Figure 2. The FMRFamide-like peptide (FLP)-induced contractions are dependent on the presence of extracellular Ca²⁺.

In normal extracellular medium containing 4.1 mM Ca²⁺, the muscle fibers contract in response to exposure to the flatworm FLP YIRFamide, depolarization with elevated extracellular K⁺ or caffeine. If the extracellular Ca²⁺ is omitted, the FLP- and the depolarization-induced contractions are significantly diminished; if 0.5 mM EGTA is included, both are completely blocked. In contrast, caffeine-induced contractions are not nearly as affected by the exclusion of extracellular Ca²⁺. Each data point is the mean ± SEM of % muscle fibers contracting, with n≥5 for each point. Each treatment is compared to control, **P<0.01, ***P<0.005.