Isoproterenol increases active lipoprotein lipase in adipocyte medium and in rat plasma

Abstract

White adipose tissue (WAT) lipoprotein lipase (LPL) activity channels diet fat towards storage in adipocytes. Adrenaline (ADR) is accepted to reduce WAT or adipocyte LPL activity (LPLa), but available data are not clear-cut regarding long exposure to ADR in vitro or in vivo. We studied the effects of long exposures to ADR or β-adrenergic agonist on LPL: in isolated rat adipocytes (3 h) and in rats (>1 day). Isoproterenol (ISO) (1 μM) did not alter LPLmRNA nor LPLa in adipocytes, but increased LPLa in medium more than twofold (3.58 ± 0.35 vs. 1.32 ± 0.35 mU/10 6 adipocytes, P < 0.001). Effect was time (not present at 1 h, clear at 2 h) and concentration dependent (high sensitivity from 10 to 100 nM, max at 1 μM). Adenylate cyclase activator or cyclic AMP (cAMP) analogue produced a similar increase. Thus in adipocytes ISO produced an increase in LPLa release and/or a decrease in extracellular LPLa degradation. ADR or ISO treated rats had a two to fourfold decrease in WAT LPLa vs. unchanged LPLmRNA. This decrease was 10-fold in WAT heparin-releasable LPLa (5.7 ± 0.6 vs. 57.3 ± 10.2 mU/g, P < 0.001), which represents peri/extracellular LPLa. Plasma LPLa was increased 11-fold by ADR (3.30 ± 0.58 vs. 0.32 ± 0.08 mU/ml, P < 0.001) whereas only threefold by ISO ( P > 0.01). We suggest that in vivo ADR increased release of active LPL to plasma from endothelial cells of LPL-rich tissue(s)—WAT was probably one of these tissues releasing LPL since it lost 90% of its peri/extracellular LPLa—and/or decreased degradation of plasma active LPL. Since liver LPLa was not increased, plasma active LPL might be kept away from hepatic degradation by binding to stabilising entities in plasma (fatty acids (FA), lipoproteins or soluble heparan sulphates (HS)). In conclusion, we believe this is the first report stating that: (a) ISO increases LPLa in isolated adipocyte medium, and (b) ADR administration to rats decreases WAT extracellular active LPL and increases preheparin plasma active LPL.