The effect of sodium, potassium and calcium on Limulus lateral eye retinular cells

Osmotic properties of Limulus seawaters and organ cultures: an unrecognized issue

Excised eyes of the horseshoe crab (Limulus polyphemus) have long provided robust preparations for fundamental studies of the visual neuroscience of photoreceptors. Such preparations are particularly useful for immersion in perfusion chambers whose perfusate can be varied as needed. Little attention has been paid in the past to the osmotic properties of such perfusates because they have generally been virtually isotonic with natural seawater. We here report that (1) some recent Limulus studies have used organ culture perfusates that are severely hypotonic and (2) that that fact has not been adequately disclosed.

Light-evoked depolarizations in the retina of Strombus: role of calcium and other divalent cations

Previous studies indicate that overlapping inward sodium and outward potassium currents play a role in generating the waveform of light-evoked depolarizations (LEDs) in one type of retinal neuron in Strombus luhuanus, a marine gastropod [Chinn, K. S., and Gillary, H. L. (1985). Comp. Biochem. Physiol. 80A:233-245]. This paper concerns the effects of divalent cations on the LED. The LED can exhibit a distinct early phase of depolarization (DE). Increasing the [Ca2+] in the artificial seawater (ASW) bathing medium reduced the amplitude of the entire LED, and omitting Ca2+ increased it. Adding 10 mM Sr2+ or 10 mM Mn2+ to either normal ASW or 0-Ca2+ ASW decreased the LED amplitude. Adding 10 mM Ba2+ to 0-Ca2+ ASW also decreased the LED amplitude, but adding Ba2+ to normal ASW selectively increased DE. Cd2+ (100 microM) selectively reduced DE when added to normal ASW but not when added to 0-Ca2+ ASW. The results show that a variety of divalent cations can alter the currents that underlie the LED. They also suggest that an inward Ca2+ current occurs during DE.

Temporal relationships of latency and magnitudes of Limulus ventral photoreceptor potentials elicited by two light pulses

As the dark interval between two stimulating light pulses increases, the latent period of the ensuing receptor potential increases to an asymptote reached about midway during the latency and is, thereafter, constant. The latent period is thus divisible into two segments, the first of which is light sensitive and the second not. The possible significance of these segments has been explored [12]. During the time the latent period is increasing toward its limiting value, the magnitudes of both components of the receptor potential are constant and maximal. The magnitude of the first component of the receptor potential begins to decline at a pulse interval of 15 msec, signify that the contribution of photons incident on the photoreceptor from the second pulse is diminishing. The magnitude of the second component of the receptor potential begins to decline at a pulse interval of 20 msec, suggesting that the contribution of photons delivered by the first pulse is diminishing. These results demonstrate that the two pulse stimulating paradigm reveals two segments of the receptor potential latent period and the ability of the photoreceptor to integrate radiant energy incident during the latent period, as measured by receptor potential magnitude.

Two latent period processes in Limulus ventral photoreceptors

The latent period of Limulus ventral photoreceptor potentials consists of two contiguous time segments, S1 and S2, which exhibit the following properties: (1) the duration of S1 is inversely dependent on the incident energy provided by the first stimulating light flash; (2) the duration of S1 is prolonged in the presence of GTP-gamma-S [guanosine-5'-0- (3-thiotriphosphate)] or papaverine; (3) the duration of S1 is abbreviated in the presence of sodium vanadate or chlorobutanol; (4) the duration of S2 is abbreviated by light adaptation; and (5) the duration of S2 is prolonged more than S1 in a low temperature environment. The existence of these two components of the Limulus ventral photoreceptor potential latent period is demonstrable both in dark adapted photoreceptors, using single constant intensity light pulses of varying duration, or in partially light adapted photoreceptors, using a stimulating conditioning pulse-test pulse sequence. On the basis of these results it is tentatively concluded that the two segments of the latent period of Limulus ventral photoreceptor potentials are occupied by different processes. The first process, occupying the S1 segment, generates a critical concentration of a photoproduct which, directly or indirectly, eventually alters the conductance of the active photoreceptor membrane. The second process, occupying the S2 segment, is concerned with the initiation of the SPFs which sum to produce the receptor potential. It is possible that the hydrolysis of GTP actively participates in the first process.

