STIMULATION OF LUTEAL CELL PROGESTERONE PRODUCTION BY LIPOPROTEINS FROM COWS FED CONTROL OR FAT-SUPPLEMENTED DIETS

Plasma lipoproteins from lactating dairy cows fed 0 or 7% supplemental fat were examined for their composition and ability to stimulate luteal cell progesterone production in vitro. Ultracentrifugation was utilized to isolate blood lipoproteins, and heparin affinity chromatography allowed separation of lipoprotein fractions based on the presence (low density lipoproteins) or absence of apolipoprotein B (high density lipoproteins). A portion of high density lipoproteins was fractionated by size, utilizing gel filtration chromatography. Slaughterhouse corpora lutea were dissociated, and plasma lipoproteins were added to the luteal cells on d 3 of culture and incubated for 48 h. In Experiment 1, blood was collected from heifers fed a diet that was not supplemented with fat. The addition of cholesterol from large, high density lipoproteins with a high cholesterol to protein ratio to luteal cultures increased progesterone production by an average of 17% compared with the addition of cholesterol from small, high density lipoproteins with a low cholesterol to Protein ratio. In Experiment 2, electrophoretic mobility, apolipoprotein composition, and size of lipoproteins from control and fat-supplemented cows were similar. Lipoproteins from cows assigned to either a control or fat-supplemented diet showed no difference in their ability to stimulate progesterone production. Increased plasma progesterone concentration in lactating dairy cows fed supplemental fat does not appear to be mediated by alterations in lipoprotein composition.