The effects of lanthanum and manganese on excitation-contraction coupling in the newborn rabbit heart.

The effects of 5 microM of lanthanum (La) and 0.5 mM of manganese (Mn) on myocardial mechanical function were studied in isolated, arterially perfused cardiac preparations of newborn and adult rabbits. The La-induced decrease in developed tension (DT) and maximal rate of tension development [+dT/dt(max)] was similar in the two age groups for the first 4 min of perfusion. La perfusion caused a significant rise in the resting tension (RT) of the newborn preparation, but not of the adult preparation. In the newborn, Mn perfusion resulted in a decrease in DT and +dT/dt (max) that was similar to the decrease induced by La. In the adult, however, the Mn-induced decrease in function occurred much more slowly than did that of La. In addition, the Mn-induced decrease in function in the newborn was significantly (p less than 0.005) greater than that of the adult. As with La, Mn caused a significant (p less than 0.001) increase in the RT of the newborn preparation only. These data show that there are significant age-related differences in the mechanical response of the myocardium to uncouplers of excitation-contraction. Furthermore, they suggest that the etiology of these differences may lie in the maturation of intracellular organelles that are responsible for the beat-to-beat regulation of myocardial contractility.