Effect of various oxygen concentrations on the ATP content in isolated hepatocytes of rats adapted and nonadapted to hypoxia

Abstract

It is shown that isolated hepatocytes are capable of perceiving slight changes in the environmental oxygen concentration. A complicated phase dependence exists between adenosine triphosphate and partial oxygen pressure, which differs in cells from animals with high and low resistance to hypoxia, the former showing a more stable and resistant energy-synthesizing function than the latter. After long-term adaptation to periodic hypoxia, the resistance of the energy-synthesizing function rises in hepatocytes from high-resistant animals, and falls in low-resistant animals suggesting a fundamentally different organization of the emergency compensatory mechanisms of the energysynthesizing function in hepatocytes of animals of these two types.Key Words: hepatocytes; hypoxia; adenosine triphosphate; resistance; adaptation It is well known that aerobic organisms respond to a decrease of environmental oxygen by a change in respiration and adenosine triphosphate (ATP) synthesis. For oxygen-dependent enzyme systems, such as cytochrome oxidase, this relationship is described by the Michaelis-Menten equation: the process is governed by zero-order kinetics within a wide range of oxygen concentrations (respiration is independent of the partial oxygen pressurepO2), but within a very narrow range of low pO2 values it obeys ftrst-order kinetics (respiration linearly decreases as pO 2 declines) [10,12]. This relationship holds true for a mitochondrial suspension. However, cell systems exhibit some deviations from this law both due to the limited oxygen diffusion across the histohematic barriers [11,12] and due to specific features of cell metabolism and energetics [5,6,14]. Nonmitochondrial oxygen-deLaboratory of Bioenergetics, Research Institute of Pharmacology, Russian Academy of Medical Sciences, Moscow pendent enzyme systems which have a lesser affinity to oxygen than cytochrome oxidase, as well as regulatory interactions between glycolysis and aerobic oxidation significantly distort this relationship [5,9]. A knowledge of how a cell or tissue responds to oxygen deficiency and of the mechanisms controlling energy synthesis in the intact cell in hypoxia is of fundamental importance, and yet the topic has hardly been addressed. Isolated cells are most suitable model systems for such study, for in then the diffusional limitations for oxygen are minimized, and energy synthesis and utilization, as well as other important metabolic processes, remain intact. In view of this, in the present investigation we studied the specifics of ATP formation in isolated hepatocytes under conditions of various oxygen concentrations in the incubation medium. The motivation for the study came from the high incidence of ischemic liver pathology, which is difficult to treat because the molecular and bioenergy mechanisms of this disorder are poorly uderstood.

Bioenergetic hypoxia: Definition, mechanisms, and methods of correction

“…The associated increase in electron transport from NAD-dependent substrates can be the cause of the increase in CCO reduction during this period, which was demonstrated for different cell models [18] but not yet explained. Moreover, this reduction correlates with an increase in the ATP content [21,35] and intensification of celhflar functional activities: pulsed activity of neurons [19,20], myocardial contractility [ 10,11,22,32], hepatocyte capacity to urea production [2], etc.…”

“…Probably, it is this mechanism that is responsible for accumulation of the Krebs' cycle NAD-dependent substmtes during the early stages of hypoxia [34]. Despite disturbed MEC I function during this period, intracellular ATP concentration and cell function are either unchanged or slightly decreased [2,[15][16][17]19,21,351. This is possible only if suppression of the main oxidation pathway involves activation of compensatory metabolic flows in the respiratory chain, preserving energy-producing function of the cytochrome site.…”

“…Individual reactions of the organism to hypoxia play an essential role in the development, course, and outcome of the resultant pathological state. Individual differences in reaction to hypoxia, assessed by changes of the functional activity, are retained at the tissue and cell levels [2,[4][5][6][7]10,[13][14][15][16][17][21][22][23]35,36,45].…”

Role of glycolysis in the maintenance of the energy-synthesizing function of hepatocytes from rats adapted and nonadapted to hypoxia at different oxygen concentrations

“…1). In the former case this correlated with stable ATP values over the entire incubation period and in the latter case with its slight increase [5].…”

“…At 890-100 ,M 02 the lactate content remained relatively constant (2-5 ~tmol/g wet tissue), which correlates with the steady-state cell level of ATP in the same region [5]. Starting from 50 ~tM 02, the lactate concentration markedly increased, this occurring against the background of just as marked a decrease in the cell ATP content (Fig.…”