Figure 4.

Variability in light-dependent channel density and use of late openings to gauge unitary conductance. A, A patch in a Hesse photoreceptor producing a few isolated channel openings, although the cell was clearly responsive to light, as indicated by the presence of the capacitative transient; the attenuation factor is indicated near each trace (V_p = +50 mV). Unattenuated light intensity 24.6 × 10¹⁵ photons · cm⁻² · s⁻¹. B, Voltage changes applied in the dark, spanning a range of 150 mV, failed to produce channel openings. C, Massive, multichannel activity in a Joseph cell, evoked by stimulating light, the intensity of which was progressively increased, as indicated (V_p = +80 mV). The brightest flashes produced responses with a shape reminiscent of the macroscopic photocurrent of the cell. Unattenuated light intensity 11.4 × 10¹⁵ photons · cm⁻² · s⁻¹. D, Light-sensitive patch in which channel openings are entirely confined to a late interval after the clearly biphasic I_C, that is, after repolarization of the receptor potential (light, 6.2 × 10¹² photons · cm⁻² · s⁻¹). E, Activity in a patch containing numerous light-dependent channels. Inset, Expansion of the recording interval 400–500 ms after light onset reveals many clearly resolvable single-channel transitions. F, G, Late channel openings recorded at different pipette potentials in two patches (light, 5.8 × 10¹³ and 6.1 × 10¹⁵ photons · cm⁻² · s⁻¹, respectively). H, I–V relationship for the events in G.