Comparison of the effects of caffeine and a 2 alkyl 1,2,3 benzotriazinium iodide on frog rectus abdominis

The mode of action of 2-n-propyl-4-p-tolylamino-1,2,3-benzotriazinium iodide (TnPBI), which induced contractures of frog rectus abdominis muscle, was investigated. TnPBI caused contractures of frog rectus abdominis when the muscle had been depolarized with potassium chloride. Experiments with TnPBI and caffeine in calcium-free Ringer suggested that both compounds produce contractures by releasing intracellular bound calcium. It is suggested that at least two calcium stores are involved, one of which is sensitive to caffeine while both are sensitive to TnPBI.