Figure 3.
Modulation of cone ICa by focal application of a high-pH solution to the cone synaptic terminal layer. Recordings were obtained from cone photoreceptors of newt retinal slices under whole-cell voltage clamp. The slices were superfused with control Ringer's solution buffered with bicarbonate and containing 100 μM picrotoxin. (A) Alkalinized Ringer's solution (pH 9.0) was focally applied to the cone synaptic terminal layer by pressure ejection (duration: 10 ms, pressure: 59 kPa; time indicated by arrow). The cell was voltage-clamped at various voltages in the range of −50 to +6 mV, in 8-mV steps. The representative four traces, voltage clamped at −42, −26, −18, and +6 mV are shown. Transient signals at the pressure ejection were artifacts produced by the valve opening. While a small inward current at −42 mV was seen in this cell, it was not seen in the other five cones tested. The current was sampled at the points marked by a symbol to construct the I-V curves shown in B. Small spot (diameter, 30 μm) illumination was maintained throughout. (B, top) Leak-subtracted I-V curve of cone ICa in normal Ringer's solution (pH 7.4, filled squares) and in response to a high-pH solution (pH 9.0, open circles). The data is from the same cell as that described in A. The leak conductance was 0.94 nS. (Bottom) Activation curves derived from the I-V curves fitted to the Boltzmann function. The midpoint of the curve (−17.7 mV; black line connecting the filled squares) as obtained in normal Ringer's solution (pH 7.4) was shifted to −28.1 mV (gray line connecting the open circles) after the application of the high-pH solution. The maximum conductance was calculated from the slope of the I-V curve in the high-pH solution, between −10 and 6 mV, and normalized to 1.0.