The perinatal rat: body weight, hematocrit and regional changes in heart weight and lactate dehydrogenase isozyme composition and activity.

Body weight (BW), hematocrit (Hct), heart weight (HW) and cardiac lactate dehydrogenase (LDH) activity and isozyme pattern were studied in the perinatal rat. BW increased linearly, from 5 days before birth till 10 days after birth, while Hct increased from 30 to 34% within 1 day of birth. HW increased in step with BW. However, the relative contribution to total HW of right ventricle (RV), interventricular septum (S) and atria declined relative to the left ventricle free wall (LVW) beginning 2 to 3 days before birth. RV/LVW declined steadily throughout the study period. LDH specific activity in LVW, RV, S and atria increased greatly prior to birth and less so afterwards, with atria showing the lowest values throughout the study. Total LDH activity in each portion of the heart paralleled the respective regional weight changes. LDH isozyme composition (percent M subunit) declined at the time of birth in LVW, RV and S from 63 to 43%; change in atrial M subunit was smaller. The change in LDH isozyme composition could be accounted for in LVW, RV and S by increasing H subunit activity alone. In atria, both M and H subunit activity increased.