Circadian rhythms of lipoprotein lipase and hepatic lipase activities in intermediate metabolism of adult rat

Although intermediate metabolism is known to follow circadian rhythms, little information is available on the variations in lipoprotein lipase (LPL) and hepatic lipase (HL) activities during the 24-h period, and there is also a lack of adequate statistical analysis. Here, adult male rats were fed ad libitum and kept at 21°C under 12:12-h light-dark cycles. They were killed in batches every 3 h over a 24-h period. Lipase activities were determined in plasma and fresh homogenates of epididymal white adipose tissue (EWAT), interscapular brown adipose tissue (IBAT), heart, skeletal muscle, and liver. Plasma insulin, corticosterone, glucose, triacylglycerol (TAG), cholesterol, glycerol, β-hydroxybutyrate, and liver and muscle glycogen were determined. Cosinor analysis was used to evaluate the presence (significance of fit of cosine curve to data and variance explained by rhythm) and characteristics of possible circadian rhythms [acrophase (φ), mesor, and amplitude]. Statistically significant circadian rhythms were detected for 1 ) all metabolites studied, except TAG, cholesterol, and liver HL activity; 2 ) LPL and HL activity in plasma (both φ in light phase); and 3 ) LPL activity in all tissues studied (φ: heart in light phase; skeletal muscle, IBAT, and EWAT in dark phase). Liver also showed a circadian rhythm of LPL activity, with φ near that in plasma. These findings demonstrate for the first time that, in physiological conditions, LPL activities in plasma and various tissues, including liver, and HL activity in plasma follow circadian rhythms. Their metabolic significance is discussed.