Calcium homeostasis in vertebrate retinal rod outer segments.
The outer segments of vertebrate retinal rod photoreceptors (ROS) exhibit dynamic Ca2+ fluxes. In darkness, Ca2+ continuously enters via the light-sensitive, cGMP-gated channels and this requires the presence of a powerful Ca2+ extrusion mechanism in the ROS plasma membrane. Our laboratory has characterized a Na/Ca+K exchanger in the ROS plasma membrane, which utilizes both inward Na+ gradient and outward K+ gradient to extrude Ca2+. Here, I review our work on the functional properties of the Na/Ca+K exchanger including the stoichiometry, ion binding sites and regulation of Ca2+ transport via Na/Ca+K exchange. Inactivation of the Ca2+ extrusion mode of the Na/Ca+K exchanger will be discussed as a mechanism to prevent lowering of cytosolic free Ca2+ to undesirably low values of < 1 nM that are expected from the coupling stoichiometry of the Na/Ca+K exchanger and that are expected to occur when Ca2+ influx via the cGMP-gated channels is interrupted during saturation of rod photoreceptors in bright light. This review also reexamines the contribution of internal Ca2+ stores (i.e. disks) to Ca2+ homeostasis in ROS.
Schnetkamp
P P
PP
Department of Medical Biochemistry, Faculty of Medicine, University of Calgary, Alberta, Canada.
D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D012160:Retina / Q000378:metabolism*; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D014714:Vertebrates; D014795:Visual Pathways
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; C074530:potassium-dependent sodium-calcium exchanger; D002118:Calcium