Fig. 5. Effects of extracellular Ca²⁺ on atrial beating. (A) Representative control response after treatment with synthetic EI-14 in normal Ringer’s solution. EI-14 (10^-7moll^-1) was applied during the period indicated by the horizontal bar. (B) Effects of EI-14 in the absence of Ca²⁺ in the bathing medium. Ca²⁺ was omitted during the period indicated by the horizontal bar (Ca²⁺-free). (C) Effects of EI-14 after pretreatment with verapamil. Verapamil (10^-5moll^-1) was applied during the period indicated by the horizontal bar.