The effects of low calcium and background light on the sensitivity of toad rods

1. We have examined the effects of decreases in extracellular Ca(2+) concentration on the intracellularly recorded light responses of rods from the toad, Bufo marinus. In agreement with previous results (Brown & Pinto, 1974; Lipton, Ostroy & Dowling, 1977), Ca(2+) concentrations below 10(-6) M produced a depolarization of rod resting membrane potential of approximately 30-40 mV and a corresponding increase in the maximum amplitude of the rod's light responses, so that saturating flashes in normal and low Ca(2+) Ringer produced hyperpolarizations to approximately the same membrane potential.2. The rod's sensitivity was reduced in low Ca(2+) Ringer by an amount dependent upon the extracellular Ca(2+) concentration. At 10(-6) M-Ca(2+), sensitivity was approximately 0.6 log units below normal. Thereafter, it dropped nearly linearly with [Ca(2+)](o) to a value approximately 4.0 log units below normal at 10(-9) M-Ca(2+). Most of the decline occurred within 1-2 min after the solution change as the membrane potential depolarized, but sensitivity continued to fall slowly with time at the lowest Ca(2+) concentrations. Exposure to low Ca(2+) solutions altered the kinetics of the receptor response to brief flashes, delaying response onset and time-to-peak but affecting the time course of decay very little.3. The sensitivity of the rod to maintained steps of light was also reduced in low Ca(2+). Furthermore, the changes in sensitivity produced by background illumination were very much smaller in low Ca(2+) than in normal Ringer. In some cases backgrounds actually increased sensitivity.4. In 10(-8) M-Ca(2+), backgrounds which themselves produced no response in the rod and no changes in rod sensitivity produced large decreases in response latency for responses of all amplitudes, and pronounced changes in time-to-peak and time-to-decay for moderate and large amplitude responses.5. Since the effects of background light and low Ca(2+) on the wave form of the rod are distinct and in some cases antagonistic, and since the changes in receptor sensitivity produced by backgrounds and low Ca(2+) are not additive, the decreases in sensitivity produced by exposure to low Ca(2+) appear to be caused by a mechanism distinct from normal light adaptation. We suggest that they are caused by an increase in the buffering capacity of the receptor cytosol for Ca(2+) and that Ca(2+) is the excitatory messenger or ;internal transmitter', as originally suggested by Yoshikami & Hagins (1971).