Morphological adaptation of the left and right coronary arteries of neonatal rats was studied using the whole body freezing technique. Newborn rats were fixed by using acetone cooled to -80°C by dry ice on 0(n=20), 1(n=13), 2(n=18),4(n=16) days after birth. The thorax of the frozen rat was sectioned on a freezing microtome in the coronal plane. These sections of the heart were photographed and the inner diameters of the two coronary arteries were measured. Biventricular masses were calculated as follows; all the cross-sectional areas of the ventricular muscles multiplied by 0.3 mm(thickness) were summed up. Left ventricular mass increased rapidly after birth (Oday; 7.9±0.2mm3, 1 day; 11.0±0.5mm3, p<0.01, 2 days; 13.8±0.5mm3,p<0.01, 4days; 22.0±0.7mm3,p<0.01). There was no significant change of the diameter of left coronary artery from 0 day after birth (122±4 μ m) to 2days after(128±2 μ m). Since then, it significantly increased(4days; 165±5 μ m, p<0.01). Right ventricular mass increased after birth (0day; 9.1±0.3mm3, 1day; 10.7±1.4 mm3, 2 days; 12.4±2.4mm3, p<0.01, 4days; 18.3±0.8mm3, p<0.01). The diameter of right coronary artery also significantly increased since 2days after birth(0day; 121±4 μ m, 2days; 123±3 μ m, 4days; 137±4 μ m, p<0.01). In neonatal rats right after birth, there were time lags in the morphological adaptation (increase of the diameters) of the coronary arteries to rapid increase of the ventricular masses.
