Actions of EGTA and high calcium on the cones in the turtle retina.

1. The effects of Ca2+ on the activity of retinal cones were investigated by recording intracellular responses from turtle retinae perfused with solutions containing different Ca2+ concentrations. 2. Lowering extracellular Ca2+ concentration with EGTA increased membrane conductance and evoked depolarization in darkness. 3. Responses to bright lights were increased by EGTA by an amount equal to the depolarization in darkness; responses to dim lights instead were usually reduced. In addition EGTA modified the time course of responses to flashes. These changes were especially evident during the later phases of the response. 4. A steady light applied during EGTA perfusion can restore the original membrane potential. The kinetics of responses in these conditions, however, differ profoundly from those prevailing Ca2+ concentrations evoked hyperpolarization of the cones and decreased the amplitude of their responses to bright lights. The time course of responses to flashes was not appreciably modified by high Ca2+. 6. A steady light which hyperpolarizes the cone membrane by the same amount as high Ca2+ has an equal effect on the amplitude of responses to bright flashes but has an entirely different action on response kinetics and on the amplitude of responses to dimmer flashes. 7. The effects of EGTA on resting potential, conductance and response amplitudecan be interpreted assuming that the drug opens channels which are normally closed by Ca2+ in darkness. Conversely, the changes caused by high Ca2+ suggest that raising the concentration of outside Ca2+ reduces the number of light-sensitive channels open in darkness. 8. The observation that Ca2+ and light have different effects on kinetics and sensitivities is difficult to reconcile with the hypothesis that Ca2+ is the substance liberated by light to cause photoresponses.