Sodium homeostasis

Toad bufadienolides occur not only by themselves but also in a conjugated form, sulfates, dicarboxylic esters and amino acid-dicarboxylic acid esters have all been reported (Steyn and van Heerden 1998). Because of the activity of the bufadienolides in inhibiting active monovalent cation transporters, it is suggested that these compounds have a role in maintaining sodium homeostasis in toads that migrate between fresh and salt water environments (Flier, I idwards, Daly and Myers 1980). 

The incidence of classic CAH-P450c21 in the United States ranges from 1 10,000 to 1 18,000, with approximately 75% of patients with CAH-P450c21 exhibiting classic salt wasting (White and Speiser, 2000). Aldosterone is essential for normal sodium homeostasis and acts to enhance sodium absorption and potassium excretion (Fig. 33-3). In the relative absence of aldosterone, there is an increase in sodium loss... 

Perhaps the main peroxide-induced alterations, within cells and tissues, are those that affect calcium and sodium homeostasis. Na, K-ATPase, which is considered as the core of the sodium pump , is strongly affected by peroxides, and especially by lipid hydroperoxides [137-139]. This implies that oxidative stress will usually be associated with cellular edema . Alternatively, activation of the Na, K-ATPase of vascular endothelia, such as the blood-brain barrier, will result in extracellular edema on the antiluminal side of the endothelium, due to massive influx of sodium ions [119]. 

Carey RM. Theodore Cooper Lecture Renal dopamine system paracrine regulator of sodium homeostasis and blood pressure. Hypertension 2001 38 297-302. 

The regulation of body fluid and sodium homeostasis requires central nervous system mechanisms to coordinate information regarding body fluid status with the appropriate physiological and behavioral responses. Mechanisms have evolved to ensure that the physiological and behavioral responses to a body deficit are appropriate. For example, during periods of water deprivation, renal mechanisms minimize loss of body water. Secretion of the pituitary hormone, arginine vasopressin (AVP), is increased and acts on... 

Fig.l. Model showing mechanisms controlling water and sodium homeostasis. Major brain nuclei and peripheral pathways are shown. See text for abbreviations. 

Influence of Omega-3 PUFAs on Body Fluid and Sodium Homeostasis... 

These results are consistent with an aberration in central osmoreceptor or sodium sensor or ANG Il-sensitive mechanisms that influence body fluid and sodium homeostasis. The results of this study suggest a potential requirement for n-3 PUFAs, early in life, for normal development and neural function. Evidence that early dietary n-3 PUFA... 

Bonventre JV, Leaf A. Sodium homeostasis Steady states without a setpoint. Kidney Int 1982 21 880-883. 

The major disease clearly linked with disordered sodium homeostasis, among other diseases, is hypertension, and this is observed in very small populations with genetic defects including glucocorticoid-responsive aldosteronism, Liddle syndrome, and apparent mineralocorticoid excess (AME) (Anke 2002). Cystic fibrosis is another genetically determined defect in the chloride channels. This leads to the secretion of sweat with high NaCl concen-... 

The major disease, though only linked in some cases with disordered sodium homeostasis, is hypertension. The undisputed facts are that there are human populations with very low salt intakes and very low prevalences of hypertension. Some of these people even fail to show the normal increase in blood pressure with age (Oliver et al. 1975). 

KInIns, human peptide hormones implicated in many physiological and pathological processes, including reduction of blood pressure and regulation of sodium homeostasis, inflammation and the cardioprotective effects of preconditioning. The kallikrein-kinin system (KKS) of the plasma generates bradykinin (BK), whereas the tissue KKS is responsible for the formation of kallidin. The kinin action is mediated via two types of kinin receptor, the type 1 (Bi) and type 2 (B2). Bi... 

Inasmuch as sodium intake may vary considerably, the maintenance of sodium homeostasis requires a broadly adjustable mechanism of renal sodium excretion and reabsorption. Regulatory mechanisms for sodium reabsorption in the kidney include (1) rapid adjustment of the levels of tubular reabsorption through the glomerular filtration rate (2) secretion of aldosterone, which stimulates sodium reabsorption (3) the possible elaboration of a natriuretic hormone that stimulates secretion. 

Lung M-L, Croning MDR, Haddad GG. Sodium homeostasis in rat hippocampal slices during oxygen and glucose deprivation role of voltage-sensitive sodium channels. Neurosci Letters 1999 275 41 4. 

ANPs play an important role in the maintenance of cardiovascular homeostasis by counterbalancing the renin—angiotensin (RAS) system. ANP, the main circulating form of the natriuretic peptides, effectively relaxes vascular smooth muscle, promotes the excretion of sodium and water, and in the CNS inhibits vasopressin release and antagonizes AT-II induced thirst. 

Clinical Presentation of Impaired Sodium and Water Homeostasis... 

Atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) are members of a family of so-called natriuretic peptides, synthesized predominantly in the cardiac atrium, ventricle, and vascular endothelial cells, respectively (G13, Y2). ANP is a 28-amino-acid polypeptide hormone released into the circulation in response to atrial stretch (L3). ANP acts (Fig. 8) on the kidney to increase sodium excretion and glomerular filtration rate (GFR), to antagonize renal vasoconstriction, and to inhibit renin secretion (Ml). In the cardiovascular system, ANP antagonizes vasoconstriction and shifts fluid from the intravascular to the interstitial compartment (G14). In the adrenal cortex, ANP is a powerful inhibitor of aldosterone synthesis (E6, N3). At the hypothalamic level, ANP inhibits vasopressin secretion (S3). It has been shown that some of the effects of ANP are mediated via a newly discovered hormone, called adreno-medullin, controlling fluid and electrolyte homeostasis (S8). The diuretic and blood pressure-lowering effect of ANP may be partially due to adrenomedullin (V5). 

Figure 11 Schematic of mucosal membrane sodium-proton exchanger and chloride-bicarbonate exchanger responsible for pH homeostasis in enterocyte cytosol. Microclimate pH is maintained by mucosal slowing of proton diffusion away from the lumenal membrane.

The BBB also has sodium- and pH-independent transporters of organic cations. They are important for the homeostasis of choline and thiamine in the brain and for the permeation of cationic drugs like propranolol, lidocaine, fentanyl, Hl-an-... 

In regards to necrosis, it is clear that the old adage an ounce of prevention is worth a pound of cure applies. Agents that stabilize ion homeostasis have proved to be effective in preventing necrosis in cell culture studies. For example, drugs that activate plasma membrane potassium ion channels or chloride ion channels can prevent membrane depolarization and so inhibit sodium and calcium ion influx. Agents that prevent large sustained increases in intracellular free calcium levels can also prevent neuronal... 

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