Figure 5.
The effect of zaprinast superfusion on the holding current of a normal bass single cone in the dark. Voltage-clamped membrane currents measured in the same cell at −40 mV holding voltage. The mean holding current in dark was 8.2 and the noise variance 0.274 pA2. (A) Photocurrent activated by 10-ms flash of 540-nm light presented at time zero. Responses generated by flashes that delivered 2.4, 10.8, 62, 242, and 1,486 photons/μm2. The light dependence of the photocurrent peak amplitude reached half maximum amplitude at σ = 146 photons/μm2 (Eq. 1.1). (B) Power spectra of the dark noise. The continuous line over the data in darkness is a function derived from a theoretical model of the molecular origin of the current fluctuations (Eq. 1.13). (C) At time = 0, the solution bathing the cone was rapidly (<50 ms) switched to one containing 200 μM zaprinast, a membrane-permeable blocker of PDE activity. The inward holding current increased linearly at first and then reached a new stationary value. The slope of the linear current change near t = 0 was −16.6 pA/s. From this slope, we computed the rate of cGMP synthesis in the dark, γdark = 4.89 μM/s (Appendix 3).