Enzymatic changes associated with the initiation and maintenance of lactation in the rat.

The changes in the activities of a number of enzymes, representing various metabolic activities, in pyridine nucleotides, in levels of deoxyriboand ribonucleic acid, and in numbers of nuclei in mammary glands removed from rats at various stages of pregnancy and lactation have been documented. The onset of lactation is accompanied by a rapid 3to 4-fold increase in numbers of nuclei and in DNA levels, followed by a rapid increase in the activities of all the enzymes investigated. The increases in enzyme activities which occur during the first 3 days of lactation appear to be of a pattern similar to that of the tissue after 15 days of lactation, indicating that the newly formed cells acquire an enzymatic complement characteristic of fully lactating tissue very soon after their formation. It is suggested that the apparent preferential increases, during lactation, in the activities of enzymes directly associated with the synthesis of milk components are due to the fact that their activities are very low in nonsecretory tissue and that a significant proportion of the enzymes extracted from mammary tissue during early lactation arises from nonsecretory tissue. Under these conditions enzyme activities present in high levels in nonsecretory tissue would not increase dramatically during lactation.