Effect of chlorobutanol on Limulus ventral photoreceptor latency: a model of a two component latent period

Chlorobutanol (1,1,1-trichlor-2-methyl-2-propanol) decreases the duration of the latent period of Limulus ventral eye receptor potentials. This effect is both concentration and intensity dependent. Between 0.05 and 1 mM chlorobutanol in sea water the extent of latency shortening effect of chlorobutanol linear function of the logarithm of the chlorobutanol concentration. The latency shortening effect of chlorobutanol diminishes with decreasing intensity of stimulation and virtually disappears at intensities just exceeding threshold. These results are explained by postulating that the latent period is composed of two processes only one of which is sensitive to chlorobutanol and, further, that the duration of the chlorobutanol-sensitive process is not appreciably affected by the stimulating intensity. This model of a two component latent period may be tested experimentally. In several experiments prolonged exposure of ventral photoreceptor cells to chlorobutanol coupled with a short exposure to calcium deficient sea water produced a collapse of the receptor potential followed by a slow recovery after returning to perfusion with normal sea water. During the recovery a slow, low amplitude positive wave exists which has been tentatively called a prepotential.

Partial voltage clamping of Limulus ventral photoreceptor potentials: evidence of programmed conductance changes

Light-initiated currents elicited by brief light stimuli from Limulus ventral photoreceptors bathed in normal sea water generally exhibit a smooth contour, although the unclamped receptor potential elicited by an identical light stimulus usually exhibits distinct C1 and C2 components. However, light-initiated currents obtained from cells exposed to chlorobutanol often exhibit two components. Data from such experiments indicate that peak C2 current is more strongly voltage dependent than peak C1 current, as in Limulus lateral eye retinular cells. The results of partial voltage clamp experiments with ventral photoreceptors in which the clamping episode terminated at different times during the receptor potential reveal relatively minor perturbations of the rebound receptor potential when compared with the unclamped control response. These findings suggest that the temporal pattern of the membrane conductance changes which underlie the receptor potential is determined prior to the occurrence of the receptor potential. It is likely that the program for these conductance changes is developed during the latent period of the receptor potential.

Two components of extracellularly-recorded photoreceptor potentials in the cephalopod retina: differential effects of Na+, K+ and Ca2+

The ERG of the isolated, superfused half-eye of the cephalopod Sepiola atlantica, evoked by a brief (10 micro second) light flash, has been studied by recording intraretinal potentials with glass microelectrodes. The intensity-response characteristics of the potentials recorded at an electrode fixed at the surface (Vs) can be fitted by a simple equation derived from an equivalent circuit model based on a sodium conductance increase mechanism. Raising the external potassium level reduces the maximal response (deltaVm), but does not alter the half-saturation intensity value (I0). Reducing external sodium does not affect deltaVm, but increase I0. Reducing external calcium also does not affect delta Vm, but decrease I0. These effects are adequately described by the model if it is also assumed (a) that changing the external sodium does not significantly alter the transmembrane sodium gradient, and (b) that sodium and calcium ions compete for the sensitivity control mechanism. Differential-depth recording between the fixed electrode at the surface and another electrode that could be moved into the retina revealed that the two component appearance of the transretinal ERG arose from the superposition of two vitreal-negative waveforms. An initial "fast" component was mainly recorded in the photoreceptive distal segments while a "slow" component was prominent in the more proximal regions of the retina. Perfusion with high K+ salines resulted in a decrease in the amplitudes of both fast and slow components of the response whereas reducing external Na+ reduced the amplitude of the fast component at all light intensities but reduced the amplitude of the slow component only at low intensities. The amplitudes of both the fast and slow components increased on reducing external calcium, but the rate of rise and fall of the fast component was independent of external calcium. The rate of rise of the slow component was also independent of the external Ca2+ level but a minimum in the recovery time (tF) was shifted to a lower intensity value at lower calcium concentrations. The shift of the minimum was to a higher intensity value with lowered sodium perfusing solutions. On the basis of the differential sensitivity of the two components to ion changes, as well as stimulus intensity and intraretinal distribution of the components, it is suggested that they reflect two distinct processes in the light-evoked potential of the photoreceptor cells.

