The effects of sodium removal on the two types of calcium currents in single frog atrial cells.

The effects of sodium removal on the two types of calcium currents were studied in enzymatically dispersed frog (Rana esculenta) atrial myocytes with the single pipette patch-clamp technique. Reduction of calcium currents was recorded when NaCl was replaced either by TEACl, LiCl, TrisCl, cholineCl or by mannitol. An involvement of the Na-Ca exchange mechanism could be ruled out since the decrease was also observed after replacing external Ca with Ba. The slight shift of the apparent reversal potential recorded in our study suggests that the inward flow of Na ions through calcium channels does not contribute significantly to the L-type calcium current. Once again, the slight negative shift of the steady-state inactivation curve of the L-type calcium current cannot explain this decrease while no shift was recorded for the T-type calcium current. Even if a TTX-resistant Na current was recorded from a few cells this current cannot explain the decrease of calcium currents which was always observed upon sodium removal. To date we have no explanation for this effect.