Can maximum ouabain-sensitive 86Rb+ uptake rate be obtained by increasing Na+ influx?

Linkage of aerobic glycolysis to sodium-potassium transport in rat skeletal muscle. Implications for increased muscle lactate production in sepsis

Although a linkage between aerobic glycolysis and sodium-potassium transport has been demonstrated in diaphragm, vascular smooth muscle, and other cells, it is not known whether this linkage occurs in skeletal muscle generally. Metabolism of intact hind-leg muscles from young rats was studied in vitro under aerobic incubation conditions. When sodium influx into rat extensor digitorum longus (EDL) and soleus muscles was facilitated by the sodium ionophore monensin, muscle weight gain and production of lactate and alanine were markedly stimulated in a dose-dependent manner. Although lactate production rose in both muscles, it was more pronounced in EDL than in soleus. Monensin-induced lactate production was inhibited by ouabain or by incubation in sodium-free medium. Preincubation in potassium-free medium followed by potassium re-addition also stimulated ouabain-inhibitable lactate release. Replacement of glucose in the incubation medium with pyruvate abolished monensin-induced lactate production but exacerbated monensin-induced weight gain. Muscles from septic or endotoxin-treated rats exhibited an increased rate of lactate production in vitro that was partially inhibited by ouabain. Increases muscle lactate production in sepsis may reflect linked increases in activity of the Na+, K+-ATPase, consumption of ATP and stimulation of aerobic glycolysis.

Pretreatment of rat brain slices with ouabain decreases chloride-dependent L-glutamate transport in synaptic membrane

Possible roles of (Na(+)-K+)-ATPase on regulation of Cl(-)-dependent L-glutamate (L-Glu) transport in rat synaptic membrane were examined. Pretreatment of rat cerebral slices with 20 micrograms/ml veratrine, 56 mM K+ and 20 microM monensin increased Cl(-)-dependent L-[3H]-Glu uptake into membrane vesicles prepared from the slices. Pretreatment with (Na(+)-K+)-ATPase inhibitors, 100 microM ouabain, 100 microM strophanthidin, 100 microM vanadate and K(+)-free medium, decreased the uptake by 25-30%, while 5 microM A23187 and 10 microM ruthenium red had no effect. Ouabain (100 microM) caused the maximal effect in 10-20 min of incubation. The effect of (Na(+)-K+)-ATPase inhibitors was reversible and characterized by a decrease in Vmax of the uptake. Addition of 5 microM ouabain abolished the increases in the uptake induced by either veratrine, 56 mM K+ or monensin. Dose-inhibition curve of ouabain on the increased Cl(-)-dependent L-[3H]-Glu uptake was bi-phasic; ouabain at 0.5-5 microM selectively diminished the stimulatory effect of veratrine and inhibited the basal uptake at higher concentrations. Veratrine had no effect on L-[3H]-Glu uptake in 5 microM strophanthidin containing or K(+)-free medium. These results suggest the involvement of (Na(+)-K+)-ATPase in the veratrine-induced increase in Cl(-)-dependent L-Glu transport in rat brain synaptic membranes.

Aging: stimulation rate on cardiac intracellular Na+ activity and developed tension

Previous reports suggested that Na,K-ATPase activity and Na(+)-pump capacity decrease with senescence in left atrial myocardium of F344 rats. Current experiments were designed to determine if this reduction in the Na(+)-pump affects free intracellular Na+ levels. Mean intracellular Na+ ion activity (aiNa) was measured with Na-selective microelectrodes in left atrial muscle isolated from hearts of 4-, 14- and 25-month-old F344 rats. Preparations were stimulated randomly at frequencies between 0 and 12 h. There were no age-associated differences in aiNa measured at any frequency or in the decay of Na+ activity following discontinuation of electrical stimulation. These data indicate that the aging-related decline in Na,K-ATPase does not result in elevated aiNa even at extremely high stimulation frequencies, thus suggesting that other routes of Na+ influx and efflux are also altered in atrial muscle.

Brain Na+,K(+)-ATPase regulation in vivo: reduction in activity and response to sodium by intracerebroventricular tetrodotoxin

We investigated the effects of intracerebroventricular infusion of tetrodotoxin on activity and function of brain Na+,K(+)-ATPase. Infusion of 1 or 3 micrograms/h for 2, 4 or 7 days by osmotic minipump reduced the number of Na+,K(+)-ATPase sites as measured by ouabain binding in cerebral cortex. Tetrodotoxin infusions substantially reduced the functional transport capacity of Na+,K(+)-ATPase, measured by the maximal increase in synaptoneurosomal 86Rb+ uptake in the presence of monensin. The effects were maximal at 4 days, with a possible partial recovery of activity at 7 days. Results of ouabain inhibition curves suggested that the effect of tetrodotoxin was not specific for enzyme with high or low affinity for ouabain.