BETA-ADRENERGIC MODULATION OF TRIGLYCERIDEMIA UNDER INCREASED ENERGY-EXPENDITURE

This study aimed to identify the metabolic steps involved in the acute hypotriglyceridemia brought about by increased energy expenditure (co ld exposure) and to assess the causative involvement of some determina nts of triglyceride (TG) metabolism as well as that of the beta-adrene rgic pathway. Rats were kept at 24 degrees C or exposed to 10 degrees C for 3 h after acute administration of the beta-adrenergic antagonist propranolol (Prop) or vehicle. Cold exposure increased the rate of TG secretion (Triton WR1339 method) into the circulation by 50% (P < 0.0 005), an effect that was blunted by Prop. The cold-induced increase in TG secretion was closely related to changes in circulating nonesterif ied fatty acid levels, but not with serum insulin concentrations. Desp ite an increase in TG secretion, serum TG levels after acute cold expo sure fell to 50% (P < 0.002) of those measured at 24 degrees C, indica ting that the lowering of serum TG; was entirely due to an increase in their rate of intravascular hydrolysis. This was confirmed by observi ng a 70% increase (P < 0.002) in the rate of clearance of an exogenous TG emulsion in cold-exposed rats compared with those kept in the warm . Prop treatment before cold exposure decreased (similar to 30%, P < 0 .005) the cold-induced stimulation of TG hydrolysis. The increased TG clearance rate in cold-exposed animals occurred in the absence of any change in the intravascular availability of lipoprotein lipase (LPL). In contrast, the activity of LPL displayed a tissue-specific response to cold exposure, being reduced by one-half in white adipose tissue (P < 0.0005) and increased in brown adipose tissue (130%, P < 0.0001) an d the heart (50%, P < 0.001). These findings show that, in the postpra ndial state, an acute increase in energy expenditure induced by cold e xposure results in a lowering of serum TG entirely due to an increase in their rate of intravascular hydrolysis and that serum TGs are lower ed despite an increase in the rate of TG secretion into the circulatio n. More efficient TG hydrolysis occurs independently of the intravascu lar availability of LPL. The study further shows that the effects of c old exposure on serum TG concentration and their rates of secretion an d clearance are in large part mediated by the beta-adrenergic pathway.