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Physiological Laboratory, University of Cambridge.

1. In order to study the role of cytoplasmic calcium concentration (Ca2+i) in rod photoreceptor light adaptation, we have attempted to prevent light-induced changes in Ca2+i 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+o) was reduced to micromolar levels. 2. With guanidinium and 1-3 microM-Ca2+o, the circulating current in darkness was maintained for a period of at least 15 s, consistent with approximate stability of Ca2+i. With Li+ rather than guanidinium most of the initial current was suppressed, but the residual current was again relatively stable. 3. During prolonged exposures (greater than 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+i. 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+i, all manifestations of light adaptation disappear, and the rod simply sums the effects of incident photons with an invariant integration time. 7. 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.(ABSTRACT TRUNCATED AT 400 WORDS)

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