Net charge transport during sodium-dependent calcium extrusion in isolated salamander rod outer segments.
The light-sensitive current and the current associated with the extrusion of internal Ca2+ in exchange for external Na+ have been recorded from detached rod outer segments from the salamander retina by the use of the whole-cell voltage clamp technique. No significant current-carrying mechanisms are present in the outer segment membrane apart from the light-sensitive conductance and the Na:Ca,K exchange, and exchange currents can therefore be recorded directly without the use of subtraction procedures or pharmacological blockers. The charge moved by the exchange was studied by loading outer segments with a known amount of calcium and then recording the exchange current on return to a Na(+)-containing solution. Calcium is not sequestered to any significant extent in a slowly exchanging internal store, as the charge recovered is unaffected if admission of the Na(+)-containing solution is delayed for 40 s. The number of charges flowing into the cell in exchange for each Ca2+ ion extruded was found not to deviate significantly from one over a wide range of ionic conditions and membrane potentials. These results show that the stoichiometry of the exchange is fixed over a wide range of conditions, and that the size of the inward exchange current is therefore directly proportional to the rate of Ca2+ efflux through the carrier.
Lagnado
L
L
Physiological Laboratory, Cambridge, United Kingdom.
McNaughton
P A
PA
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000557:Ambystoma / Q000502:physiology*; D000818:Animals; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002118:Calcium / Q000378:metabolism*; D004566:Electrodes; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D012374:Rod Cell Outer Segment / Q000187:drug effects / Q000502:physiology*; D012964:Sodium / Q000378:metabolism*
D012964:Sodium; D011188:Potassium; D002118:Calcium; D015056:1-Methyl-3-isobutylxanthine