Fig. 4. Fluorescence ratio (F₃₄₀/F₃₈₀) measurements of cytoplasmic Ca²⁺ concentration of an isolated iridophore cell (concentrations of drugs given below; see text for details). Cytoplasmic [Ca²⁺] increases transiently in response to (A) potassium chloride (KCl), (B) caffeine and (C) ACh, but not nicotine. (D) Muscarine evokes a transient increase in cytoplasmic Ca²⁺, which is blocked by atropine. After washing for 15 min, the muscarine response is regained. (E) Ca²⁺ increases transiently in response to ACh and caffeine in the absence of Ca²⁺ in the external solution (EGTA Ca²⁺ free ASW). (F) Changing the osmolarity of the external solution has no effect on cytoplasmic Ca²⁺ concentration, although ACh evokes a strong increase in cytoplasmic [Ca²⁺]. s-ASW, ASW + 50 mmol l^-1 sorbitol (1112 mOsm kg^-1); 75% ASW, 769 mOsm kg^-1. Concentrations of drugs: KCl, 20 mmol l^-1; caffeine, 5 mmol l^-1; Ach, 50 µmol l^-1; nicotine, 2 mmol l^-1; muscarine, 50 µmol l^-1; atropine, 50 µmol l^-1.