Potassium extracellular concentration

An intracellular to extracellular difference in sodium and potassium ion concentrations is essential to the function of nerves, transport of important nutrients into the cell, and maintenance of proper cell volume. 

Solis JM, Herranz AS, Herreras O, Munoz MD, Martin del Rio R, et al. 1986. Variation of potassium ion concentrations in the rat hippocampus specifically affects extracellular taurine levels. Neurosci Lett 66(3) 263-268. 

Lantos J., Temes G., andTorokB. (1986) Changes during ischaemia in extracellular potassium ion concentration of the brain under nitrous oxide or hexobarbital-sodium anaesthesia and moderate hypothermia. Acta Physiol. Hung. 67,141-153. 

The total body potassium of a 70 kg subject is 3.5 mol (40 to 59 mmol/kg) of which only 1.5% to 2% is present in the ECF. Nevertheless, plasma K is a relatively good indicator of total K stores with only a few exceptions. Because extracellular concentrations are maintained at the expense of the intracellular supply, plasma concentration can initially be normal and belie a total body deficit of up to 200 mmol. 

This pump does not operate equally in both directions, and two to three sodium ions are transported out of the cell for each potassium ion that enters the cell. Also, cell membranes are almost 100 times more permeable to potassium ions than to sodium ions, so that it is reasonable to assume the potassium ion concentration difference across a cell is an equilibrium state. On average, for a resting nerve fiber, the sodium ion concentrations are 10 millimolar (mM) within cells and 142 rruVI outside the cell, while the potassium ion concentrations are 5 mM outside the cell (in the extracellular fluid, ECF) and 140 mM inside the cell. (Note that the actual situation is further complicated by the fact that the ion channels open and close depending on the physiological situation. For example, during a nerve impulse the permeability of the membrane to sodium ions can increase by a factor of a thousand, almost eliminating the sodium ion concentration difference.)... 

Lithium readily enters excitable cells via sodium channels. It is not. however, removed well by the sodium/potassium ATPase exchange mechanism. It therefore accumulates within the cell. Thus, the intracellular concentration of potassium is decreased as the influx of potassium is reduced, both through inhibition of active transport (by Na+/K+ ATPase) and the decrease in the electrical gradient for potassium. Extracellular potassium therefore increases. The reversal in potassium levels results in a decrease in neuronal excitability, producing a therapeutic calming effect. 

It would appear that changes in the intracellular or extracellular concentration of potassium ions markedly alter the resting membrane potential. For this reason, neurophysiologists treated an excitable cell as if it were an electrochemical, or Nernst, cell. The resting potential for one permeant species could therefore be explained by the familiar Nernst equation ... 

At either end of the selectivity pore, a partially solvated is seen to be coordinated by four carbonyl groups, while the other half of the ion is bound by four waters. This represents either the dehydration step as the ion enters the filter, or the solvation step as it emerges into the extracellular or cavity environments, depending on the direction of flow. This directionality suggests that gating must occur and the structural mechanisms for this, which ultimately depend on local potassium concentrations and binding affinities, have begun to be addressed." Ultimately, the structure, and therefore coordination motif, is dependent on potassium ion concentration. ... 

As expected from the push-pull and dialysis data, measurement of electroactive substances in extracellular fluid has indicated that the composition of the primary detectable species are ascorbate and dihydroxy-phenylacetlc acld[31,32]. These measurements were made in the caudate nucleus, a region of the brain known to contain large amounts of dopamine, in the hope that this substance would be determined. The first report of the release of dopamine which was confirmed by voltammetry came as a result of injecting potassium at a distance remote from the electrode into this caudate nucleus region[33]. The basis of this experiment is that the potassium would cause the depolarization of many neurons resulting in an increased extracellular concentration of dopamine. This is indeed the observed result. 

Figure 3j5. A. The movements of hydrogen and potassium ions between intra- and extracellular fluid compartments produced by a fall in hydrogen ion concentration of the extracellular fluid. B. Similarly for a fall in potassium ion concentration of the extracellular fluid. C. Movements of ions between the renal tubular fluid and the renal interstitial fluid sodium, chloride and bicarbonate are reabsorbed, hydrogen and potassium ions are secreted.

Figure 3.6. Diagram to illustrate the fact that changes in the extracellular concentration of hydrogen ions are usually in the same direction as those of potassium concentration and vice versa. Each horizontal line joins corresponding values of extracellular concentrations in a given subject at a given time. The arrows indicate the directions of change of this horizontal line in various conditions considered in the text.

A summary of the inter-relationship between hydrogen ion concentration and potassium ion concentration in the extracellular fluid is shown in Figure 3.6. The horizontal line which moves up and down represents the fact that changes in the concentrations of the two species occur together in the same direction. 

As already explained, the renal response to loss of extracellular fluid volume results in a low plasma potassium concentration. As shown in Figure 3.8B, the lowering of the extracellular concentration of potassium results in an increased potassium concentration gradient from intra- to extracellular fluid. Consequently, potassium difl uses from the intracellular to the extracellular compartment. To maintain electrochemical neutrality, hydrogen ions move in the opposite direction, from extracellular to intracellular fluid. The extra-... 

The potassium leaving cells and entering the extracellular fluid partially replaces the potassium lost into the renal tubular fluid and is of itself just a redistribution of potassium within difleient fluid compartments in the body. It is not lost from the body. However, the potassium moving from the intra- to the extracellular compartment bolsters the extracellular concentration of potassium, and, by the electroneutrality effect described earlier, becomes available for excretion in the urine, thereby being lost to the body entirely. The overall effect is a dramatic lowering of total potassium content of the body. 

Ionic concentration gradients exist across cell membranes and are responsible for maintaining the resting membrane potential. Intracellular and extracellular Na+ are approximately 10 mM and 140 mM, respectively. The converse is true for K% with an intracellular concentration of 130-140 mM and an extracellular concentration of 4-5 mM. Sodium enters the cells of excitable tissue during the up stroke of the action potential and potassium leaves... 

The ventricular action potential is depicted in Fig. 6-2.2 Myocyte resting membrane potential is usually -70 to -90 mV, due to the action of the sodium-potassium adenosine triphosphatase (ATPase) pump, which maintains relatively high extracellular sodium concentrations and relatively low extracellular potassium concentrations. During each action potential cycle, the potential of the membrane increases to a threshold potential, usually -60 to -80 mV. When the membrane potential reaches this threshold, the fast sodium channels open, allowing sodium ions to rapidly enter the cell. This rapid influx of positive ions... 

With active transport, energy is expended to move a substance against its concentration gradient from an area of low concentration to an area of high concentration. This process is used to accumulate a substance on one side of the plasma membrane or the other. The most common example of active transport is the sodium-potassium pump that involves the activity of Na+-K+ ATPase, an intrinsic membrane protein. For each ATP molecule hydrolyzed by Na+-K+ ATPase, this pump moves three Na+ ions out of the cell and two K+ ions into it. As will be discussed further in the next chapter, the activity of this pump contributes to the difference in composition of the extracellular and intracellular fluids necessary for nerve and muscle cells to function. 

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