Control of hepatic nitrogen metabolism and glutathione release by cell volume regulatory mechanisms

Abstract

1. Urea synthesis was studied in isolated perfused rat liver during cell volume regulatory ion fluxes following exposure of the liver to anisotonic perfusion media. Lowering of the osmolarity in influent perfusate from 305 mOsm/l to 225 mOsm/l (by decreasing influent [NaCl] by 40 mmol/l) led to an inhibition of urea synthesis from NH4CI (0.5 mmol/l) by about 60% and a decrease of hepatic oxygen uptake by 0.43 f 0.03 pmol g-min-[from 3.09 f 0.13 pmol g-' min-' to 2.66 0.12 pmol g -l min-' (n = 9)]. The effects on urea synthesis and oxygen uptake were observed throughout hypotonic exposure (225 mOsm/l). They persisted although volume regulatory K+ efflux from the liver was complete within 8 min and were fully reversible upon reexposure to normotonic perfusion media (305 mOsm/l). A 42% inhibition of urea synthesis from NH4Cl (0.5 mmol/l) during hypotonicity was also observed when the perfusion medium was supplemented with glucose (5 mmol/l). Urea synthesis was inhibited by only 10-20% in livers from fed rats, and was even stimulated in those from starved rats when an amino acid mixture (twice the physiological concentration) plus NH4C1 (0.2 mmol/l) was infused.2. The inhibition of urea synthesis from NH4C1 (0.5 mmol/l) during hypotonicity was accompanied by a threefold increase of citrulline tissue levels, a 50 -70% decrease of the tissue contents of glutamate, aspartate, citrate and malate, whereas 2-oxoglutarate, ATP and ornithine tissue levels, and the [3H]inulin extracellular space remained almost unaltered. Further, hypotonic exposure stimulated hepatic glutathione (GSH) release with a time course roughly paralleling volume regulatory K + efflux. NH4Cl stimulated lactate release from the liver during hypotonic but not during normotonic perfusion. In the absence of NH4Cl, hypotonicity did not significantly affect the lactate/pyruvate ratio in effluent perfusate. With NH4C1 (0.5 mmol/l) present, the lactate/pyruvate ratio increased from 4.3 to 8.2 in hypotonicity, whereas simultaneously the 3-hydroxybutyrate/acetoacetate ratio slightly, but significantly decreased.3. Addition of lactate (2.1 mmol/l) and pyruvate (0.3 mmol/l) to influent perfusate did not affect urea synthesis in normotonic perfusions, but completely prevented the inhibition of urea synthesis from NH4Cl (0.5 mmol/l) induced by hypotonicity. Restoration of urea production in hypotonic perfusions by addition of lactate and pyruvate was largely abolished in the presence of 2-cyanocinnamate (0.5 mmol/l). Addition of 3-hydroxybutyrate (0.5 mmol/l), but not of acetoacetate (0.5 mmol/l) largely reversed the hypotonicity-induced inhibition of urea synthesis from NH4Cl.4. The data are consistent with a cell-swelling-induced inhibition of the transfer of malate and reducing equivalents into the mitochondria1 compartment, thereby inhibiting urea synthesis due to a limitation of aspartate supply for argininosuccinate synthesis. They suggest that alterations of liver cell volume and/or volume regulatory responses exert marked effects on hepatic ammonia, amin...

Regulation of liver cell volume and proteolysis by glucagon and insulin

“…In normo-osmotic perfusion the intracellular water space was 548 + 10 1/g (n = 44). This value is close to that reported by others in freeze-clamp studies [22] and corresponds well to an extracellular space in freezeclamped perfused rat livers of about 35 % [12,13]. Changes in the intracellular water-space after hypo-osmotic exposure or insulin were in close agreement with the cell-volume effects determined in isolated hepatocyte suspensions by the 'hepatocrit method'.…”

“…In addition, the increase in the intracellular water-space after hypo-osmotic exposure could account for about 90 % of the simultaneously observed liver mass increase. In line with previous data [13], the present approach also shows that liver mass changes after aniso-osmotic exposure reliably reflect volume changes of liver cells in the intact organ. Liver mass recordings, however, are not suitable to monitor cell volume changes in response to hormones ( Table 1).…”

Albumin gene expression is down-regulated by albumin or macromolecule infusion in the rat.

“…Recently, a modulatory activity of atriopeptins on vasopressin release from the hypophysis has been proposed (43). The interplay of these vasoactive peptides on a systemic basis may, therefore, influence the metabolic function of specific organs, as recently suggested for volume regulatory responses modulating hepatic ammonia metabolism (44). However, since an inhibitory effect on albumin synthesis by colloids has also been described in ex vivo systems (16)(17)(18) (47)(48)(49).…”

Short- and Long- Term Effects of Cigarette Smoke Exposure on Glutathione Homeostasis in Human Bronchial Epithelial Cells

“…Some of the proteins responsible for GSH export from mammalian cells have recently been identified [15][16][17], and there is increasing evidence that these GSH exporters are multispecific and multifunctional, regulating a number of key biological processes. GSH extrusion has been reported to be induced or enhanced by a variety of stimuli including hyposmotic stress [18], and the involvement of anionic channels such as CFTR [16] or swelling activated channels in GSH release has been demonstrated [19]. GSH intracellular levels are also influenced by the mucolytic drugs N-acetylcysteine and S-carboximethilcysteine, Lysine salt (S-CMC-Lys).…”