Figure 3.

Dark noise and photocurrents in a bass single loaded with 10 mM BAPTA at 400 nM free Ca²⁺ and briefly treated with hydroxylamine to decrease the number of dark VP molecules. (A) Comparison of the power spectra of the dark noise (continuous line) and the dim light photocurrent (dotted line) in the same cell. The lines are redrawn from C and D but scaled to match their zero frequency amplitude. The panel graphically demonstrates the difference between the two power spectra. (B) Photocurrent activated by 10-ms flash of 540-nm light presented at time zero. Responses generated by flashes that delivered 233, 6,417, and 25,546 photons/μm². At the brightest light tested, the peak amplitude was only ∼15% of that measured in untreated cells. The loss in light sensitivity demonstrates that the treated cell lost >1/10³ VP molecules in the dark. (C) Power spectra of dark current fluctuations in the same hydroxylamine-treated cone. The continuous line over the data is an optimally fit theoretical function based on a model of the molecular origin of current fluctuations (Eq. 1.8). (D) Power spectra of the photocurrent generated by the 25,546 photon/μm² photon flash shown in B. The continuous line over the data is an optimally fit product of three identical Lorentzians with α = 1.75 Hz. For comparison, open circles illustrate the function that best describes the mean dim light photocurrent in untreated cones (α = 2.17 Hz). The near identity of the two spectra shows that transient hydroxylamine treatment does not affect the photoresponse of bass cones, although they are much less sensitive to light.