Cobalt ions enhance light responsiveness of carp cone horizontal cells in low calcium

Effects of cobalt ions (Co2+) on horizontal cells in low extracellular calcium were examined in isolated, superfused carp retinas. While 0.1 mmol/L Co2+ completely suppressed both rod- and cone-driven horizontal cells in normal Ringer's solution, it enhanced light responses of cone horizontal cells in low (0.1 mmol/L) calcium. The enhancement of the cone horizontal cell response by Co2+ was not caused by changes in light responsiveness of cone photoreceptors. Moreover, application of 50 mumol/L IBMX, an inhibitor of phosphodiester enzyme, reduced the suppressive effect of 0.1 mmol/L Co2+ in normal Ringer's solution. In consequence, the above-described enhancement of the cone horizontal cell light responsiveness may be due to a depolarization of cones caused by low calcium, which increases the activity of voltage-dependent calcium channels at cone terminals.