Fig. 10. Light-induced Ca2+ signals and simultaneously recorded whole-cell currents (lower traces) measured in the absence of both extracellular Ca2+ and Na+. In contrast to Figure 9, the Ca2+ rise was virtually abolished when Na+ was substituted withN-methyl-d-glucamine (NMDG) with 0 Ca2+ and 2 mm EGTA applied by rapid perfusion from a puffer pipette. a, Response to a 1 sec saturating illumination in a cell perfused with NMDG solution, originally bathed in Ca2+-free, Na+-containing bath. b, Response from a photoreceptor perfused with NMDG, but this time after being initially bathed in normal Ringer’s (1.5 mmCa2+); again there was little or no increase in Ca2+. Both cells, clamped at −70 mV, produced small outward currents, as NMDG does not permeate the light-sensitive channels; c, Light-induced responses from the same cell as in Figure 11b to a weak 20 msec LED flash before (left), after (right), and during (middle) rapid perfusion with the Ca2+-free NMDG solution. The cell was clamped at −70 mV. After perfusion with NMDG, the response reversed and became slower (because of the absence of Ca2+-dependent feedback effects), but recovered completely after returning to normal Ringer’s. The Ca2+ measurement was made ∼60 sec after these responses were recorded, after the cell had been perfused again with NMDG for ∼30 sec.