 Caffeine, which stimulates intracellular Ca# + release channels known as ryanodine receptor (RyR) channels, induces contraction of individual muscle fibres dissociated from the trematode Schistosoma mansoni, and the turbellarians Dugesia tigrina and Procerodes littoralis. Caffeine is much more potent on S. mansoni fibres (EC &! 0n7 m) than those from D. tigrina or P. littoralis (3n2 m and 4n6 m, respectively). These caffeine-induced contractions are blocked by ryanodine, confirming

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... e presence of functional RyR channels in these flatworm muscles. However, the contractions are not blocked by typical RyR channel blockers ruthenium red or neomycin, indicating that there may be important pharmacological differences between the RyR channels in this early-diverging phylum and those of later animals. These studies demonstrate that RyR channels are present in the muscle of these flatworms, and that the sarcoplasmic reticulum stores sufficient Ca# + to support contraction.