Figure 4.

Dark noise in bass single cones loaded with 10 mM BAPTA at 400 nM free Ca²⁺ and different cyclic guanosine nucleotides. Voltage-clamped membrane current measured at −40 mV holding voltage. (A) Cone loaded with a solution lacking cGMP, GTP, and ATP. Only voltage-gated ion channels are active in this cell. The mean holding current in dark was 68.5 pA. Light did not change the current. The power spectrum of the current fluctuations is shown in the bottom panel. It is well described by the dashed, straight line. (B) Cone loaded with a solution containing 2 μM 8-cpt-cGMP and lacking GTP or ATP. cGMP-gated ion channels are active in this cell, but the cyclic nucleotide is not hydrolyzed by PDE. The mean holding current in dark was −82 pA, but it is essentially noiseless in the 0–30 Hz range. The cell does not respond to light. The power spectrum of the current fluctuations is shown in the bottom panel. It is well described by a straight line, not different from the spectrum in the complete absence of nucleotides. (C) Cone loaded with the normal 1 mM GTP and 3 mM ATP. Holding current in dark was −54 pA. The bottom panel illustrates the power spectrum of the dark noise. It is well described by Eq. 1.8, the continuous line optimally fit to experimental data. Dark noise in BAPTA-loaded cones is detected only in the presence of cGMP and originates from fluctuations in PDE activity.