Electrogenic Na-Ca exchange clears Ca2+ loads from retinal amacrine cells in culture.
Calcium influx into cultured retinal amacrine cells is followed by a small, slow, inward current that we show here results from the operation of electrogenic Na-Ca exchange. The activity of the exchanger is shown to correlate with the magnitude of the Ca2+ load and to depend on both the Ca2+ and Na+ gradients. Li+ is unable to substitute for Na+ and in the absence of Na+, slow tail currents are almost entirely suppressed. A rapid change in [K+]o does not affect the activity of the exchanger, suggesting that only Na+ and Ca2+ are transported. The ratio of charge entering as Ca2+ current to the charge entering as exchange current is highly variable between cells. We suggest that variability results from a variable fraction of Ca2+ load, we estimate typically 40%, being removed by a process other than Na-Ca exchange. This process is likely to involve internal buffering or sequestration since inhibition of the plasmalemmal Ca(2+)-ATPase does not increase the fraction of Ca2+ expelled by the exchanger. Ca2+ loading performed in the absence of Na+o generates smaller exchange charge the longer the delay in returning Na+o to the neuron. About 30% of exchange charge is lost for a delay of 1 sec.
Gleason
E
E
Division of Biological Sciences, University of California, Davis 95616, USA.
Borges
S
S
Wilson
M
M
D000818:Animals; D001640:Bicuculline / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002352:Carrier Proteins / Q000187:drug effects / Q000378:metabolism*; D002478:Cells, Cultured; D002642:Chick Embryo; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D012160:Retina / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D012165:Retinal Ganglion Cells / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger; D013779:Tetrodotoxin / Q000494:pharmacology; D013997:Time Factors
D002352:Carrier Proteins; D019831:Sodium-Calcium Exchanger; D013779:Tetrodotoxin; D004533:Egtazic Acid; D012964:Sodium; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D011188:Potassium; D002118:Calcium; D001640:Bicuculline