Cytoplasmic calcium as the messenger for light adaptation in salamander rods.

Abstract

SUMMARY1. In order to study the role of cytoplasmic calcium concentration (Ca2+) in rod photoreceptor light adaptation, we have attempted to prevent light-induced changes in Ca 2' by minimizing calcium fluxes across the outer segment plasma membrane. This was achieved by exposing the outer segment to a low-Ca2 , 0-Na+ solution, in which sodium was replaced with either guanidinium or lithium and the external calcium concentration (Ca2+) was reduced to micromolar levels.2. With guanidinium and 1-3 ,sM-Cao , the circulating current in darkness was maintained for a period of at least 15 s, consistent with approximate stability of Ca 2+ With Li+ rather than guanidinium most of the initial current was suppressed, but the residual current was again relatively stable.3. During prolonged exposures (> 30 s) to low-Ca2 , 0-Na+ solution followed by dim illumination, the circulating current did not remain constant but slowly increased. Incorporation of calcium buffer into the cytoplasm greatly reduced the rate of change of current, consistent with the idea that the increase arose from a gradual decrease in Ca2+.4. Light responses of rods exposed to low-Ca2+, 0-Na+ solution in darkness were altered in a characteristic manner. Although the initial rising phase of the light response was little changed, the peak amplitude of the response was larger and occurred later, and the response decayed more slowly than in control. The response-intensity relation was steepened and was shifted towards lower intensities both for flashes and for steps of light. The normal sag in the response to steps disappeared, and the waveform of the step response could be predicted to a close approximation from the integral of the dim flash response.5. Presentation of background illumination in Ringer solution produced a marked acceleration of the response to a subsequent bright flash. No such acceleration was observed if the background was given in low-Ca2+, 0-Na+ solution.6. The results described in paragraphs 4 and 5 indicate that, under conditions expected to minimize changes in Ca2+, all manifestations of light adaptation disappear, and the rod simply sums the effects of incident photons with an invariant integration time.t To whom correspondence should be addressed. G. L. FAIN AND OTHERS7. Exposure of a light-adapted rod to low-Ca2 , 0-Na+ solution altered the responses to superimposed test flashes in much the same way as for rods in darkness. The initial rising phases in low-Ca2+, 0-Na+ solution were unchanged, but the responses were larger, reached peak later and decayed more slowly. Nevertheless, when the low-Ca2+, 0-Na+ solution was presented on adapting backgrounds of increasing intensity, the time-to-peak of responses shortened in a graded manner.8. Extinction of the background in low-Ca2 , 0-Na+ solution elicited large lightsuppressible currents, presumably as a result of increased cytoplasmic cyclic GMP concentration. The rate of increase of these currents was graded with the intensity of the prior background.9. The results described in para...

Toward a unified model of vertebrate rod phototransduction

“…The discrepancy between the salamander rod recovery phases of the Fain et al (1989) data and the model may reflect, in part, species differences since actual toad rod data to which we fitted the model (Fig. 4) recovered from saturation with kinetics closer to our model (black curve).…”

“…Step-flash paradigm of Fain et al (1989)-Fain et al introduced a paradigm demonstrating Ca 2+ -mediated light adaptation in salamander rods (Fain et al, 1989;Matthews, 1995). They presented a saturating flash (of fixed intensity) to a rod at the offset of 7-s steps of light presented at different intensities.…”

“…Specifically, if the model could account for single-photon response features, but failed to reproduce realistic responses in the high light regime, including light adaptation (LA), the model would have to be modified in a fundamental way. Toward this end, we adjusted the model in an attempt to account for six important classes of rod data: (1) SPR waveforms and statistics (as in Hamer et al, 2003), including SPR reproducibility; (2) a full dark-adapted flash-response series (obtained in the present study), from dim-flash to saturating, bright flash levels, obtained from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al, 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods (Forti et al, 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout (KO) rod preparations.…”

“…A dark-adapted flash series, and stimulus paradigms from the literature eliciting various degrees of light adaptation (LA), were simulated using a full differential equation version of the model that included the addition of Ca 2+ -feedback onto rhodopsin kinase via recoverin. With this model, using a single set of parameters, we attempted to account for (1) SPR waveforms and statistics (as in Hamer et al, 2003); (2) a full darkadapted flash-response series, from dim flash to saturating, bright flash levels, from a toad rod; (3) steady-state LA responses, including LA circulating current (as in Koutalos et al, 1995) and LA flash sensitivity measured in rods from four species; (4) step responses from newt rods (Forti et al, 1989) over a large dynamic range; (5) dynamic LA responses, such as the step-flash paradigm of Fain et al (1989), and the two-flash paradigm of Murnick and Lamb (1996); and (6) the salient response features from four knockout rod preparations. The model was able to meet this stringent test, accounting for almost all the salient qualitative, and many quantitative features, of the responses across this broad array of stimulus conditions, including SPR reproducibility.…”

Deactivation of Phosphorylated and Nonphosphorylated Rhodopsin by Arrestin Splice Variants

“…This degeneration has been attributed to excessive signaling of the phototransduction cascade. It can be dramatically accelerated by exposing mice to constant light or prevented by raising the mice in constant darkness (Fain et al, 1989;Xu et al, 1997;. To assess the ability of either p48 or m44 to prevent light damage in Arr Ϫ/Ϫ retinas, we compared the retinal morphology of mice reared under normal 12 h dark/light cycles.…”