Ca2+ fluxes and channel regulation in rods of the albino rat.
By use of microelectrodes, changes in the receptor current and the Ca2+ concentration were measured in the rod layer of the rat retina after stimulation by flashes or steady light. Thereby light induced Ca2+ sources, and sinks along a rod were determined in dependence of time. Thus, the Ca2+ fluxes across the plasma membrane of a mammalian rod could be studied in detail. By light stimulation, Ca2+ sources are evoked along the outer segment only. Immediately after a saturating flash, a maximum of Ca2+ efflux is observed which decays exponentially with tau = 0.3 s at 37 degrees C (4.2 s at 23 degrees C). During regeneration of the dark current, the outer segment acts as a Ca2+ sink, indicating a restoration of the Ca(2+)-depleted outer segment. These findings agree with earlier reports on amphibian rods. Further experiments showed that the peak Ca2+ efflux and tau are temperature dependent. The peak amplitude also depends on the external Ca2+ concentration. In contrast to the reports on amphibian rods, only a part of the Ca2+ ions extruded from the outer segment is directly restored. Surprisingly, during steady light the Ca2+ efflux approaches a permanent residual value. Therefore, in course of a photoresponse, Ca2+ must be liberated irreversibly from internal Ca2+ stores. There is certain evidence that the inner segment acts as a Ca2+ store. Our results show that the Ca2+ fraction of the ions carrying the dark current is proportional to the extracellular Ca2+ concentration. This indicates that the Ca2+ permeability of the plasma membrane of the rod outer segment is independent of the Ca2+ concentration.
Knopp
A
A
Max Volmer Institut für Biophysikalische and Physikalische Chemie, Technische Universität Berlin, Germany.
Rüppel
H
H
D000417:Albinism / Q000378:metabolism / Q000503:physiopathology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism / Q000502:physiology*; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D004594:Electrophysiology; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D008839:Microelectrodes; D008954:Models, Biological; D010775:Photic Stimulation; D010786:Photoreceptor Cells / Q000502:physiology; D051381:Rats; D017208:Rats, Wistar; D017948:Retinal Rod Photoreceptor Cells / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D012374:Rod Cell Outer Segment / Q000378:metabolism
D015220:Calcium Channels; D002118:Calcium