Effects of extracellular calcium on signal transmission between photoreceptors and horizontal cells (HCs) are studied in superfused isolated retina of the larval tiger salamander. Horizontal cell light response is optimal when extracellular Ca2+ is maintained between 1–2 mM. Ca2+ levels beyond this range in either direction significantly reduce the HC light response amplitude. When extracellular Ca2+ is lowered from 2 mM to 0.5 mM, the rod input to HCs is reduced whereas the cone input is not affected. In comparison, the peak voltage responses of rods are not changed whereas the cone voltage responses are enhanced in 0.5 mM Ca2+. The selective suppression of rod input to HCs is probably due to the interplay of three factors: (1) the photocurrents, (2) voltage- and time-dependent membrane currents in photoreceptors, and (3) the Ca2-dependent synaptic gain between photoreceptors and HCs.
