Developmental changes in mitochondria during the transition into lactation in the mouse mammary gland. II. Membrane marker enzymes and membrane ultrastructure

Abstract

The activity of cytochrome oxidase (an inner mitochondrial membrane marker) in mouse mammary gland homogenates was found to increase five- to sixfold from late pregnancy to day 8 of lactation, while that of monoamine oxidase (an outer membrane marker) increased only about 25%. The specific activity of cytochrome oxidase in the isolated mitochondria decreased slightly over the same period while the specific activity of monoamine oxidase decreased fivefold. This reflects the fact that both cytochrome oxidase and mitochondrial protein are increasing at a much greater rate than is monoamine oxidase activity. Mixing experiments preclude the possibility that the release or removal of an inhibitor or stimulator produces the changes in enzymatic activity. The cytochrome oxidase to monoamine oxidase ratio was followed throughout the pregnancy-lactation cycle in total mammary homogenates, isolated mammary parenchymal cells, and isolated mammary mitochondria. In each preparation the pattern was the same with little change in the ratio until late pregnancy; and then a three- to fourfold increase occurred and the values reached a maximum by day 8 of lactation. These experiments were interpreted as demonstrating that the observed enzymatic changes are reflective of alterations in the mitochondria of the mammary parenchymal cell population. Electron micrographs of mid-pregnant and mid-lactating mammary parenchymal cells in situ were prepared, and distinct changes in the mitochondrial morphology noted. The most significant and obvious change is the large increase in the number of inner membrane cristae and an increase in matrix density in the lactating gland cell. Therefore, both enzymatic and morphological studies support the concept of an expansion of the mitochondrial inner membrane during presecretory differentiation in the mouse mammary parenchymal cell.

PIP: The enzyme markers for mitochondrial inner and outer membranes throughout the pregnancy-lactation cycle in the mouse were compared. The ultrastructural changes of the organelle during the transitions were studied by electron microscopy. The activity of cytochrome oxidase in mouse mammary gland homogenates was found to increase 5- to 6-fold from late pregnancy to Day 8 of lactation, while that of monoamine oxidase in the isolated mitochondria decreased slightly over the same period while the specific activity of monoamine oxidase decreased 5-fold. The cytochrome oxidase to monoamine oxidase ratio was followed throughout the pregnancy-lactation cycle in total mammary homogenates, isolated mammary parenchymal cells, and isolated mammary mitochondria. The pattern was the same in each preparation with little change until late pregnancy and then a 3- to 4-fold increase occurred and values reached a maximum by Day 8 of lactation. Electron micrographs of midpregnant and midlactating mammary parenchymal cells in situ were prepared, and changes in the mitochondrial morphology noted. The most significant change is the large increase in number of inner membrane cristae and an increase in matrix density in the lactating gland cell.