Sodium physiological importance

Dihydromuscimol (49) is a conformationally restricted analogue of the physiologically important neurotransmitter y-aminobutyric acid (GABA) and has been prepared using the cycloaddition of dibromoformaldoxime to A-Boc-allylamine followed by N-deprotection with sodium hydroxide (Scheme 6.52) (278). The individual enantiomers of dihydromuscimol were obtained by reaction of the bromonitrile oxide with (5)-( + )-l,2-0-isopropylidene-3-butene-l,2-diol, followed by separation of the diastereoisomeric mixture (erythro/threo 76 24), hydrolysis of respective isomers, and transformation of the glycol moiety into an amino group (279). 

The expression of the sodium iodide symporter is perhaps nowhere more important than in the thyroid gland. A complete review of the physiological importance of the thyroid is beyond the scope of this chapter. It is sufficient to say that the symporter provides the iodine needed for normal thyroid function. Once the symporter has been trafficked to the basolateral surface of the thyrocyte, it can transport iodine from the blood into the cell. Once inside the cells, iodine is transported to the apical membrane where it is organified through attachment to a tyrosine residue and incorporated into the thyroid hormone thyroglobulin. The thyroglobu-lin is then stored inside thyroid follicles as colloid, to be released into the bloodstream as thyroid hormones (thyroxine and triiodothyronine) via TSH stimulation. 

The ionophore N,N -diheptyl-N,N -dimethylsuccinic acid diamide (9) is considered the best available lipophilic ligand regarding selectivity for magnesium over the physiologically important ions of sodium, potassium and calcium (37). Glass microelectrodes filled with (9) (20 mass %), propylene... 

Besides intestinal absorption, another important determinant of bioavailability is secondary active transport of ascorbate in the kidney. Most AA circulates in the blood in the form of the ascorbate anion. The ascorbate in the blood plasma is freely filtered at the renal glomerulus, but much of it is reabsorbed in the proximal tubule. Ascorbate uptake across the luminal membranes of renal proximal tubule cells occurs through sodium-ascorbate cotransport. The amount of ascorbate lost in the urine rises when the plasma ascorbate concentration exceeds the renal threshold. Above this threshold the tubular reabsorptive capacity is overwhelmed. The renal threshold for AA is reported to be slightly higher in men than in women (plasma ascorbate concentrations of 86 and 71 pM, respectively), but the underlying mechanism and physiological importance of this difference are unknown (Oreopoulos etal., 1993). 

The list of salty tasting substances contains numerous organic and inorganic salts including some toxic substances (e.g., lithium chloride), but also some nutritionally important cations and anions. Most abundant of all of these compounds is sodium chloride which, in addition to its physiological importance, was also part of the medium (sea water) in which many phyla evolved. For marine and terrestrial animals alike, the most common salty compound is sodium chloride with the toxic ones much less common. The distribution of NaCl is more patchy for terrestrial than for marine animals, however (21). Since NaCl is an absolute requirement for complex organisms, it is not surprising that nearly all species tested respond to it. 

Sodium imaging is relatively time consuming and cannot be performed on standard clinical scanners without specialized hardware and software upgrades. Nevertheless, the unique physiologic information provided by sodium imaging may make this technique an important tool in acute stroke imaging in years to come. 

The ion-channel blocking mechanism has been widely tested and found to be important in both pharmacology and physiology. Examples are the block of nerve and cardiac sodium channels by local anesthetics, or block of NMDA receptor channels by Mg2+ and the anesthetic ketamine. The channel-block mechanism was first used quantitatively to describe block of the squid axon K+ current by tetraethylammonium (TEA) ions. The effects of channel blockers on synaptic potentials and synaptic currents were investigated, particularly at the neuromuscular junction, and the development of the single-channel recording technique allowed channel blockages to be observed directly for the first time. 

Clinical chemistry, particularly the determination of the biologically relevant electrolytes in physiological fluids, remains the key area of ISEs application [15], as billions of routine measurements with ISEs are performed each year all over the world [16], The concentration ranges for the most important physiological ions detectable in blood fluids with polymeric ISEs are shown in Table 4.1. Sensors for pH and for ionized calcium, potassium and sodium are approved by the International Federation of Clinical Chemistry (IFCC) and implemented into commercially available clinical analyzers [17], Moreover, magnesium, lithium, and chloride ions are also widely detected by corresponding ISEs in blood liquids, urine, hemodialysis solutions, and elsewhere. Sensors for the determination of physiologically relevant polyions (heparin and protamine), dissolved carbon dioxide, phosphates, and other blood analytes, intensively studied over the years, are on their way to replace less reliable and/or awkward analytical procedures for blood analysis (see below). 

Although its mode is uncertain, sodium nitroprusside (SNP) is one of the most valuable vasodilators. Its use in clinical practice is suspect as the cyano-ligands render cyanide poisoning a possibility. However for ex vivo experiments this consideration is less important but the possibility of some biological action due to these ions remains. The mechanism by which SNP acts as a vasodilator is not fully understood. With the discovery of a physiological role for NO there has been renewed interest in mechanistic studies of reactions involving SNP and a re-examination of studies of SNP undertaken before 1987. So far, only one simple reaction leading to the release... 

Buffer solutions that are isosmotic with respect to some standard, typically chosen such that suspended cells will neither shrink nor expand. Sodium chloride solutions (0.90% weight/volume or 0.155 M) at 37°C is often used to represent physiological conditions. These buffer systems are also important in studies of intact cells and membranal organelles likewise, many pharmaceutical formulations must be prepared as isotonic solutions. Most enzyme-catalyzed reactions are affected by ionic... 

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