Vasopressins assay

Figure 50-13 Retention of plasma vasopressin to osmolality. Regions I and 2 represent the normal reference interval and values seen in subjects with psychogenic polydipsia. Region 3 indicates central diabetes Insipidus, Region 4 is NDl, and Region 5 is SIADH. LD denotes the lowest detectable plasma vasopressin correlation with this assay. (Modified and abstracted from Zerbe RL, Robertson GL A comparison of plasma vasopressin measurements with a standard indirect test in the differential diagnosis of polyuria.

Measurement of Arginine Vasopressin Numerous immunoassays for measuring AVP in plasma or urine have been described. However, their routine clmical application has been hampered because of method complexity and lack of assay sensitivity and specificity. With most plasma assays, a prefiminary extraction procedure is required, not only to concentrate die minute amount of hormone that is present in the specimen, but also to remove nonspecific interfering substances. 

Mice are pretreated with to label the thyroid, 1 yg thyroxine, and codeine prior to injection of the material to be tested. The test material must be free of TSH, vasopressin, and MSH, which exert effects directly on the thyroid (R4). Codeine is given to sensitize (R4) the pituitary, and the small dose of thyroxine is given to partially suppress endogenous TSH secretion and establish basal conditions. Blood levels obtained 2 hours after injection are proportional to the amount of TRF injected (R4). Plasma TSH can be measured directly by use of the McKenzie assay (Mil), but this requires another group of animals. Pituitary TSH depletion can be measured directly by biossay of TSH in the pituitary of treated mice (S3). TSH assays will be discussed in Section 3.4. 

Pituitary glands are incubated in tissue culture medium and TSH measured in the medium after introduction of the TRF. The presence of calcium in the medium is necessary (VI). TSH is measured either by bioassay or more recently by radioimmunoassay (W2). In vitro methods require only a few rat pituitary glands, and incubation times are short. TSH release into the medium is a linear fimction of the log dose (GIO). Crude hypothalamic extracts can be assayed in this way, since contaminating vasopressin does not stimulate TSH release from the pituitary (R2). This latter point has been recently questioned, however, by Krass et al. (K5). Vasopressin may stimulate TSH release in vitro. 

In a series of studies, Sliwa [146-149, 151] demonstrated mouse sperm accumulation in acetylcholine, adrenaline, calcitonin, -endorphin, oj ocin, and substance P. Negative mouse sperm accumulation (i.e., apparent repulsion) was demonstrated with glucagon and vasopressin. However, since only a single assay was used in these studies (a choice assay that did not distinguish between chemotaxis and trapping), the significance of these observations with respect to chemotaxis is not clear. 

The foremost advantage of the chemical assay is that the specificity of the analysis can be tested with the most rigid criteria. In the pituitary assay of oxytocin and vasopressin (Gruber et a/., 1976), specificity was demonstrated by the following four criteria (1) The oxytocin and vasopressin peaks were identified by their retention times. (2) Addition of synthetic oxytocin or vasopressin to the pituitary extract resulted in a quantitative increase in the height of the specific peak with no noticeable peak broadening. (3) After purification by reverse-phase chromatography, each of the nonapeptide fluorophors was collected, hydrolyzed in 6 N HCl, and then characterized... 

The specificity of the antigen-antibody reaction is rather high, and lysine vasopressin, for example, cross-reacted very poorly in the assay system. A variety of structural analogues of oxytocin varied in their cross-reactivity from almost zero to a reactivity actually exceeding that of oxytocin itself (table 1). 1-... 

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