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Departament de Bioquímica i Biologia Molecular, Facultat de Biologia, Universitat de Barcelona, 08028 Barcelona, Spain
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, 
-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 (
), 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 in light phase); and 3)
LPL activity in all tissues studied (: heart in light phase;
skeletal muscle, IBAT, and EWAT in dark phase). Liver also showed a
circadian rhythm of LPL activity, with 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.
glucose; glycogen; cholesterol; glycerol; triacylglycerol; DNA; ketones; insulin; corticosterone; glucose paradox; epididymal white adipose tissue; brown adipose tissue; liver; skeletal muscle; plasma; heart; cosinor; mesor; acrophase
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