Effect of extracellular calcium on myocardial mechanical function in the neonatal rabbit.

The effects of extracellular calcium ([Ca2+]0) and barium ([Ba2+]0) on myocardia mechanical function were studied in the isolated arterially perfused neonatal and adult rabbit heart. The force of cardiac contraction per gram tissue in the newborn was approximately one third that in the adult for [Ca]0 of 1.5 mM. One half maximal values of developed tension (DT) and maximal tension first derivative [+dT/dt (max)] in the newborn were observed at [Ca2+]0 that were significantly higher than those in the adult. The maximal inotropic effect of calcium was observed at [Ca2+]0 of 10 mM in the adult and 30 mM in the newborn. Calcium binding to the isolated sarcolemma in the newborn and adult were identical. Cellular calcium in the newborn was significantly less than that in the adult. Barium infusion caused equal contracture in the newborn and adult, and the time to half maximal contracture was not different in the two age groups. These data indicate that (1) sarcolemmal permeability to divalent cations is similar in the newborn and adult rabbit, and (2) although calcium influx is similar in the two age groups, the amount of contractile calcium in the newborn is less than that in the adult for all [Ca]0.