Figure 4. Inhibition of tyrosine phosphatase activity blocks potentiation by dim background light.

Whole-cell voltage clamp recordings from ‘on’ bipolar cells in dogfish retinal slices, on equilibration with 200 μM orthovanadate (A) or 100 μM RK682 (B). The cells were clamped to their dark potentials of -40 and -35 mV, respectively; the zero-current level is indicated by the dashed lines. The I-R relationship for each cell was measured in the dark and against dim backgrounds by raising the flash intensity sequentially by factors of 2, at 10 s intervals, which elicited ramps of flash responses saturating at 8 Rh**. The dimmest test flash timing (0.0625-0.5 Rh**) is marked by the diamonds above the trace. C and D show the I-R relationships derived from three ‘on’ bipolar cell recordings with 200 μM orthovanadate (OV) and three ‘on’ bipolar cell recordings with 100 μM RK682. Flash responses measured in the dark were obtained just after establishing the whole-cell configuration (♦, continuous curve), on equilibration with OV or RK682 in the dark (▪, dashed curve) and on equilibration against 0.2 Rh** s⁻¹ backgrounds (▵). The slopes of the dark control curves were 1.8, and showed no significant change with OV, RK682 or background light on equilibration with these tyrosine phosphatase inhibitors.