MATURATION AND SECRETION OF RAT HEPATIC LIPASE IS INHIBITED BY ALPHA(1B)-ADRENERGIC STIMULATION THROUGH CHANGES IN CA2- THAPSIGARGIN AND EGTA BOTH MIMIC THE EFFECT OF ADRENALINE( HOMEOSTASIS )

In rats, the daily changes in hepatic lipase (HL) activity in the live r follow the diurnal rhythm of the catecholamines. To study the underl ying mechanism, the effect of adrenaline on maturation and secretion o f HL was determined in freshly isolated rat hepatocytes. Adrenaline (1 0 mu M) acutely inhibited the secretion of HL. This effect was abolish ed by 0.1 mu M prazosin, but not by 1 mu M propranolol, indicating the involvement of the alpha(1)-adrenergic pathway. Prazosin was at least 1000-fold more potent than WB4101, a selective alpha(1A)-antagonist. Adrenaline had no effect on HL secretion in hepatocytes pretreated wit h chloroethylclonidine, an irreversible alpha(1B)-selective antagonist . Inhibition of HL was not induced by 10 mu M methoxamine, a alpha(1A) -selective agonist. Thus, adrenaline inhibited HL secretion through ac tivation of the alpha(1)-adrenoceptors subtype B, which have been show n to signal through Ca2+ as well as cAMP. A similar reduction in HL se cretion was induced by the Ca2+-mobilizing hormones angiotensin II (10 0 nM) and vasopressin (12 nM), the Ca2+ ionophore A23187 (2 mu M), and by thapsigargin (1 mu M), which inhibits the ER Ca2+-ATPase pump. HL secretion was unaffected by elevating cAMP with 10 mu M forskolin or 1 mu M 8-Br-cAMP. These results suggest that the alpha(1B)-adrenergic e ffects on HL expression are mainly mediated through elevation of intra cellular 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 H L from the 53 kDa, Endo H-sensitive precursor to the Endo H-resistant, catalytically active protein of 58 kDa. In addition, adrenaline induc ed intracellular degradation of newly synthesized HL. Similar post-tra nslational effects, both qualitatively and quantitatively, were observ ed 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+.