Barium delays the onset of rapid dark adaptation in bullfrog cones.

The late receptor potential of the excised, perfused bullfrog retina was isolated with sodium aspartate. By employing a three-flash technique, cone responses were monitored without interference from rods. In cones barium ions were found to delay the onset of rapid dark adaptation, but the rate of recovery, once begun, was unaffected. We propose that barium ions act directly upon the enzyme system postulated to govern the onset of rapid dark adaptation of cones. In addition, barium was found to affect the amplitude of the rod receptor potential differently from that of cones, increasing the former but decreasing the latter. The effect of barium upon photoreceptor potential amplitude is discussed in terms of a reduction in the potassium conductance of the photoreceptors and the mechanisms postulated for photoreceptor excitation and rapid dark adaptation.