The latent period of the Limulus lateral eye receptor potential: action of chlorobutanol

Excised lateral eye retinular cells of Limulus polyphemus exposed to chlorobutanol were examined using microelectrode techniques. The effects observed were: (1) a marked shortening of the receptor potential latent period; (2) a decrease in the effective input resistance of retinular cells to hyperpolariziog currents; and (3) a reduction in magnitude of retinular cell receptor and membrane potential. The magnitude of the changes were concentration dependent and reversible and were only produced by chlorobutanol and closely related compounds. Chlorobutanol does not alter the sodium permeability of the receptor cell membrane but it may increase the rate of the process or processes occurring during the latent period of the receptor potential.

Kinetics of 42K and 86Rb loss from the crayfish retina in the dark and the effect of light on the rate of isotope loss

86Rb and 42K loss from non-illuminated crayfish retinas is described by a sum of two exponential functions with rate constants lambda1 approximately 0.12 min-1 and lambda 2 approximately 0.006 min-1. 86Rb and 42K movements distinguish mainly by the lambda2 rate constants, lambda2Rb/lambda2K = 0.7. Oubain causes a delayed increase in the rate of isotope loss from non-illuminated retinas (lambda approximately 0.009 min-1 after 40 min). Light stimuli evoke a temporary increase in the rate of both 86Rb and 42K loss by the same factor (2.7 under these conditions). In the presence of ouabain the light-induced isotope loss is abolished after 40 min if the retina is periodically illuminated but not if the retina is kept in the dark.

A study of light-initiated currents in Limulus lateral eye retinular cells

Light-initiated currents in Limulus lateral eye retinular cells were studied using the voltage clamp technique. To assess the validity of such current measurements, the isopotentiality of retinular cells was determined on triply impaled cells and the effect of voltage clamping one retinular cell on adjacent retinular cells and on the eccentric cell in the same ommatidium was determined. The results of the experiments are: (1) retinular cells are isopotential at loci 100 micron apart; (2) appreciable steady state current during the clamping episodes leaks into neighboring retinular cells and the eccentric cell; (3) light-initiated currents exhibit two components; (4) there is a dynamic change in the resistance of the photoreceptor membrane during development of the receptor potential; (5) suppression of the rising phase (C1) of the receptor potential does not affect subsequent voltage changes; (6) suppression of the sodium influx which normally produces C1 has only minor effects on subsequent voltage changes; (7) reduced [KC1]out increases and increased [CK1]out decreases the reversal potential of light-initiated currents; and (8) reduced [NaC1]out reduces the magnitude and the reversal potential of light-initiated currents.

Effects of removing extracellular Ca2+ on excitation and adaptation in Limulus ventral photoreceptors

In Limulus ventral photoreceptors, removing extracellular calcium (Ca2+o) increases the median latency of light-evoked discrete waves. Removal greatly lengthens the time-to-peak of responses in the dark-adapted cell, but not in the light-adapted cell. Removal does not block light-adaptation or the light-induced rise in intracellular calcium (Ca2+i). These results are interpreted in terms of the hypothesis that both sensitivity and the kinetics of excitation are dependent on Ca2+i, and that Ca2+i is dependent on Ca2+o in the dark-adapted cell, but in the light is dependent largely on Ca2+ released from intracellular compartments.