Caffeine-induced depression is higher for increased basal calcium. A–C, [Ca²⁺]_i responses of a rod inner segment to bath-applied caffeine in 2 mm (A), 30 mm (B), and 90 mm(C) KCl. The magnitudes of the transient peak and depression were positively correlated to basal [Ca²⁺]_i before caffeine. The decay from the initial peaks was fitted with single-exponential functions having time constants of 97, 42, and 40 sec.