Potassium ions transport across membranes

Soil as an Electrolyte Most soils are conductive due to the presence of dissolved ions, such as calcium, magnesium, sodium, potassium, (bi) carbonate, some soluble fatty acids, nitrate, phosphate, sulfate, and chloride ions. Most seemingly, dry soils have more than 5% moisture, sufficient to provide a continuous path for these ions to move. This is essential for plants as the roots need access to these nutrients and ion transport across membranes is the means with which they extract them. The most signihcant feature of natural soils, with respect to their contamination and subsequent remediation, is the high ion exchange capacity (Table 33.1). 

Before leaving the subject of the mechanism of metal-ion transport across membranes, a cautionary result and a by-product of these investigations might be reported. Hall has found that the antibiotic monamycin forms complexes in solution with K+, Rb+, and Cs+, but not with Na+ or Li+. However, it seems that their antibacterial action is due to their lytic effects on cell membranes rather than to any ion-transporting properties they may possess. The metal-ion selectivity of nonactin and valino-mycin has been put to use in the design of potassium-selective membrane electrodes. These contain the antibiotics as components of the membrane, and can be used to estimate K+ in the presence of Na+, for example in human serum. 

One may find the correlation between the naked ions and the transport behaviour surprising because ions in aqueous medium are generally believed to be strongly hydrated so that the transport behaviour might be rather determined by the hydrodynamic radius, which decreases from lithium to potassium, for example. However, our results clearly indicate that the hydration only plays a minor role. Obviously, the interactions of the water molecules within the membrane with the polyelectrolyte chains are so strong that the available energy is not sufficient to also cause a hydration of the metal ions. Similar arguments were already previously considered to explain the ion transport across membranes of macroscopic thickness [14]. 

An essential requirement for diffusion of Na+ ions is the creation of a concentration gradient for sodium between the filtrate and intracellular fluid of the epithelial cells. This is accomplished by the active transport ofNa+ ions through the basolateral membrane of the epithelial cells (see Figure 19.4). Sodium is moved across this basolateral membrane and into the interstitial fluid surrounding the tubule by the Na+, K+-ATPase pump. As a result, the concentration of Na+ ions within the epithelial cells is reduced, facilitating the diffusion of Na+ ions into the cells across the luminal membrane. Potassium ions transported into the epithelial cells as a result of this pump diffuse back into the interstitial fluid (proximal tubule and Loop of Henle) or into the tubular lumen for excretion in the urine (distal tubule and collecting duct). 

As detailed in chapter 17, biological membranes are basically lipid—think fat or oil—in nature with some attached proteins. As such, these thin sheets of phospholipids and proteins are nearly impermeable to charged particles such as sodium, potassium, or chloride ions. While the isolation of the cell interior from the exterior ionic environment is critical in many ways, it is also true that controlled permeability to ions may be critical. In fact, it is the near-impermeability of biological membranes to ions that permits control of ion transport across them by certain, specific proteins. 

Fig. 4.3 Uptake of glutamate is associated with ion transport across neural membranes. Ion transport results in entry of 3 sodium ions and 1 proton in the cell whilst 1 potassium ion is transported out (modified from Attwell, 2000)...

Potassium bromide appears to stabilize neuronal cell membranes by interfering with chloride ion transport across them. It potentiates the effect of GABA by hyperpolarizing the membrane. Other drugs with GABAergic activity, such as the barbiturates, may act synergistically with potassium bromide to raise the seizure threshold. 

One of the most common control mechanisms for enzymes is by phosphorylation. The side-chain hydroxyl groups of serine, threonine, and tyrosine can all form phosphate esters. Transport across membranes provides an important example, such as the sodium-potassium ion pump, which moves potassium into the... 

T. Hill and Y. Chen, Cooperative Effects in Models of Steady-State Transport Across Membranes Simulation of Potassium Ion Transport in Nerve, Proc. Natl Acad. Scl USA 66(3), 607-614 (1970). 

A well-known example of active transport is the sodium-potassium pump that maintains the imbalance of Na and ions across cytoplasmic membranes. Flere, the movement of ions is coupled to the hydrolysis of ATP to ADP and phosphate by the ATPase enzyme, liberating three Na+ out of the cell and pumping in two K [21-23]. Bacteria, mitochondria, and chloroplasts have a similar ion-driven uptake mechanism, but it works in reverse. Instead of ATP hydrolysis driving ion transport, H gradients across the membranes generate the synthesis of ATP from ADP and phosphate [24-27]. 

In sum, the natural tendency will be for sodium, calcium, and chloride ions to flow into the neuron and for potassium ions to flow out, and in so doing to reduce the membrane potential to zero. In reality, this is not so easy. The plasma membrane of the neuron is not very permeable to these ions. If it were, it would be impossible to sustain concentration gradients across it. The rate of passive diffusion of these ions across this membrane is very slow, though not zero, and different for each ion. So how do ions get across the neuronal plasma membrane rapidly There are two ways gated channels and active transport by pumps. 

The most universal transport systems are those involved in the transport of the ubiquitous inorganic ions, sodium, potassium and calcium1. The sodium pump counteracts passive water movement across the cell membrane by removing sodium ions together with chloride or other anions from the cytoplasm to lower its content of osmotically active substances. In most cells, however, the elimination of sodium ions is connected with an accumulation of potassium ions6. For three sodium ions leaving the cell two potassium ions are taken up9,10). The resulting concentration... 

Uncomplexed valinomycin has a more extended conformation than it does in the potassium complex.385,386 The conformational change results in the breaking of a pair of hydrogen bonds and formation of new hydrogen bonds as the molecule folds around the potassium ion. Valinomycin facilitates potassium transport in a passive manner. However, there are cyclic changes between two conformations as the carrier complexes with ions, diffuses across the membrane, and releases ions on the other side. Tire rate of transport is rapid, with each valinomycin molecule being able to carry 104 potassium ions per second across a membrane. Tlius, a very small amount of this ionophore is sufficient to alter the permeability and the conductance of a membrane. 

Sixliultt ion acts in concert with other electrolytes, in particular K. to regulate the osmotic pressure and to maintain the appropriate water and pi I balance ot the body. Homeostatic control of these functions is accomplished by the lungs and kidneys inlereciing by way of the blood. Sodium is essential for glucose absorption and transport of other substances across cell membranes. It is also involved, as is KJ. ill transmitting nerve impulses and in muscle relaxation. Potassium ion acts as a catalyst in the intracellular fluid, in energy metabolism, and is required for carbohydrate and protein metabolism. 

---