Plasma lipids and lipoproteins of fasted ponies

SUMMARY Plasma lipids and lipoproteins were studied in a group of healthy ponies in which food was withheld for 208 hours. Increases in plasma triglycerides and cholesterol and loss of weight were consistent with increases in free fatty acids circulating to the liver of these animals due to lipid mobilization from adipose tissue. Plasma triglycerides underwent the greatest absolute increase, with 30 times higher levels at the end of the fasted period than those of the fed control animals. Plasma cholesterol concentrations increased to a lesser extent and appeared to plateau at a level twice that of the control group. Total and direct plasma bilirubin concentrations increased to about 5 times their normal value by day 2 of the fast and seemed to plateau at this time. Separation of the plasma lipoproteins on agarose-gel electrophoresis indicated an increase of the pre-β band in the fasted animals. Isolation of the lipoproteins at the commonly used density ranges for human lipoproteins revealed the pre-β band as having a hydrated density of < 1.006 g/ml which is consistent with a very low-density lipoprotein (vldl) fraction. Compositional analysis of the various lipoproteins indicated no appreciable differences among the low-density lipoprotein (ldl) and high-density lipoprotein (hdl) fractions in fed or fasted animals. The vldl from fasted ponies, however, although containing about the same relative amount of triglyceride, had 50% less protein and 50% more cholesterol than did the vldl of fed ponies. Consequently, vldl in fasted ponies was triglyceride-enriched when compared with its lower protein content. The concentration of this lipoprotein fraction also was appreciably higher in the fasted ponies. Although the equine liver appears able to secrete large amounts of triglyceride in the form of vldl in response to a large fatty acid influx, interferences with the normal metabolism of this vldl may be occurring leading to the hyperlipoproteinemia observed in the present studies.