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vasopressin bioassy

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  1. 1. 43 Section ofEndocrinology 1069 Meeting February 23-24 1971 with the Societyfor Endocrinology Water Metabolism [Abridged]Dr Mary L Forsling immunoassay were consistently some 20-30(Department of ChemicalPathology, microunits/ml higher. This difference was foundSt Bartholomews Hospital, London ECI) even when no biological activity was present and probably represents nonspecific interference ratherBioassay and Radioimmunoassay of than the presence of biologically inactive frag-Vasopressin in Relation to Water Metabolism ments. Thus the radioimmunoassay at present is not suitable for the determination of basal levels Several clinical disorders are associated with ab- of AVP in plasma.normal secretion of the antidiuretic hormone, argi- Patterns of urinary excretion were followed innine vasopressin (AVP), so that the ability to assay an attempt to circumvent this problem. Sincethis peptide routinely in plasma and urine would urinary levels are generally higher than those in be of considerable value. However, this is not plasma they may provide a more sensitive indexalways possible since the basal circulating levels of the basal secretion of AVP. The peptide can beof the hormone, which are of the order of extracted from urine and assayed using the same 1 microunit/ml (approximately 25 pg/ml), are method as employed for plasma. The expectedbelow the level of sensitivity of most assay patterns of excretion were found following waterprocedures. loading in patients with the syndrome of inappro- A relatively specific bioassay employing the priate secretion of antidiuretic hormone, andwater-loaded rat anesthetized with alcohol has water loading, dehydration and smoking inbeen used for many years, but is not sufficiently normal subjects. Experiments using both exo-sensitive for use with unextracted plasma. An genous AVP (Fabian et al. 1969) and exogenousextraction and concentration procedure developed oxytocin (Boyd et al. 1971) indicate that thefor radioimmunoassay (Edwards et al. 1970), concentration of the hormone in the urine maybased on the adsorption of AVP to porous glass, provide a better indication of plasma levels thanmay be used with the bioassay provided a pre- the total amount excreted. If the same holds forliminary precipitation step is carried out. The endogenous AVP, then measurement of urinarybioassay may then be used to detect levels of concentration without reference to clearance05 microunit/ml plasma. Nevertheless, the should be a simple guide to its secretion rate.capricious nature of the assay and the small REFERENCESnumber of samples that can be processed limits Boyd N R H, Jackson D B, Hollingsworth S, Forsling M Lits wide clinical application. More recently & Chard T (1971) J. Endocr. (in press)radioimmunoassays have been developed for Edwards C R W, Chard T, Kitau M J & Forsling M L (1970) J. Endocr. 48, xiAVP which are easier to perform and more Fabian M, Forsling M L, Jones J J & Pryor J Sspecific, in that many biologically active sub- (1969)J. Physiol. (Lond.) 204, 653 Robertson G L, Klein L A, Roth J & Gorden Pstances, including related peptides such as (1970) Proc. Nat. Acad. Sci. (Wash.) 66, 1298arginine vasotocin, show little cross-reaction.Their sensitivity varies according to the antiserumused, and for some (Robertson et al. 1970) iscomparable with the most sensitive bioassays. Dr M A Barraclough Plasma levels of AVP following anesthesia, (St Thomass Hospital Medical School,surgery and hemorrhage have been studied in London SEJ)several species by both bioassay and radio-immunoassay. The results obtained by these two Inappropriate Secretion of Antidiuretictechniques correlate closely. For example, in one Hormone and Potassium Depletionseries of studies on the release of vasopressin inthe cat following hemorrhage, the coefficient of The syndrome of inappropriate secretion oflinear correlation was found to be 0-97 (P<0{001, antidiuretic hormone (SIADH) is characterizedn = 18). However, it was found in most of these by the combination of hyponatremia, failure tostudies that the levels determined by radio- excrete an appropriately dilute urine in the