Quantitative Influence of Blood Volume Expansion on the Osmotic Threshold for Vasopressin Release1

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Abstract
The level of plasma osmolality at which hydrated human subjects first respond to an intravenous infusion of 5% saline with a statistically significant fall in free water clearance (without a fall in osmolal clearance or creatinine excretion) has been called the osmotic threshold for vasopressin release. The mean osmotic threshold in 9 normal subjects was found to be 285.32 ±2.93 (sd) mOsm/kg. Expansion of the blood volume resulting from the infusion of 6% dextran in saline was found to increase the osmotic threshold to 288.12 ±4.44 mOsm/kg (p<.01). Blood volume expansion prolonged the duration of the 5% saline infusion from its onset until attainment of the osmotic threshold from 53.6 to 83.6 min (p <.005). The endogenous release of vasopressin at the time of the osmotic threshold reduced the free water clearance to an extent which was approximated by intravenous infusion of vasopressin at the rate of 12 mU/hr. The fall in free water clearance brought about by the infusion of 12 mU/hr of vasopressin was not delayed by dextran-induced blood volume expansion. It appears from these data that the expansion of blood volume by the infusion of 500 ml of 6% dextran in saline reduces the effectiveness of an osmotic stimulus to vasopressin release, as reflected in a delay in attaining and a 2.80 mOsm/kg rise in the osmotic threshold.