Electrical and adaptive properties of rod photoreceptors in Bufo marinus. I. Effects of altered extracellular Ca2+ levels.

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Abstract
The effects of altering extracellular Ca2+ levels on the electrical and adaptive properties of toad rods were examined. The retina was continually superfused in control (1.6 mM Ca2+) or test Ringer''s solutions, and rod electrical activity was recorded intracellularly. Low Ca2+ Ringer''s (10-9 M Ca2+) superfused for up to 6 min caused a substantial depolarization of the resting membrane potential, an increase in light-evoked response amplitudes and a change in the waveform of the light-evoked responses. High Ca2+ Ringer''s (3.2 mM) hyperpolarized the cell membrane and decreased response amplitudes. Under conditions of either low or high Ca2+ superfusion for up to 6 min, in both dark-adapted and partially light-adapted states, receptor sensitivity was virtually unaffected; i.e., the V-log I [light intensity] curve for the receptor potential was always located on the intensity scale at a position predicted by the prevailing light level, not by Ca2+ concentration. Cytosol Ca2+ concentration may be capable of regulating membrane potential levels and light-evoked response amplitudes, but not the major component of rod sensitivity. Low Ca2+ Ringer''s also shortened the period of receptor response saturation after a bright but nonbleaching light flash, hence accelerating the onset of both membrane potential and sensitivity recovery during dark adaptation. Exposure of the retina to low Ca2+ (10-9 M) Ringer''s for long periods (7-15 min) caused dark-adapted rods to lose responsiveness. Response amplitudes gradually decreased, and the rods became desensitized. These severe conditions of low Ca2+ caused changes in the dark-adapted rod that mimic those observed in rods during light adaptation. Loss of receptor sensitivity during prolonged exposure to low Ca2+ Ringer''s may result from a decrease of intracellular (intradisk) stores of Ca2+; i.e., less Ca2+ is thereby released per quantum catch.

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