Force-frequency relationship, contraction duration and recirculating fraction of calcium in postnatally developing rat heart ventricles: correlation with heart rate.

Heart rates (HR) of awake unrestrained animals, isometric contraction duration and force-frequency relationship of ventricular tissue were determined in adult and postnatally developing rats. Resting HR was lowest in newborns (256 beats min-1), reached maximum at the age of 2.5 weeks (506 beats min-1) and then declined to the level of adult rats (381 beats min-1). Duration of isometric contraction correlated negatively with HR. Time to peak tension (TPT) was 185 ms in newborns but fell rapidly during the first days of post-natal life. Minimum was attained at the age of 2.5 weeks (TPT = 98 ms), followed by a slight prolongation towards adulthood. Recirculating fraction of activator Ca2+ increased parallel with HR, being 6% in newborns, 33% in 11-day-old pre-weanlings, and 87% in adult rats. Similar developmental pattern of the parameters suggests that a post-natal increase in HR and a shortening of contraction duration are closely associated with a shift from extracellular to intracellular source of activator Ca2+. Force-frequency curves were similar at different developmental stages and consisted of three phases; a negative staircase between 0.05 and 1.0 Hz, a positive staircase between 1.0 and 4.0 Hz, and a secondary decline above 4.0 Hz. In adult rats the positive force staircase was weak or absent. Furthermore, our results show that negative staircase is not only a property of adult rat heart but is present, and even more pronounced, in pre-weanling and weanling rat heart. Therefore negative staircase is not solely explained by quantitative changes in the contribution of sarcoplasmic reticulum (SR) to contractile activation, but rather by the mechanisms which regulate loading and/or release of sarcoplasmic reticular Ca2+.