Multiple conductance states of the light-activated channel of Limulus ventral photoreceptors. Alteration of conductance state during light.

Abstract

A B S T R A C T The properties of light-dependent channels in Limulus ventral photoreceptors have been studied in cell-attached patches. Two sizes of single-channel events are seen during illumination. Previous work has characterized the large (40 pS) events; the goal of the current work was to characterize the small (15 pS) events and determine their relationship to the large events. The small events are activated by light rather than as a secondary result of the change in membrane voltage during light. The mean open time of the small events is 1.34 -+ 0.49 ms (mean_ SD, n = 15), ~50% of that of the large events. The large and small events have the same reversal potential and a similar dependence of open-state probability on voltage. Evidence that these events are due to different conductance states of the same channel comes from analysis of relatively infrequent events showing a direct transition between the 15 and 40-pS levels. Furthermore, large and small events do not superpose, even at positive voltages when the probability of being open is very high, as would be predicted if the two-sized events were due to independent channels. Expression of the different conductance states is not random; during steady illumination there are alternating periods of several hundred milliseconds in which there are consecutive, sequential large events followed by periods in which there are consecutive, sequential small events. At early times during the response to a step of light, the large conductance state is preferentially expressed. At later times, there is an increase in the relative contribution of the low conductance state. These findings indicate that there is a process that changes the preferred conductance state of the channel. This alteration has functional importance in the process of light adaptation.Address reprint requests to Dr.

Light-activated ion channels in solitary photoreceptors of the scallop Pecten irradians.

“…In Limulus, small, light-dependent channel openings have also been described (Bacigalupo and Lisman, 1983;Nagy 1990;Nagy and Stieve, 1990). Johnson et al (1991) reported that the incidence of small events tends to become higher sometime after light onset, and suggested that transition from large to small events could contribute to the decreased amplitude of the photocurrent during the slow phase of light adaptation. A quantitative analysis of such apparent nonstationary behavior is difficult, particularly without voltage-clamping the cell (see Discussion).…”

“…In the bottom part of the figure a segment of the recording was expanded, and straight lines with slopes of 50 and 17 pS were fitted to the large and the small events, respectively. scheme involving slowly changing gating modes (Johnson, Bacigalupo, Vergara, and Lisman, 1991). Unfortunately, while small-amplitude, light-induced openings were observed on many occasions, the presence of muhiple channels in those patches precluded statistical analysis.…”

“…In Limulus, Nagy and Stieve (1990) and Nagy (1990) reported up to three different sizes of unitary currents activated by light, and suggested---on the basis of their activation latencies--that separate classes of ion channels may exist, gated by different internal transmitter systems. Johnson et al (1991) provided evidence favoring the interpretation of a single class of ion channel proteins possessing subconductance states, and speculated that transition between different "modes" of gating involving the large vs. the small conductance state may be responsible at least in part for the decay of the photocurrent during the late phase of light adaptation. Some of the present results (see Fig.…”

Photoreceptor cells dissociated from the compound lateral eye of the horseshoe crab, Limulus polyphemus, II: Function

“…The procedures for obtaining gigaseals on the R-segment membrane are similar to those described for experiments on the R-lobe of ventral photoreceptor cells (Johnson et al, 1991). The patch electrodes used were fabricated from Corning #7052 glass (Garner Glass, Claremont, CA), coated with Sylgard, and firepolished.…”

“…We have been able to make whole-cell recordings from isolated cells and have found that their responses to light are similar to those of cells in vivo, and that currents recorded under voltage clamp in the dark resemble currents recorded with the two-electrode voltage clamp in ventral photoreceptor cells (Lisman et al, 1982). We have also been successful in making gigaseals onto both the rhabdomeral membrane of the R-segment and the arhabdomeral membrane of the A-segment in isolated cells (Chamberlain & Barlow, 1984): voltage-activated channel activity can be observed in patches of A-segment membrane, while patches of R-segment membrane contain light-activated channels which resemble those observed in the light-sensitive membrane of ventral photoreceptors (Bacigalupo & Lisman, 1983;Nagy & Stieve, 1990;Johnson et al, 1991).…”