Maturation and secretion of rat hepatic lipase is inhibited by α1B-adrenergic stimulation through changes in Ca2+ homoeostasis: thapsigargin and EGTA both mimic the effect of adrenaline

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 μM) acutely inhibited the secretion of HL. This effect was abolished by 0.1 μM prazosin, but not by 1 μM propranolol, indicating the involvement of the α₁-adrenergic pathway. Prazosin was at least 1000-fold more potent than WB4101, a selective α_1A-antagonist. Adrenaline had no effect on HL secretion in hepatocytes pretreated with chloroethylclonidine, an irreversible α_1B-selective antagonist. Inhibition of HL was not induced by 10 μM methoxamine, a α_1A-selective agonist. Thus, adrenaline inhibited HL secretion through activation of the α₁-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 μM), and by thapsigargin (1 μM), which inhibits the ER Ca²⁺-ATPase pump. HL secretion was unaffected by elevating cAMP with 10 μM forskolin or 1 μM 8-Br-cAMP. These results suggest that the α_1B-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⁺.