Hypoxic responses of Na+/K+ ATPase in trout hepatocytes

Abstract

SUMMARY Reduction in oxygenation induces inhibition of Na+/K+ATPase in a number of cells and tissues, including hepatocytes. When not reversed, decrease in Na+/K+ pump activity leads to a gradual Na+ accumulation, cell swelling and death. However, when accompanied by suppression of dissipative cation pathways, it has also been shown to be a beneficial adaptive strategy used by some hypoxia-tolerant species to reduce ATP consumption during prolonged periods of anoxia. This study aims to investigate acute hypoxic responses of the Na+/K+ ATPase in primary cultures of trout hepatocytes. Gradual decrease in oxygenation was followed by an instantaneous transient dose-dependent downregulation of the Na+/K+ ATPase transport activity, but was without an effect on hydrolytic function of the enzyme. Hypoxia-induced inhibition of active K+ influx was reversed spontaneously when hypoxic incubation time exceeded 20 min. The stimulating effect of prolonged hypoxic exposure on the Na+/K+ pump is most probably secondary to hypoxia-induced activation of the Na+/H+ exchanger with the following Na+accumulation leading to Na+/K+ ATPase activation. Hypoxia-induced inhibition of the Na+/K+ pump was not caused by ATP depletion or global oxidative stress. However, local controlled production of reactive oxygen species seems to play an important role in hypoxia-induced regulation of the Na+/K+ ATPase. Treatment of cells with mercaptopropionyl glycine (MPG), a scavenger of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(\mathrm{O}\mathrm{H}^{.-}\) \end{document}, abolished hypoxia-induced inhibition of the Na+/K+ ATPase. Earlier on we have shown that activation of Na+/H+ exchanger under hypoxic conditions can be opposed by MPG treatment as well. Taken together our results suggest that regulation of both oxygen-sensitive transporters may be accomplished by local changes in free radical production.

Moderate altitude but not additional endurance training increases markers of oxidative stress in exhaled breath condensate

“…Hypoxic exposure did lead to an increase in GSH-concentration after 24 hours; and, GSH levels decreased to normoxic levels during the stay at altitude. Earlier we described a rapid increase in mouse erythrocytes GSH levels on 0.5% O 2 hypoxic exposure in vitro (Bogdanova et al 2003), and others showed an increase in mouse muscle reduced GSH with acute hypoxia equivalent to an altitude of 7000 m (Magalhaes et al 2004), a result that correlates with the observation of suppressed ROS production in hepatocytes primary cultures in response to acute hypoxia (Bogdanova et al 2005). Chronic hypoxic exposure reduces erythrocytes GSH content (Singh et al 2001) and triggers activation of glutathione cycle related enzyme activity (Magalhaes et al 2005) leading to improved antioxidant capacity of blood.…”

Rapid regulation of Na+fluxes and ammonia excretion in response to acute environmental hypoxia in the Amazonian oscar,Astronotus ocellatus

“…1E). An important caveat here is the recent demonstration that in trout hepatocytes, low PO 2 can reduce the transport activity of Na ϩ -K ϩ -ATPase before its ATP hydrolytic capacity (the parameter measured in the assay) is affected (5). It must be remembered, that Na ϩ -K ϩ -ATPase activity is assayed under optimal conditions in vitro, whereas in vivo, the intracellular milieu of the enzyme may change during hypoxia.…”

Involvement of oxidative stress in hypoxia-induced blood–brain barrier breakdown

“…Cellular ROS generation and GSH measurements ROS formation was quantified by fluorometry techniques based on DCF oxidation, whereas GSH measurement was obtained using colorimetry technique with protocols previously published in the literature (Bogdanova et al, 2005;Petrushanko et al, 2006).…”