43 Section ofEndocrinology 1069

Meeting February 23-24 1971with the Societyfor Endocrinology

Water Metabolism [Abridged]

Dr Mary L Forsling(Department ofChemicalPathology,St Bartholomew's Hospital, London ECI)

Bioassay and Radioimmunoassay ofVasopressin in Relation to Water Metabolism

Several clinical disorders are associated with ab-normal secretion ofthe antidiuretic hormone, argi-nine vasopressin (AVP), so that the ability to assaythis peptide routinely in plasma and urine wouldbe of considerable value. However, this is notalways possible since the basal circulating levelsof the hormone, which are of the order of1 microunit/ml (approximately 25 pg/ml), arebelow the level of sensitivity of most assayprocedures.A relatively specific bioassay employing the

water-loaded rat anesthetized with alcohol hasbeen used for many years, but is not sufficientlysensitive for use with unextracted plasma. Anextraction and concentration procedure developedfor radioimmunoassay (Edwards et al. 1970),based on the adsorption of AVP to porous glass,may be used with the bioassay provided a pre-liminary precipitation step is carried out. Thebioassay may then be used to detect levels of05 microunit/ml plasma. Nevertheless, thecapricious nature of the assay and the smallnumber of samples that can be processed limitsits wide clinical application. More recentlyradioimmunoassays have been developed forAVP which are easier to perform and morespecific, in that many biologically active sub-stances, including related peptides such asarginine 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.Plasma levels of AVP following anesthesia,

surgery and hemorrhage have been studied inseveral species by both bioassay and radio-immunoassay. The results obtained by these twotechniques correlate closely. For example, in oneseries of studies on the release of vasopressin inthe cat following hemorrhage, the coefficient oflinear correlation was found to be 0-97 (P<0{001,n= 18). However, it was found in most of thesestudies that the levels determined by radio-

immunoassay were consistently some 20-30microunits/ml higher. This difference was foundeven when no biological activity was present andprobably represents nonspecific interference ratherthan the presence of biologically inactive frag-ments. Thus the radioimmunoassay at present isnot suitable for the determination of basal levelsofAVP in plasma.

Patterns of urinary excretion were followed inan attempt to circumvent this problem. Sinceurinary levels are generally higher than those inplasma they may provide a more sensitive indexof the basal secretion of AVP. The peptide can beextracted from urine and assayed using the samemethod as employed for plasma. The expectedpatterns of excretion were found following waterloading in patients with the syndrome of inappro-priate secretion of antidiuretic hormone, andwater loading, dehydration and smoking innormal subjects. Experiments using both exo-genous AVP (Fabian et al. 1969) and exogenousoxytocin (Boyd et al. 1971) indicate that theconcentration of the hormone in the urine mayprovide a better indication of plasma levels thanthe total amount excreted. If the same holds forendogenous AVP, then measurement of urinaryconcentration without reference to clearanceshould be a simple guide to its secretion rate.REFERENCESBoyd NR H, Jackson D B, Hollingsworth S, ForslingML& Chard T (1971) J. Endocr. (in press)Edwards CR W, Chard T, KitauMJ& Forsling ML(1970) J. Endocr. 48, xiFabian M, ForslingM L, Jones J J & Pryor J S(1969)J. Physiol. (Lond.) 204, 653Robertson G L, Klein L A, Roth J& Gorden P(1970) Proc. Nat. Acad. Sci. (Wash.) 66, 1298

DrMA Barraclough(St Thomas's Hospital Medical School,London SEJ)

Inappropriate Secretion ofAntidiureticHormone and Potassium Depletion

The syndrome of inappropriate secretion ofantidiuretic hormone (SIADH) is characterizedby the combination of hyponatremia, failure toexcrete an appropriately dilute urine in the