Figure 5.
The cone ICa and its surround response recorded in a superfusate enriched with Tris A cone photoreceptor in the retinal slice was recorded under whole-cell voltage-clamp. The slice was superfused with control Ringer's solution (buffered with bicarbonate) and a solution enriched with Tris to elevate the pH-buffering capacity. (A a) Effects of 15-mM Tris-enriched buffer (with 15 mM Tris) on the cone ICa and surround response. Diffuse light (duration, 400 ms) illumination was given during the step depolarization (shorter bar) in the presence of a small spot light (diameter, 30 μm; top bar). The cone was held at −40 mV and polarized to voltages ranging from −50 to +6 mV in 8-mV steps. Representative traces at −34 and −18 mV are shown. (From the left to the right column) The current traces before (external HCO3 −: 22 mM) and during application of bicarbonate solution plus 15 mM Tris. All the recordings were from the same cell. The recording sequence was: the left column (−34 mV, −18 mV) followed by the right column (the same command voltage sequence as for the recording in the left column). The leak conductance of 2.0 nS did not change in the Tris-containing solution (see, e.g., the current traces at −34 mV). In the Tris-containing solution, the ICa in darkness increased and the surround response was suppressed (see the current traces at −18 mV). Symbols denote the sampling points for the calculation of the I-V curves of ICa (B b). In the control solution, the inward current produced by ICa was counterbalanced by the outward leak current (at −18 mV in control). A current dip appeared toward the time of switch-off of the surround illumination (arrow). (b) I-V curve of the ICa of the same cone photoreceptor shown described in A a. Filled squares, in the control solution without surround illumination; open squares, in the control solution during surround illumination; filled gray circles, in the Tris-containing solution without surround illumination; open gray circles, in the Tris-containing solution during surround illumination. Inset shows the voltage dependence of the surround response in the control solution (filled squares) and in 15 mM Tris-containing solution (open circles). (c) Activation curves fitted to the Boltzmann function derived from the data in A b. (B) Effect of Tris on the response of a HC to the flash of a large (diameter, 4,000 μm; duration, 100 ms) light spot. The recording in the left column was taken in the control Ringer, that in the middle column was recorded during application of 15 mM Tris-containing solution, and the right record that in the right column was recorded after the return of the slice to the control Ringer's solution.