The Ventral Photoreceptor Cells of Limulus

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.

Section: Light Activates Unitary Channel Currents Of Two Distinct Ampsupporting

confidence: 91%

Section: Introductionmentioning

confidence: 99%

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

Johnson

Bacigalupo

Vergara

et al. 1991

“…Ventral nerves from Limulus were pinned into a plexiglas chamber and superfused with artificial seawater (ASW) as described previously (Millecchia and Mauro, 1969;Levy and Fein, 1985). For measurements with Ca-selective electrodes, cells were stimulated with white light from a 45-W tungsten lamp that was brought through a shutter into the specimen plane using an optic fiber (3 mm diameter).…”

Section: Methodsmentioning

confidence: 99%

A lingering elevation of Cai accompanies inhibition of inositol 1,4,5 trisphosphate-induced Ca release in Limulus ventral photoreceptors.

Levy

Payne

1993

Injection of inositol 1,4,5 trisphosphate (ImPs) into Limulus ventral photoreceptors causes an elevation of intracellular free Ca concentration (Cai) and depolarizes the photoreceptors. When measured with the photoprotein aequorin, the InsPs-induced Cai increase follows the time course of depolarization and declines within 1-2 s. However, sensitivity to further injections of InsP3 remains suppressed for several tens of seconds. The possibility that the suppression of Ca release (feedback inhibition) is due to a small lingering elevation of Cai, below the existing detection limit of aequorin, was investigated by measuring Cai with Ca-sensitive electrodes. Double-barreled, Ca-selective microelectrodes were used to pressure inject InsPs and measure Cai at the same point. Light or InsPs injections into the light-sensitive compartment depolarized the photoreceptors and induced an elevation of Cai that persisted for tens of seconds. Injections of InsP3 during the decay of Cai showed that sensitivity to InsP3 recovered as resting Cai approached the prestimulus level. The relationship between elevated Cai and feedback inhibition was very steep. An elevation of Cai of 1 o.M or more was associated with inhibitions of 79 +-12.4% (SEM; n = 7) for the InsP~-induced Cai increase and of 76 ± 8% for depolarizations. With a residual Cai elevation of 0.01 o.M or less, the mean inhibition was 10 ± 7.4% for InsPs-induced Cai increase and 6.6 ± 4% for InsPz-induced depolarization. Injections of InsP3 into a light-insensitive compartment within the cell induced elevations of Cai with no associated depolarizations or feedback inhibition. To verify that a sustained elevation of Ca i is necessary for inhibition of InsPz-induced Cai increase and depolarization, we injected ethyleneglycol-bis-(13-aminoethylether)-N,N'-tetraacetic acid (EGTA) between two injections of InsP3. Injection of 1 mM EGTA or the related Ca chelator BAPTA, delivered 750 ms after the first injection of InsPs, restored the peak depolarization caused by the second injection of InsP3 to > 80 ± 3% of control, compared with 13 ± 8% without an intervening injection of EGTA. Measurement of Cai with aequorin showed that an intervening injection of EGTA partially restored the InsP3-induced Ca i increase.

“…However, removal of Ca from the normal extracellular solution had little or no effect on V,-e,. (Millecchia and Mauro, 1969;Brown and Mote, 1974); by contrast, a significant shift was reported in Balanus photoreceptors (Brown et al, 1971). A complementary observation in these two species is that the intracellular rise in Ca that accompanies the light response was drastically reduced in low-Ca external solution in Balanus but was only attenuated in Limulus (Brown and Blinks, 1974), suggestive of differences in the extent to which Ca influx may occur during the photoresponse.…”

Section: Introductionmentioning

confidence: 94%

“…The process of ion permeation through light-activated channels, by contrast, has received relatively less attention since the seminal work by Millecchia and Mauro (1969) and Brown and Mote (1974) in Limulus ventral photoreceptors, and by Brown et al (1971) in Balanus photoreceptors. Those authors had concluded that, under physiological condi-tions, the inward current that underlies the depolarizing receptor potential is carried by sodium ions.…”

Section: Introductionmentioning

confidence: 99%

Ion permeation through light-activated channels in rhabdomeric photoreceptors. Role of divalent cations.

Gómez

Nasi

1996

A B S T RA C T The receptor potential of rhabdomeric photoreceptors is mediated primarily by a Na influx, but other ions must also permeate through light-dependent channels to account for some properties of the photoresponse. We examined ion conduction in macroscopic and single-channel light-induced currents of Lima and Pecten photoreceptors. In the absence of Na, a fivefold change in extracellular K shifted the reversal voltage of the photocurrent (V~v) by ~ 27 mV. Because the dependency of Vre v on [K] o was sub-Nernstian, and Vre v in each condition was more positive than EK, some other ion(s) with a positive equilibrium potential must be implicated, in addition to K. We assessed the participation of calcium, an important candidate because of its involvement in light adaptation. Three strategies were adopted to minimize the impairments to cytosolic Ca homeostasis and loss of responsiveness that normally result from the required ionic manipulations: (a) Internal dialysis with Na-free solutions, to prevent reverse operation of the Na/Ca exchanger. (b) Rapid solution changes, temporally limiting exposure to potentially detrimental ionic conditions. (c) Single-channel recording, exposing only the cell-attached patch of membrane to the test solutions. An inward whole-cell photocurrent could be measured with Ca as the only extracellular charge carrier. Decreasing the [Ca]o to 0.5 mM reduced the response by 43% and displaced the reversal potential by -4.3 mV; the shift was larger (AVr~v = -44 mV) when intracellular permeant cations were also removed. In all cases, however, the current carried by Ca was < 5% of that measured with normal [Na]o. Unitary light-activated currents were reduced in a similar way when the pipette contained only divalent cations, indicating a substantial selectivity for Na over Ca. The fall kinetics of the photoresponse was slower when external Ca was replaced by Ba, or when the membrane was depolarized; however, dialysis with 10 mM BAPTA failed to antagonize this effect, suggesting that mechanisms other than the Ca influx participate in the modulation of the time course of the photocurrent.