MODULATION OF LIPOPROTEIN-LIPASE ACTIVITY IN THE RAT BY THE BETA-2-ADRENERGIC AGONIST CLENBUTEROL

This study evaluated the effects of beta2-adrenoceptor stimulation on some determinants of triglyceride metabolism. Male Sprague - Dawley rats were injected twice daily with clenbuterol (30 mug . kg-1) for 7 days, or with an equivalent volume of vehicle. Serum triglycerides, hepatic triglyceride secretion rate, and lipoprotein lipase activity in white and brown adipose tissues as well as in red vastus lateralis muscle and heart were evaluated in the fasting state and following a fat-free, high-sucrose meal, 3 h after the last agonist injection. In rats killed in the fasting and postprandial states, clenbuterol reduced the mass of white adipose tissue (-25 and -12%, respectively; p < 0.02), whereas it increased the mass of vastus lateralis muscle (+11 and +7%; p < 0.002) and heart (+13 and +10%; p < 0.0001). In vehicle-injected animals, the fasting state was associated with lower lipoprotein lipase activity in white and brown adipose tissues, and higher enzyme activity in vastus lateralis and heart, compared with the postprandial state. Postprandially, treatment with clenbuterol reduced lipoprotein lipase activity in white adipose (-24%), whereas it increased enzyme activity in brown adipose (+107%) as well as in vastus lateralis (+35%). In fasted animals, no significant variation of enzyme activity in these tissues was observed following clenbuterol treatment, whereas in the heart, a decrease of lipoprotein lipase activity was observed (-22%). Clenbuterol lowered serum triglycerides significantly (-23%), but not their rate of secretion, whereas the agonist decreased the insulin to glucagon ratio only in the postprandial state. These results suggest that modulation of triglyceride-rich lipoprotein metabolism, particularly tissue-specific alterations in lipoprotein lipase activity, may be one of the pathways through which clenbuterol affects partitioning of lipid substrates.