Plasma expansion does not precipitate the fall in plasma vasopressin in humans drinking isotonic fluids.

Abstract

SUMMARY1. In a group of healthy humans, plasma vasopressin (AVP) levels fell on drinking either Tyrode or mannitol solutions isosmotic with plasma. Both the timing and magnitude of the fall were appropriate to account for the transient diuresis which followed the drinking.2. Although plasma expansion follows drinking Tyrode solution it occurred too late to account for the fall in plasma AVP. It was also too small to inhibit AVP secretion.3. Even though plasma volume tended to contract on drinking isosmotic mannitol solution a fall in plasma AVP and a diuresis occurred, similar to those found after drinking Tyrode solution.4. These findings appear to eliminate plasma volume expansion as the stimulus for the fall in plasma AVP and the associated diuresis on drinking isotonic fluids.5. In a further group of human subjects, bypassing the oropharynx by intragastric infusion resulted in a slower onset of diuresis after a water load.6. We suggest that receptors, as yet undefined, in the upper gastrointestinal tract contribute to the early stages of a water diuresis and account for the apparently inappropriate transient diuresis which follows the drinking of isotonic fluids.

The location of the receptors involved in the human diuretic response to drinking an isotonic electrolyte solution.

“…Since Tyrode solution is slowly absorbed activation of systemic volume receptors (Dunn, Brennan, Nelson & Robertson, 1973) could possibly augment or sustain the effects of the oropharyngeal signals on plasma AVP levels. In the present study we did not measure a fall in plasma AVP levels following the drinking of Golytely even though the assay used in this study was the same as that used in the study of Cotter et al (1986) and sufficiently sensitive to measure changes in plasma AVP of 0 6 pg ml-'. Furthermore measured levels of plasma AVP agreed well with those predicted by Robertson's equation (Robertson et al 1976) on the control morning and for the predrinking period on the test morning.…”

“…The excretion of a dilute urine is not only attributed to but has been shown to be due to a decrease in plasma AVP levels. A fall in plasma AVP has been observed following the drinking of Tyrode solution, though not sufficient to account for the magnitude of the diuresis (Cotter et al 1986). This fall was attributed to stimulation of oropharyngeal receptors which have been shown by a number of workers to decrease plasma AVP (Thrasher et al 1980;Geelan et al 1984).…”

“…10.40 and 12.00 h, and on the 'drinking' morning plasma AVP was measured at the start of drinking (0 min) and 20, 30 and 120 min later. This sampling protocol was adopted in the light of the results of previous experiments in which Tyrode solution was drunk (Cotter et al 1986). AVP was measured by the method of Ang (1984).…”

Expansion of plasma volume by intragastric isotonic saline inhibits supraoptic neurones in rats

“…Cotter et al (1986) and Thrasher, Keil & Ramsey (1987) concluded that the effect of fluid on the oral cavity and oesophagus was the major factor in mediating this response. Our data support the contention that gastric distension may also be important, at least in rats.…”

“…The present experiments were undertaken to investigate this inter¬ action further and to establish whether increasing plasma volume by administration of isotonic saline would decrease the activity of cells in the supraoptic nucleus. Measurement of changes in haematocrit is an established method for detecting changes in plasma volume (Cotter, Gebruers, Hall & O'Sullivan, 1986).…”