Adapting lights and lowered extracellular free calcium desensitize toad photoreceptors by differing mechanisms.

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Abstract
Extracellular recordings were made across the outer segment layer of isolated superfused toad retinas. Under these recording conditions, the photovoltage reflects primarily the current flowing through the outer-segment membrane of red rods. In normal toad Ringer solution, a dim conditioning flash desensitized a test flash response. The desensitization reached a peak 1.8-2.0 s after the conditioning flash and then declined approximately as an exponential with time constant 6 s. Lowered extracellular Ca, [Ca2+]o, desensitized the photoresponse. It required .apprx. 10 times more light to reach a half-maximal response for each 10-fold change in [Ca2+]o from 10-6-10-9 M. When [Ca2+]o was < 10-7 M, substitution of Li+ for Na+ as the predominant monovalent cation in the superfusate permitted responses to continue and a resensitization of up to .apprx. 1 log unit was observed. The effects of lowered [Ca2+]o on response kinetics were markedly different from the effects of background lights producing a comparable desensitization. Low [Ca2+]o increased absolute latency and time-to-peak of the flash response. Background lights decreased time-to-peak, leaving latency unchanged. The effects of background lights and lowered [Ca2+]o are not additive. Moderate backgrounds had little effect on the intensity/response function in low [Ca2+]o. Conditioning flashes facilitated the test flash response in 10-7 M-[Ca2+]o superfusate. These results can be understood in terms of the Ca2+ hypothesis of transduction (Hagins et Yoshikami, 1974) if it is assumed that lowered [Ca2+]o exposes an endogenous Ca2+ buffer. The data also provide evidence for a role of Na+/Ca2+ exchange in regulating intracellular Ca2+ concentration in the toad photoreceptor. A quantitative model based on these assumptions is derived and compared with the experimental data.