Stimulation of glucose production from glycogen by glucagon, noradrenaline and non-degradable adenosine analogues is counteracted by adenosine and ATP in cultured rat hepatocytes

Abstract

The glycogenolytic potency of adenosine and ATP was studied in adult rat hepatocytes and compared with the action of glucagon and noradrenaline. In cells cultured for 48 h, adenosine and ATP as well as their analogues 2-chloroadenosine, phenylisopropyladenosine, N-ethylcarboxamidoadenosine and 6,y-methylene-substituted ATP (p[CH2] [14C]Glucose production from glycogen was stimulated only 3-fold by ATP and adenosine, compared with a 7-fold increase produced by the hormones. Stimulation of glucose production by glucagon or noradrenaline was almost completely abolished by ATP or adenosine, with half-maximal effects at around 10 /SM. The non-degradable adenosine analogues were equipotent with glucagon with respect to stimulation of glucose production, and their action was also inhibited by adenosine. ATP and p[CH2]ppA, which were both degraded to adenosine, showed comparable metabolic effects, whereas the a,,l-methylene analogue was without biological action and also was not degraded to adenosine. In the presence of the adenosine transport inhibitor nitrobenzyl thioinosine (NBTI), adenosine exerted an increased glycogenolytic potency, reaching 80 % of the maximal stimulation obtained by glucagon. The glucagon-antagonistic effect of adenosine could be completely abolished by NBTI, but was not affected by phenyltheophylline. It is concluded that, in the hepatocyte culture system, adenosine and ATP decrease the catalytic efficiency of phosphorylase a through signals arising from their uptake into the cell. INTRODUCTIONThe activation of hepatic glycogenolysis by adenosine and ATP has recently received much attention. It is well documented that both purines activate glycogen phosphorylase [1][2][3][4] and increase glucose production in perfused livers from fed rats, an action comparable with the effects of noradrenaline and glucagon and of hepatic nerve stimulation [5][6][7]. The effects of adenosine are thought to be mediated by an increase in cyclic AMP concentration via P1 receptors [3,8,9], whereas the signal chain initiated by ATP appears to involve the breakdown of phosphatidylinositol to InsP3 and mobilization of intracellular Ca2+ via P2y receptors in a cyclic AMP-independent manner [10][11][12][13][14][15]. A number of studies with hepatocyte suspensions report ATP-and adenosine-dependent activation of glycogen phosphorylase [3,4,13,15,16], but very rarely has glucose production also been determined in this isolated cell system. There are few and conflicting reports on the glycogenolytic potency of adenosine and ATP in hepatocyte suspensions. Basal glycogenolysis has been reported to be not affected [17], slightly decreased [18] or increased [19,20] by these purines.During an investigation into the interaction of ATP and insulin in their effects on glycogen metabolism, we found that in the system used, the primary cultured adult hepatocyte, ATP inhibited glucose production from glycogen and at the same time activated glycogen phosphorylase to the same degree as did glucagon. The present study investigates this p...

The diacylglycerol and protein kinase C pathways are not involved in insulin signalling in primary rat hepatocytes

“…Hepatocytes from fed male Wistar rats (of weight 180–250 g) were isolated by recirculating collagenase perfusion in situ , purified by centrifugation through Percoll and cultured in M199 medium on 6‐cm plastic dishes [28]. For the first 3 h, medium contained 4% newborn calf serum, 1 n m insulin and 0.1 µ m dexamethasone.…”

Nucleoside uptake in rat liver parenchymal cells

“…Nevertheless NBTI-binding had been reported in earlier studies in liver plasma membrane vesicles [22,23]. Indeed, this analogue was extensively used as a nucleoside transport inhibitor in metabolic studies designed to determine the role of intracellular nucleosides in the control of key pathways such as glycogen synthesis and ureogenesis [24][25][26]. The first evidence of concentrative uptake of nucleosides was provided by using isolated rat hepatocytes and monitoring thymidine uptake [27].…”

Pharmacological Agents That Directly Modulate Insulin Secretion

“…The action of methylxanthines is to relax the smooth muscles of the bronchi and therefore produce a definite increase in vital capacity of the lungs. There are three known potential molecular mechanisms of action of this class of drug: they are an ability to 1) translocate intracellular calcium; 2) inhibit phosphodiesterase and thereby increase intracellular concentrations of cAMP and cGMP; and 3) bind to and antagonize the known P 1 purinoceptors, and consequently raise intracellular cAMP levels in ␤-cells, which contain A 1 receptors (Hillaire-Buys et al, 1994), and are negatively coupled to AC and prevent the rise of cAMP in hepatocytes, which contain A 2 receptors (Oetjen et al, 1990), and are positively coupled to AC. With aminophylline doses used therapeutically (plasma theophylline levels of the drug should be between 5 and 15 M) it is only the latter of the three that is considered to be operative.…”