1) Two techniques, bioassay and radioimmunoassay, are used to measure levels of the antidiuretic hormone arginine vasopressin (AVP) but radioimmunoassay is more sensitive and specific. 2) Basal circulating levels of AVP are very low, below the detection limit of most assays, so urinary concentrations are studied instead as a proxy for plasma levels. Patterns of urinary excretion correlate with known stimuli and disorders of AVP secretion. 3) The two techniques generally show close correlation, though radioimmunoassay sometimes detects slightly higher levels, possibly due to non-specific interference rather than biologically inactive fragments.

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 found
St Bartholomew's Hospital, London ECI) even when no biological activity was present and
probably represents nonspecific interference rather
Bioassay 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 in
nine vasopressin (AVP), so that the ability to assay an attempt to circumvent this problem. Since
this 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 index
always possible since the basal circulating levels of the basal secretion of AVP. The peptide can be
of 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 expected
below the level of sensitivity of most assay patterns of excretion were found following water
procedures. loading in patients with the syndrome of inappro-
A relatively specific bioassay employing the priate secretion of antidiuretic hormone, and
water-loaded rat anesthetized with alcohol has water loading, dehydration and smoking in
been 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 exogenous
extraction and concentration procedure developed oxytocin (Boyd et al. 1971) indicate that the
for radioimmunoassay (Edwards et al. 1970), concentration of the hormone in the urine may
based on the adsorption of AVP to porous glass, provide a better indication of plasma levels than
may be used with the bioassay provided a pre- the total amount excreted. If the same holds for
liminary precipitation step is carried out. The endogenous AVP, then measurement of urinary
bioassay may then be used to detect levels of concentration without reference to clearance
05 microunit/ml plasma. Nevertheless, the should be a simple guide to its secretion rate.
capricious nature of the assay and the small REFERENCES
number of samples that can be processed limits Boyd N R H, Jackson D B, Hollingsworth S, Forsling M L
its 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, xi
AVP which are easier to perform and more Fabian M, Forsling M L, Jones J J & Pryor J S
specific, in that many biologically active sub- (1969)J. Physiol. (Lond.) 204, 653
Robertson G L, Klein L A, Roth J & Gorden P
stances, including related peptides such as (1970) Proc. Nat. Acad. Sci. (Wash.) 66, 1298
arginine vasotocin, show little cross-reaction.
Their sensitivity varies according to the antiserum
used, and for some (Robertson et al. 1970) is
comparable with the most sensitive bioassays. Dr M A Barraclough
Plasma levels of AVP following anesthesia, (St Thomas's 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 Antidiuretic
techniques correlate closely. For example, in one Hormone and Potassium Depletion
series of studies on the release of vasopressin in
the cat following hemorrhage, the coefficient of The syndrome of inappropriate secretion of
linear correlation was found to be 0-97 (P<0{001, antidiuretic hormone (SIADH) is characterized
n = 18). However, it was found in most of these by the combination of hyponatremia, failure to
studies that the levels determined by radio- excrete an appropriately dilute urine in the