Another possibility may be that the lower activity of myosin ATPase and abnormal myosin isoenzyme distribution could be responsible for the depressed contractile function in diabetic myocardium or that diabetic Ca2+ sensitivity changes in the myocardium are coupled with troponin T alterations . However, no data are available on intracellular Ca2+ homeostasis in the neonatal hearts of rats from diabetic mothers.Our results confirm the previously published observation that cardiac development during the early postnatal period – at least in rats – is not linear. Two different phases can be distinguished: before and after the fourth postnatal day. This common trend, not markedly changed in the offspring of diabetic mothers, is already reflected in the time course of cardiac growth. The heart weight to body weight ratio increases from birth to day 4, when it attains a maximum value in the rat . The biphasic time course of the inotropic response to Ca2+ may be hypothetically explained by a higher availability of intracellular Ca2+ on day 1 than on day 4 due to the disproportionate development of the two different membrane transport systems during this period: a decreasing leakage of immature cell membrane and a slowly increasing number of functional Ca2+ channels. The critical day 4 coincides closely with the age at which Anversa et al and Olivetti et al reported that large structural developmental differences occur between the left and the right ventricular myocardium. Recently, Lie et al observed that day 4 is decisive for the mode of cardiac growth because the proliferation activity of the cardiac myocytes approaches nil.
Our data have shown that body and heart growth as well as cardiac contractile function and inotropic responsiveness to extracellular Ca2+ in offspring from diabetic mothers are significantly influenced during the first postnatal week. Whether these changes are only transient and limited to the suckling period remains to be clarified. Moreover, our results point to the possible negative consequences of the clinical use of inotropic drugs in infants from diabetic mothers during early phases of postnatal development