Increased intracellular sodium mimics some but not all aspects of photoreceptor adaptation in the ventral eye of Limulus.

Abstract

A B S T R A C T The effects of the intracellular iontophoretic injection of Na + ions have been quantitatively compared with adaptation in ventral photoreceptors of Limulus. We find that: (a) both light adaptation and sodium injection are associated with a decrease in the variability of the threshold response amplitude; (b) both light adaptation and sodium injection are associated with a decrease in the absolute value of the temporal dispersion of the threshold response time delay; (c) the same template curve adequately fits the intensity response relationships measured under light adaptation and Na + injection; (d) both light adaptation and Na + injection produce a fourfold decrease in response time delay for a desensitization of 3 log units; (e) the time course of light adaptation and dark adaptation is significantly faster than the onset of and recover)' from desensitization produced by Na + injection; Or) unlike local illumination, Na + injection does not produce localized desensitization of the photoreceptor. These findings suggest that a rise in intracellular Na + concentration makes at most only a minor contribution (probably less than 5%) to the total adaptation of these receptors in the intensity range we have examined (up to 3 log units above absolute threshold). However, changes in intracellular Na + concentration may contribute to certain components of light and dark adaptation in these receptors.

A quantitative comparison of the time-course of sensitivity changes produced by calcium injection and light adaptation in Limulus ventral photoreceptors

“…2, one must decide whether the differences or the similarities between the effects of light and Ca++ injection are most important. Based on other experiments (Fein and Charlton, 1977c) where we have compared the effects of light and Na+ injection, we find the similarities in Fig. 2 striking.…”

“…1 B allowed us to measure the photoresponse under voltage clamp and then use the same intracellular electrodes for injecting Ca++ by passing current between the two intracellular electrodes. (Fein and Charlton, 1977c) Light intensities are given as log10 I/Jo where Io is the intensity of the unattenuated beam of white light. The intensity of the unattenuated beam was found to be equivalent to 1.2 x 10i5 520 nm photons/cm2-s (Fein and Charlton, 1977a) RESULTS Fig.…”

“…The current clamp circuit (set for zero membrane current) insured that the injection current passed between the two intracellular electrodes, rather than across the cell membrane. (Fein and Charlton, 1977c) In this figure and all subsequent figures the peak amplitude of the response to each test flash is plotted without any averaging or smoothing of the data. OFF FIGURE 2 The effects of illumination and Ca+ injection.…”

Chemical excitation of Limulus photoreceptors. I. Phosphatase inhibitors induce discrete-wave production in the dark.

“…It is unlikely that some nonspecific ionic effect can account for the induction of discrete waves. Induction does not appear to depend on the counterion in the electrode because K+ does not induce discrete waves when ejected from conventional KCl pipettes, and because Na' has been reported to desensitize Limulus receptors, but not to induce discrete waves (Lisman and Brown, 1972;Fein and Charlton, 1977b) . A variety of anions have been injected into ventral photoreceptors in earlier studies, but they were not reported to induce the production of discrete waves .…”

The initiation of excitation and light adaptation in Limulus ventral photoreceptors.

“…These models are of particular interest in connection with the role of calcium in light adaptation. Several lines of evidence indicate that a rise in intracellular free calcium (Cai) mediates light adaptation in Limulus ventral photoreceptors (Lisman and Brown, 1972 a, b;Brown and Blinks, 1974;Lisman and Brown, 1975 b;Fein and Lisman, 1975;Brown and Lisman, 1975;Fein and Charlton, 1977 a). Direct measurement of Ca~ indicates that Ca~ goes up in the light (Brown and Blinks, 1974;Brown et al, 1977).…”