Is vasopressin in the circulation of the dog freely filterable?

Studies were carried out to determine whether endogenously secreted vasopressin in the circulation of the dog is freely filterable. Blood from anaesthetized dogs was pumped through a dialysis coil and ultrafiltration was achieved by increasing the outflor pressure from the coil. In some cses, the dogs were hydrated before the experiment. A specific, precise radioimmunoassay was used to measure the concentration of vasopressin in the ultrafiltrate and the simultaneously sampled plasma. The concentration of vasopressin in the ultrafiltrate was consistently lower than that in the plasma at concentrations ranging from 1.7 to 335 micronmicron./ml in the latter, and the extent of the binding was lower in the hydrated than in the non-hydrated dogs. At plasma levels of vasopressin of 2-20 micrommicron./ml, vasopressin binding averaged only about 12%, whereas it averaged 40% at plasma concentrations greater than 20 micronmicrom./ml. This binding was not an artifact caused by some limitation of the ultrafiltration system because vasopressin dissolved in a salt solution and subjected to ultrafiltration in the system was freely filterable.