Abstract
1. The effects of caffeine on isometric contractions of right-ventricular strips during the postnatal development of the rat heart were studied. 2. Caffeine (2–10 mM) had a positive inotropic effect on ventricular strips of 3–15-days-old rats but a negative inotropie effect on the muscles of rats older than 22 days. 3. Rest-twitch potentiation was most prominent in the muscles of 3–15-days-old rats but weakened clearly after that age. The potentiation was eliminated by 5 and 10mM caffeine. 4. An abrupt increase in frequency from 0.2 to 2 Hz caused a positive tension-staircase in 3–15-days-old rats but a negative staircase in older rats, the latter effect being eliminated by caffeine. 5. It is suggested that the observed changes during the third postnatal week are due to a shift from extracellular to intracellular Ca 2+ as the main source of this cation.
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