Morphological changes of the plasma membrane in the white adipose cell associated with lipid mobilization were assessed qualitatively and quantitatively on freeze-fracture replicas of epididymal adipose tissue from fasted and from streptozotocin-diabetic rats. The number of plasma membrane invaginations and intramembranous particles were evaluated per square micrometer of membrane and per entire adipocyte. These two determinations show that the number per square micrometer (local concentration) of both structural features progressively increases with the duration of diabetes and fasting, while that at the same time their number per entire cell (total content) remains unchanged. These data thus show: (a) a reorganization of the adipose cell plasma membrane during lipolysis; and (b) that this reorganization can be detected only by determining the concentration and the total content of the structural features of the membrane involved.Previous studies of the changes induced by lipolysis in the morphology of the white adipose cell have shown an increase in the number of microvesicular invaginations (so-called "pinocytosis") of the plasma membrane during lipolysis (13,14,16,19,22,23). However, these data remained essentially qualitative since they were based on the examination of conventional thin sections which were not assessed by a morphometric approach. The application of the freeze-fracturing technique (11) to the adipose tissue has made it possible to study large areas of the cell membrane and quantitatively to assess structural features of the adipocyte plasma membrane such as membrane invaginations and intramembranous particles under various experimental conditions. For example, we have been able to demonstrate quantitative changes in the organization of the adipose cell plasma membrane induced by lipolytic and lipogenic hormones applied to the adipose tissue in vitro (3). In the present work, we have extended these studies by assessing quantitatively, in freezefracture replicas, the adipose cell membrane organization in the course of an acute lipolysis induced in vivo by fasting or streptozotocin diabetes.
MATERIALS AND METHODS
AnimalsYoung male Wistar rats (200-290 g) were used throughout.Fasting 20 animals were placed in individual cages containing water but no food while 6 others used as controls were fed normally with Purina Chow. Six rats of the fasting group were killed after 3 days, six after 6 days, and four after 9 days (four died at the 8th day of fasting).
Diabetes18 rats were injected in the tail vein with streptozotocin (75 mg/kg) kindly supplied by Dr. W. Dulin (The 104