“Log dose-response curves for caffeine and theophylline in presence of practolol showed that both methylxantines caused a decrease in systolic tension, in diastolic relaxation and in net force of contraction (ED 50 1–2 mM and 3–5 mM for caffeine and theophylline respectively).”
“ Practolol (0·07 mM) abolished on the other hand arrhytmia induced by calcium (50 mM) or caffeine (0· mM).”