Role of glucose in modulating Mg²⁺homeostasis in liver cells from starved rats

Abstract

α1- and β-Adrenoceptor stimulation elicits Mg2+ extrusion from liver cells in conjunction with hepatic glucose output (T. Fagan and A. Romani. Am J Physiol Gastrointest Liver Physiol 279: G943–G950, 2000.). To characterize the role of intrahepatic glucose on Mg2+ transport, male Sprague-Dawley rats were starved overnight before being anesthetized and used as organ donors. Perfused livers or collagenase-dispersed hepatocytes were stimulated by α1 (phenylephrine)- or β (isoproterenol)-adrenergic agonists. Mg2+ extrusion was assessed by atomic absorbance spectrophotometry. In both experimental models, the administration of pharmacological doses of adrenergic agonists did not elicit Mg2+ extrusion. The determination of cellular Mg2+ indicated an ∼9% decrease in total hepatic Mg2+ content in liver cells after overnight fasting, whereas the ATP level was unchanged. Hepatocytes from starved rats accumulated approximately four times more Mg2+ than liver cells from fed animals. This enlarged Mg2+ accumulation depended in part on extracellular glucose, since it was markedly reduced in the absence of extracellular glucose or in the presence of the glucose transport inhibitor phloretin. The residual Mg2+ accumulation observed in the absence of extracellular glucose was completely abolished by imipramine or removal of extracellular Na+. Taken together, these data indicate 1) that hepatic glucose mobilization is essential for Mg2+ extrusion by adrenergic agonist and 2) that starved hepatocytes accumulate Mg2+ via two distinct pathways, one of which is associated with glucose transport, whereas the second can be tentatively identified as an imipramine-inhibited Na+-dependent pathway.

Cell (patho)physiology of magnesium

“…These results indicate that extracellular and/ or intracellular Mg may modulate glucose transport and/or utilization [50]. In hepatocytes, the same authors showed that glucose mobilization was essential for Mg extrusion induced by adrenergic agonists and that Mg 2+ transport through the hepatocyte plasma membrane was altered under diabetic conditions as a result of a decrease in intracellular glucose [51,52].…”

Biphasic effect of extra-reticular Mg2+ on hepatic G6P transport and hydrolysis

“…Hepatocytes were isolated as previously described [6]. Fed male Sprague-Dawley rats (230-250 g body weight) were anesthetized by i.p.…”

“…Following isolation, hepatocytes were washed twice in a medium containing 120mM NaCl, 3mM KCl, 1.2mM KH 2 PO 4 , 1.2mM CaCl 2 , 1.2mM MgCl 2 , 10mM HEPES, 10mM glucose, 12mM NaHCO 3 (pH 7.35) equilibrated with O2: CO2 (95:5 v/v), and resuspended therein, at a final volume of 30 ml. After passage through a Percoll gradient [6] to maximize cell viability, the hepatocytes were resuspended in the medium described above, at a final concentration of 1 × 10 5 cells/ml. Trypan blue exclusion test indicated a viability of 95±3% (n=10), which did not change significantly over 2 hours (94±4%, n= 10).…”

“…The list includes enzymes regulating glycolysis and glucose homeostasis [4], as well as regulatory enzymes such as adenylyl cyclase [4]. The existence of a 'link' between glucose utilization and Mg 2+ homeostasis is supported by the observation that liver cells mobilize Mg 2+ in addition to glucose in response to the exogenous administration [5] or the endogenous release [6] of glucagon or catecholamine. Technical limitations, however, have hampered our ability to properly relate changes in total or free cellular Mg 2+ content with variations in Mg 2+ content within specific cellular compartments, de facto preventing us from establishing cause-effect correlation between hormone-mediated Mg 2+ fluxes at the cellular level with changes in Mg 2+ concentration and activity of specific enzymes within cellular compartments.…”

Reduced cellular Mg2+ content enhances hexose 6-phosphate dehydrogenase activity and expression in HepG2 and HL-60 cells

“…Catecholamine and glucagon, hormones that stimulate glycogenolysis and glucose output from the liver to maintain euglycemia, also elicit Mg 2+ extrusion from liver cells [5,6]. Conversely, hormones promoting glucose entry into the hepatocyte (e.g.…”