Rat posterior pituitaries were extracted in
acid and total rat
neurophysins were isolated. Preparative disc gel electrophoresis separated the total
neurophysins into three main
peptides of differing electrophoretic mobility.
Antisera raised in rabbits recognized a common antigenic site in the three
peptides and identical radioimmunoassay standard curves were obtained with each of the isolated rat
neurophysins. A homologous rat
neurophysin radioimmunoassay was utilized to measure
neurophysin in samples of unextracted rat plasma. Basal
neurophysin levels, 3.7 +/- 0.2 ng/ml (mean +/- SEM), did not differ in samples collected by
decapitation, carotid artery cannulation, or tail vein
bleeding. Water-loading caused a significant reduction in
neurophysin, 2.8 +/- 0.1 ng/ml, while hypertonic saline and
dehydration caused a significant elevation, 10.4 +/- 2.1 and 8.0 +/- 1.4 ng/ml, respectively. A step-wise decrease in blood volume caused a step-wise increase in plasma
neurophysin concentrations which returned to baseline with reinfusion of the withdrawn blood. A second
hemorrhage caused an even greater release of
neurophysin indicating large
neurophysin reserve in the pituitary. In periodic tail vein samples over 23 days of pregnancy a rise in plasma
neurophysin was found from day 14 continuing to parturition with a peak value of greater than 13 ng/ml by day 21. Two days postpartum the value was 4.6 +/- 0.3 ng/ml. With this homologous assay, the basal levels of plasma
neurophysin are lower and the stimulated values higher than with previously reported heterologous assays. Therefore, the relative change with physiologic maneuvers is distinctly increased.