ANALYSIS OF FLUCTUATIONS IN THE CGMP-DEPENDENT CURRENTS OF CONE PHOTORECEPTOR OUTER SEGMENTS

We measured cGMP-dependent currents, under voltage clamp, in membrane patches detached from the outer segment of single-cone photoreceptors isolated from the retina of striped bass. We analyzed the variance of the current about its mean and the spectral density distribution of th e current fluctuations. From the analysis of variance, we determined t hat the cGMP-gated channels increase their probability of opening with increasing cGMP up to a maximum value of 0.87 +/- 0.03. The dependenc e on cGMP of the probability of opening is well described by a Hill eq uation with K-m= 60.2 +/- 3.7 mu M and n = 2.33 +/- 0.32 at -50 mV. At the same voltage, the spectral density distribution is well fit by th e sum of two Lorentzians with corner frequencies at 26 +/- 18 and 318 +/- 58 Hz. The single-channel conductance calculated from the current noise by two different methods suggests that the most frequently occup ied conductance state has an amplitude of about 18 pS.