A new method of measuring vasopressin activity is described. It depends on the finding that the Na+-K+-ATPase activity, measured oytochemically, in the thick ascending limb of the loop of Henlo in rat renal tissue maintainedin vitro, responded to increasing concentrations of synthetic arginine vasopressin in a log-dose related fashion. The limit of sensitivity was 0.002 pg/ml (2#xD7;10#x2212;15mol/1). The bee-responses were reproducible; the inter-assay coefficient of variation was 6.4 at a vasopressin concentration of 0.02 pg/ml. Normal plasma stimulated this Na+-K+-ATPase activity, the stimulation being reduced by 98 when the plasma had been treated with an antiserum specific for vasopressin. Measured in this system, the circulating levels of plasma vasopressin, in healthy adults after 18h dehydration, was 4.0#xB1;0.3 pg/ml (mean #xB1; SEM; n=4) and fell to 0.6#xB1;0.1 pg/ml following a water load. Absolute plasma vasopressin values obtained by the oytochemical bioassay were comparable to those measured by radioimmunoassay (r = 0.97, p 0.001).
展开▼
