The stimulation of glycogenolysis in isolated hepatocytes by opioid peptides

Abstract

The opioid peptides [Leu]enkephalin and dynorphin-(1-13) were shown to enhance glycogen breakdown when added directly to hepatocytes. This was the result of a concerted effect on the enzymes of glycogen metabolism, with a stimulation of glycogen phosphorylase activity and a simultaneous decrease in glycogen synthase I activity. The latter only became significant when the enzyme was activated by incubating the cells in presence of 20 mM- or 40 mM-glucose. The effect of the opioid peptides was independent of an increase in cyclic AMP or any change in the activity ratio of the cyclic AMP-dependent protein kinase and was abolished by depleting the cells of Ca2+. Both [Leu]enkephalin and dynorphin-(1-13) produced a significant decrease in cyclic AMP formation, suggesting that in liver, as in neuronal tissue, they may act by inhibiting adenylate cyclase activity.

Changes in free cytosolic calcium and accumulation of inositol phosphates in isolated hepatocytes by [Leu]enkephalin

“…The refractoriness of the cells to [Leu]enkephalin noted previously with glycogen mobilization (Leach & Titheradge, 1986) was clearly evident in quin-2-loaded hepatocytes, in that, after stimulation by [Leu]enkephalin, a further addition of the opioid peptide, phenylephrine or vasopressin failed to increase [Ca2+]1, even when the cytosolic concentration of Ca2+ had returned to resting values, unless a sub-maximal dose of the enkephalin was used (results not shown).…”

“…The water-soluble derivatives of the myo-[2-3H(n)]inositol were separated by loading 0.5 ml of the neutralized cell extracts in 10 ml of water on to 7 cm columns of Dowex 1 (X1O; 100-200 mesh; formate form) prepared as described by Creba et al (1983). The columns were sequentially eluted with 20 ml of water to remove free inositol, 20 ml of 60 mM-ammonium formate to remove glycerophosphoinositol ('GroPIns fraction'), 20 ml of 180 mM-ammonium formate to remove inositol monophosphate ('InsP fraction'), 30 ml of400 mM-ammonium formate/0.1 M-formic acid to remove inositol bisphos- Previous studies have suggested that the effects of the enkephalins and dynorphin-(1-13) on hepatic carbohydrate metabolism are mediated by a Ca2+-dependent mechanism (Allan et al, 1983;Leach et al, 1985;Leach & Titheradge, 1986). We have therefore examined the effect of addition of [Leu]enkephalin to isolated hepatocytes on the mobilization of intracellular Ca2+, as judged by the increase in fluorescence of the Ca2+ chelator quin-2.…”

“…We have previously postulated that the effect of the enkephalins and dynorphin-(1-13) on glycogenolysis is the result of a Ca2+-dependent activation of phosphorylase a, presumably mediated through phosphorylase kinase (Leach et al, 1985;Leach & Titheradge, 1986). The time of onset for the increase in phosphorylase a and…”

“…In previous studies, we have shown that challenging hepatocytes with [Leu]enkephalin results in a stimulation of glycogenolysis and gluconeogenesis through a cyclic AMP-independent mechanism (Allan et al, 1983;Leach et al, 1985;Leach & Titheradge, 1986). The effect is dependent on the availability of extracellular Ca2+, and therefore it has been suggested that the opioid peptides act in the liver by a similar mechanism to that employed by the vasoactive peptides angiotensin and vasopressin (Leach& Titheradge, 1986). To investigate thispossibility, the effect of [Leu]enkephalin on Ca2+ mobilization and inositol phosphate production has been examined.…”

Opioid peptides and metabolic regulation

“…These findings were not confirmed by Brubaker et al [25] who, evaluating the effects of physiological and supraphysiological concentrations of ]3-endorphin on isolated rat hepatocytes, were unable to show any influence of the opioid on both hepatic glucose production and glycogen phosphorilase a activity. Differences in the method of cell preparation and maintenance might have been important to explain the divergent findings obtained in the aforementioned studies and in other reports of the literature [26,27]. Met-enkephalin and Leu-enkephalin have also been reported to stimulate glucose production by rat hepatocytes, but only at very high concentrations [27].…”