Syndrome Of Inappropriate Antidiuretic Hormone Secretion Siadh

A variety of tumours, e.g. oat-cell lung cancer, can make vasopressin, and of course they are not subject to normal homeostatic mechanisms. SIADH also occurs in some CNS and respiratory disorders (infection). Dilutional hyponatraemia follows, i.e. low plasma sodium with an inappropriately low plasma osmolality and high urine osmolality. When the plasma sodium approaches 120 mmol/1 treatment should be with fluid restriction (< 500 ml/day). Treatment is primarily of the underlying disorder accompanied by fluid restriction. Chemotherapy to the causative tumour or infection is likely to be the most effective treatment. Demeclocycline, which inhibits the renal action of vasopressin, is useful Infusion of isotonic or hypertonic saline must be reserved for extreme emergencies, associated with stupor, and undertaken with great caution. Rapid correction of hyponatraemia must be avoided because of the risk of central pontine myelinolysis; the rate of correction must not exceed 12 mmol/1 per 24 h.

Oxytocin: see page 731.

Steroid hormone receptors (for gonadal steroids and adrenocortical steroids) are complex proteins inside the target cell. The steroid penetrates, is bound and translocates into the cell nucleus, which is the principal site of action and where RNA/protein synthesis occurs. Compounds that occupy the receptor without causing translocation into the nucleus or the replenishment of receptors act as antagonists, e.g. spironolactone to aldosterone, cyproterone to androgens, clomiphene to oestrogens.

Selectivity. Many synthetic analogues, although classed as, e.g. androgen, anabolic steroid, progestogen, are nonselective and bind to several types of receptor as agonist, partial agonist, antagonist.

The result is that their effects are complex, as will be seen in the following account.

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