Effects of caffeine on the mechanical properties of developing rat heart ventricles

The effects of caffeine on isometric contractions of right-ventricular strips during the postnatal development of the rat heart were studied. Caffeine (2-10 mM) had a positive inotropic effect on ventricular strips of 3-15-days-old rats but a negative inotropic effect on the muscles of rats older than 22 days. 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 10 mM caffeine. 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. It is suggested that the observed changes during the third postnatal week are due to a shift from extracellular to intracellular Ca2+ as the main source of this cation.