Biochemical aspects of the visual process. X. Distribution and movements of ions in the dark-adapted retina of the cuttlefish (Sepia officinalis)

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.

Explanation for constancy of intracellular chloride concentration with respect to membrane potential in guinea pig mammary gland

1. The relative permeabilities for sodium, potassium and chloride in guinea pig mammary gland slices are determined by means of ion flux studies with radioisotopes. 2. Assuming that there are no significant electrogenic potential components, we calculate permeability ratios PNa/PK = 0.97 and Pc1/PK = 1.25. 3. Substitution of these values in the Goldman equation yields membrane potentials of--15 mV before and--13 mV after ouabain treatment. 4. This small change in membrane potential explains the absence of a significant change in chloride content upon ouabain application, which leads to large changes in intracellular sodium and potassium concentrations.

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.

Efflux of potassium from the isolated frog retina: a study of the photic effect

1. Illumination of the isolated frog retina reduces the efflux of (42)K (and (86)Rb).2. Perfusion with a solution containing glutamate ions does not abolish the effect, although the electroretinogram is modified and the b-wave absent in this condition.3. Perfusion with a solution with only 2.36 mM sodium abolishes the effect of light on the efflux of potassium (and rubidium).4. The amplitude of the photic effect, measured in the first 2 min of illumination, is in linear proportion with the logarithm of the concentration of extracellular sodium ions.5. A steady rate of absorption of a few quanta per rod and per second yields a just detectable effect on the efflux of potassium (and rubidium).6. Strong illuminations followed by darkness reduce the efflux of rubidium for a period of time roughly comparable to the time of dark adaptation.7. The interpretation is proposed that the photoreceptor cells (and possibly other retinal cells too) cause the effect, and that the retention of potassium induced by light is secondary to the hyperpolarization of their membrane.