Brain soluble fractions which modulate Na+, K+-ATPase activity likewise modify muscarinic receptor

Na+/K+-pump and neurotransmitter membrane receptors

Na⁺/K⁺-pump is an electrogenic transmembrane ATPase located in the outer plasma membrane of cells. The Na⁺/K⁺-ATPase pumps 3 sodium ions out of cells while pumping 2 potassium ions into cells. Both cations move against their concentration gradients. This enzyme's electrogenic nature means that it has a chronic role in stabilizing the resting membrane potential of the cell, in regulating the cell volume and in the signal transduction of the cell. This review will mainly consider the role of the Na⁺/K⁺-pump in neurons, with an emphasis on its role in modulating neurotransmitter receptor. Most of the literature on the modulation of neurotransmitter receptors refers to the situation in the mammalian nervous system, but the position is likely to be similar in most, if not all, invertebrate nervous systems.

Repeated electroconvulsive shock induces changes in high-affinity [3H]-ouabain binding to rat striatal membranes

Repeated electroconvulsive shock is an effective treatment for affective disorders. Striatum, hippocampus and brainstem are involved in affective disorders. Sodium-potassium/ATPase is of paramount importance for the proper functioning of the brain and its involvement in the affective disorders has been claimed for a long time. Sodium-potassium/ATPase has an extracellular regulatory binding site to which cardiotonic glycosides, such as ouabain, bind to, thus regulating the activity of the enzyme. Endogenous "ouabain-like" substances exist in the brain and their actions on the sodium-potassium/ATPase resemble ouabain biological properties. The aim of this work was to determine if electroconvulsive shock (ECS) would induce changes in the high-affinity binding of ouabain to the sodium-potassium/ATPase from rat brain regions. Adult, male Wistar rats received one (ECSx1 group) or seven electroshocks (ECSx7 group) delivered daily through ear-clips electrodes. Control rats received the same manipulations; however, no current was delivered through the electrodes (SHAMx1 and SHAMx7 groups). All groups were sacrificed 24 h after the last ECS session. The B (max) and K (D) of high-affinity [(3)H]-ouabain binding were determined in crude membrane preparations from the striatum, hippocampus and brainstem. The results obtained showed a statistically significant increase in the affinity of [(3)H]-ouabain (lower K (D)) to striatal membranes in those rats receiving seven ECS. In the striatum there was no change in the K (D) after one ECS; as well as there was no change in the B (max) after a single or seven ECS. High-affinity [(3)H]-ouabain binding to hippocampus and brainstem did not reveal any significant differences either in K (D) or B (max) after one or seven ECS. The increased affinity of ouabain to the striatal sodium-potassium/ATPase induced by repeated ECS suggests an increased interaction in vivo of the endogenous "ouabain-like" substances with the enzyme and the involvement of the extracellular regulatory allosteric ouabain binding site in the striatal sodium-potassium/ATPase in the effects of electroconvulsive shock.

Rapid eye movement sleep deprivation induces changes in the high-affinity binding of [3H]-ouabain to the rat cortical membranes

Rapid eye movement sleep (REMS) suppresses seizures. On the other hand, REMS deprivation (REMSD) increases brain susceptibility to seizures. Sodium-potassium/ATPase is involved in the control of brain excitability. Ouabain, a cardiotonic glycoside, binds to a regulatory extracellular allosteric site in the sodium-potassium/ATPase inhibiting/stimulating its activity depending on its concentration. Endogenous ouabain-like substances exist in the brain; therefore, changes in the ouabain binding site may be involved in the increased brain excitability induced by REMSD. Adult, Wistar male rats were deprived of REMS for 96 hours by the flower-pot method (REMSD). A stress control group was kept in the same environment on a larger platform (LP). A third group of rats was kept in the same room in their home-cages (CONTROL). After REMSD all rats were sacrificed by decapitation and their cerebral cortex dissected. High-affinity [3H]-ouabain binding was carried out in cortical crude membrane preparation using 8 concentrations of [3H]-ouabain (1-24 nM). The results show a statistically significant increase of KD in the REMSD rats compared to both CONTROL and LP groups. There were no statistically significant differences in the Bmax among the experimental groups. There was also no change either in cortical activity of K+ stimulated p-nitrophenylphosphatase, the dephosphorylation reaction of phosphorylated sodium-potassium/ATPase or in Mg2+-stimulated p-nitrophenylphosphatase. An increase in the KD of [3H]-ouabain binding to the sodium-potassium/ATPase in REMSD rats indicates a lower affinity to the endogenous inhibitors/stimulators of the enzyme. Therefore, this decreased affinity of the endogenous ouabain-like substances may be involved in the increased excitability induced by REMSD.

How many endobains are there?

Oxidative metabolism is very active in brain, where large amounts of chemical energy as ATP molecules are consumed, mostly required to maintain cellular Na+/K+ gradients through the participation of the sodium pump (Na+,K+-ATPase), whose activity is selectively and potently inhibited by the alkaloid ouabain. Na+/K+ gradients are involved in nerve impulse propagation, in neurotransmitter release and cation homeostasis in the nervous system. Likewise, enzyme activity modulation is crucial for maintaining normal blood pressure and cardiovascular contractility as well as renal sodium excretion. The present article reviews the progress in disclosing putative ouabain-like substances, examines their denomination according to different research teams, tissue or biological fluid sources, extraction and purification, assays, biological properties and chemical and biophysical features. When data is available, comparison with ouabain itself is mentioned. Likewise, their potential action in normal physiology as well as in experimental and human pathology is summarized.