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

The Journal of Physiology
R E Greenblatt

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 calcium, [Ca2+]o, desensitized the photoresponse. It required approximately ten times more light to reach a half-maximal response for each ten-fold change in [Ca2+]o from 10(-6) to 10(-9) M. When [Ca2+]o was less than 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 approximately 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 ...Continue Reading

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