-induced Ca2+ release. We conclude that Na+-Ca2+ exchange in bovine rod photoreceptors operates at an electrogenic stoichiometry of 4 Na+:(1 Ca2+ + 1 K+). In isolated ROS and in the absence of an external electrical shunt, Na+-Ca2+ exchange operated at an electroneutral stoichiometry of 3 Na+:(1 Ca2+ + 1 K+).
Schnetkamp
P P
PP
Department of Medical Biochemistry, University of Calgary, Alberta, Canada.
Basu
D K
DK
Szerencsei
R T
RT
D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone / Q000494:pharmacology; D002352:Carrier Proteins / Q000378:metabolism*; D002417:Cattle; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D007202:Indicators and Reagents; D007700:Kinetics; D010786:Photoreceptor Cells / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism / Q000494:pharmacology*; D015221:Potassium Channels / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism* / Q000494:pharmacology; D019831:Sodium-Calcium Exchanger
D002352:Carrier Proteins; D007202:Indicators and Reagents; D015221:Potassium Channels; D019831:Sodium-Calcium Exchanger; D002259:Carbonyl Cyanide p-Trifluoromethoxyphenylhydrazone; D004533:Egtazic Acid; D012964:Sodium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D011188:Potassium; D002118:Calcium