Abstract
In rats, the daily changes in hepatic lipase (HL) activity in the liver follow the diurnal rhythm of the catecholamines. To study the underlying mechanism, the effect of adrenaline on maturation and secretion of HL was determined in freshly isolated rat hepatocytes. Adrenaline (10 microM) acutely inhibited the secretion of HL. This effect was abolished by 0.1 microM prazosin, but not by 1 microM propranolol, indicating the involvement of the alpha1-adrenergic pathway. Prazosin was at least 1000-fold more potent than WB4101, a selective alpha1A-antagonist. Adrenaline had no effect on HL secretion in hepatocytes pretreated with chloroethylclonidine, an irreversible alpha1B-selective antagonist. Inhibition of HL was not induced by 10 microM methoxamine, a alpha1A-selective agonist. Thus, adrenaline inhibited HL secretion through activation of the alpha1-adrenoceptors subtype B, which have been shown to signal through Ca2+ as well as cAMP. A similar reduction in HL secretion was induced by the Ca2+-mobilizing hormones angiotensin II (100 nM) and vasopressin (12 nM), the Ca2+ ionophore A23187 (2 microM), and by thapsigargin (1 microM), which inhibits the ER Ca2+-ATPase pump. HL secretion was unaffected by elevating cAMP with 10 microM forskolin or 1 microM 8-Br-cAMP. These results suggest that the alpha1B-adrenergic effects on HL expression are mainly mediated through elevation of intracellular Ca2+. Chelation of extracellular Ca2+ and subsequent lowering of intracellular Ca2+ with EGTA also inhibited HL secretion. In pulse-chase experiments, adrenaline was shown to inhibit the maturation of HL from the 53 kDa, Endo H-sensitive precursor to the Endo H-resistant, catalytically active protein of 58 kDa. In addition, adrenaline induced intracellular degradation of newly synthesized HL. Similar post-translational effects, both qualitatively and quantitatively, were observed with A23187, thapsigargin and EGTA. We conclude that the inhibition of HL maturation and increase in intracellular degradation induced by catecholamines, A23187, thapsigargin and EGTA is evoked by changes in Ca2+ homoeostasis, possibly through lowering ER Ca2+.