Diurnal Changes in Plasma and Liver Lipids and Lipoprotein Lipase Activity in Heart and Adipose Tissue in Rats Fed a High and Low Fat Diet

In order to evaluate a) the respective roles of adipose and muscle lipoprotein lipase (LPL) in the clearing of alimentary lipemia and b) the role of the resulting nonesterified fatty acids (NEFA) in controlling hepatic ketogenesis and liver triglyceride content, a number of parameters related to lipid metabolism were studied over the 24 hour period (the dark period being from 1930 to 0730 hours), in rats ad libitum fed either a low-fat (LF) or a high-fat (HF) diet containing respectively 1.1% and 41.5% lard. During spontaneous feeding (from 1500–1800 hours onwards), LPL activity in LF rats increased in adipose tissue and decreased in heart; in rats fed the HF diet for 3 weeks, the postprandial rise in adipose LPL was smaller, and there was no decrease in heart LPL before 2100 hours. In HF rats, unlike the LF, feeding resulted in a large increase in circulating NEFA and total ketone concentrations, an increase in liver β-hydroxybutyrate dehydrogenase activity, and a decrease in hepatic triglyceride content. The findings clearly indicate that in HF rats, muscle LPL controls the postprandial rise in plasma NEFA concentrations, which in turn appear to determine the extent of ketonemia and liver triglyceride changes. The possible control of these metabolic events by insulin is discussed.