Glycogenolytic effects of the calcium ionophore A23187, but not of vasopressin or angiotensin, in foetal-rat hepatocytes

Glycogen autophagy in glucose homeostasis

Glycogen autophagy, the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective, hormonally controlled and highly regulated process, representing a mechanism of glucose homeostasis under conditions of demand for the production of this sugar. In the newborn animals, this process is induced by glucagon secreted during the postnatal hypoglycemia and inhibited by insulin and parenteral glucose, which abolishes glucagon secretion. Hormonal action is mediated by the cAMP/protein kinase A (induction) and phosphoinositides/mTOR (inhibition) pathways that converge on common targets, such as the protein phosphatase 2A to regulate autophgosomal glycogen-hydrolyzing acid glucosidase and glycogen autophagy. Intralysosomal phosphate exchange reactions, which are affected by changes in the calcium levels and acid mannose 6- and acid glucose 6-phosphatase activities, can modify the intralysosomal composition in phosphorylated and nonphosphorylated glucose and promote the exit of free glucose through the lysosomal membrane. Glycogen autophagy-derived nonphosphorylated glucose assists the hyaloplasmic glycogen degradation-derived glucose 6-phosphate to combat postnatal hypoglycemia and participates in other metabolic pathways to secure the fine tuning of glucose homeostasis during the neonatal period.

Glycogen autophagy

Glycogen autophagy, which includes the sequestration and degradation of cell glycogen in the autophagic vacuoles, is a selective process under conditions of demand for the massive hepatic production of glucose, as in the postnatal period. It represents a link between autophagy and glycogen metabolism. The formation of autophagic vacuoles in the hepatocytes of newborn animals is spatially and biochemically related to the degradation of cell glycogen. Many molecular elements and signaling pathways including the cyclic AMP/cyclic AMP-dependent protein kinase and the phosphoinositides/TOR pathways are implicated in the control of this process. These two pathways may converge on the same target to regulate glycogen autophagy.

Switching from alpha 1- to beta-subtypes in adrenergic response during primary culture of adult-rat hepatocytes as affected by the cell-to-cell interaction through plasma membranes

The alpha 1-adrenergic response was predominant over the beta-adrenergic one in adult rat hepatocytes, when the responses were measured as the agonist-induced generations of Ins(1,4,5)P3 and cyclic AMP, respectively. During primary culture of the adult rat hepatocytes, the beta-adrenergic response developed rapidly, whereas the alpha 1-response decreased gradually. Such receptor-subtype switching did not occur unless the cells were cultured under conditions favourable for cell growth, i.e. at low cell density (10(4) cells/cm2). The switching was prevented progressively as the cell culture density was increased up to 20-fold or the low-density culture was achieved by addition of increasing amounts of liver plasma membranes. The gradual decrease in alpha 1-response was accounted for by a concurrent decrease in the receptor site density, whereas rapid development of the beta-response definitely preceded the increase in beta-ligand binding sites during the culture. This rapid development of the beta-response reflected enhanced coupling of the receptor to G-protein during the early stage of culture, as evidenced by the progressively developed ability of GTP to lower the affinity of beta-agonist binding to membranes prepared from these short-time-cultured hepatocytes.

Differences in the accessibility of the beta-adrenergic receptor in isolated hepatocytes from foetal and adult rats

Beta-receptor number (measured by [3H]-CGP 12 177 binding) and beta-adrenergic response (measured by isoproterenol stimulated glucose liberation and isoproterenol stimulated adenylate cyclase activity) were compared in hepatocytes isolated from foetal (on day 22 of gestation), adult female and adult male rats. Beta-receptor numbers in crude membrane preparations of hepatocytes from adult female and adult male rats were found to be nearly equal (15.5 and 15.1 fmol/mg), but in crude membrane preparations of foetal rats beta-adrenergic receptor number was significantly higher (34.3 fmol/mg). Determination of number of beta-adrenergic surface receptors of intact hepatocytes showed relative high values in foetal rats (about 22,000/cell) and adult female rats (about 20,000/cell), but in male rats the number was less (about 6500/cell). Glucose liberation was stimulated by isoproterenol to the same extent in hepatocytes isolated from adult female and foetal rats (about 150% over basal), whereas no effect was found in hepatocytes isolated from adult male rats. Dose-response curves showed that in foetal rat hepatocytes glucose release was already increased by 10(-8) M isoproterenol, whereas in female rat hepatocytes at least 10(-6) M isoproterenol was required. Adenylate cyclase was stimulated by isoproterenol in lysates of hepatocytes from adult female rats by about 180% and from foetal rats by about 250%. No effects were observed using lysates of hepatocytes from adult male rats. We interpret the observed differences of beta-adrenergic responses between adult female and male rats as being primarily caused by different accessibility of the beta-receptor to the beta-agonist isoproterenol in intact hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

cAMP-independent stimulation of glycogen phosphorylase in newborn rat hepatocytes

Postnatal development of glycogen phosphorylase activation by the cAMP-independent pathway was examined in isolated rat hepatocytes from control and propylthiouracil-treated congenital hypothyroid rat pups. At 5 days postnatum there was complete phosphorylase activation by beta-adrenergic stimulation, glucagon, and the calcium ionophore A23187, but no activation by alpha-adrenergic stimulation. Activation of phosphorylase by angiotensin or vasopressin was less than in hepatocytes from adult rats (P less than 0.01). At 28 days postnatum activation by all of these hormones was complete. In the propylthiouracil-treated group hormone responsiveness was similar to the control at 5 days postnatum. However, alpha-adrenergic (P less than 0.025), angiotensin, and vasopressin (P less than 0.05) activation was decreased at 28 days postnatum, and beta-adrenergic, glucagon, and A23187 activation was complete. The attenuated responses were restored by thyroxine replacement from 15 days postnatum. [32P]Pi incorporation into phosphatidylinositol by epinephrine and vasopressin in 28-day propylthiouracil-treated rats was lower than the control (P less than 0.01). We speculate that the diminished phosphorylase response of hepatocytes to alpha-adrenergic, vasopressin, or angiotensin stimuli in the early neonatal period could be related to low receptor numbers and the weaker phosphoinositide response during this period. Also, the depressed phosphorylase response to alpha-adrenergic, vasopressin, and angiotensin stimulation in congenital hypothyroidism at 28 days postnatum could be related to a decrease in number of plasma membrane receptors for these agonists.