Changes in the composition of mammary tissue, liver and muscle of rat dams during lactation and after weaning.

This investigation was conducted to evaluate the changes in the total content of protein and RNA in liver and muscles of rat dams before and after acute separation from their litters. Groups of 8-12 rats fed ad libitum were killed on the 12th (group L12) and 20th d (group L20) of lactation and on the 1st (group W1) and 7th d (group W7) after weaning. Nonpregnant, nonlactating rats paired for age served as controls. Dry matter, protein, DNA and RNA levels of the mammary gland, liver and muscles of the right hindlimb were determined. Wet weight and total organ or tissue protein, DNA and RNA were higher in mammary glands of L12 and L20 than in age-matched controls. These values were lower in groups W1 and W7 than in the lactating groups. No changes were noted in the total liver protein or DNA content, but total liver RNA was greater in groups L12 and L20 than in controls or group W1. Total muscle dry matter, DNA and RNA were significantly lower in groups L12 and L20 than in groups W1 and W7. Muscle protein content increased progressively from the 12th to 20th d of lactation to a peak in group W1, and it decreased to values found in age-matched controls in group W7. Although the muscle protein mass of the hindlimb during peak lactation (group L12) was only 63% of that in nonlactating control rats, within 1 d of weaning it was significantly higher than in nonlactating rats. Similar changes in RNA suggest that these changes in protein content are related to an adaptative mechanism designed to handle the surplus of plasma amino acids not used by the mammary gland after weaning.