AN INCREASE IN THE SERUM-LIPIDS INCREASES LUTEAL LIPID-CONTENT AND ALTERS THE DISAPPEARANCE RATE OF PROGESTERONE IN COWS

To determine whether an increase in serum lipids alters the area occup ied by lipid droplets in steroidogenic luteal cells and(or) clearance rates of progesterone from serum, pregnant beef heifers received contr ol (n = 6) or treatment (n = 5) diets. To increase serum lipids, the t reatment diet contained calcium soaps of fatty acids. Control and trea tment diets were formulated to be isocaloric and isonitrogenous. Feedi ng of diets was initiated approximately 100 d before parturition and c ontinued through the third postpartum estrous cycle. On d 12 or 13 of the third postpartum cycle, corpora lutea were collected by ovariectom y and a center slice was processed for electron microscopy. Eight samp les fr om each slice were sectioned, stained, and examined at a magnif ication of 2,500x. Five micrographs per sample were analyzed for area occupied by small (SLC) and large (LLC) luteal cells, percentage of th e area of each steroidogenic cell type occupied by lipid, and total st eroidogenic area (SLC + LLC) occupied by lipid. Jugular blood was coll ected before and after ovariectomy, and progesterone, cholesterol, lip oprotein (HDL), and low-density (LDL) were quantified. Cows consuming treatment diets had approximately twice (P < .05) the concentration of cholesterol, HDL, and progesterone in serum that controls had. The pe rcentage of the area of SLC, LLC, and total area occupied by lipid was greater(P < .05) in treated than in control cows. The average time re quired for serum concentrations of progesterone to decrease by 50% aft er ovariectomy was greater (P < .05) in treated than in control cows ( 170 +/- 16 vs 113 +/- 15 min). Concentrations of progesterone in serum were correlated(P < .05) with the total steroidogenic area occupied b y lipid and with serum concentrations of cholesterol, HDL, and LDL. In summary, adding calcium soaps of fatty acids to diets increased conce ntrations of cholesterol, HDL, and progesterone in serum. The increase d progesterone was associated with increased lipid accumulation within the corpus luteum and a slower rate of disappearance of progesterone from serum.