Thirst perception, plasma osmolality and estimated plasma arginine vasopressin concentration in dehydrated and oral saline loaded subjects.

Infusion of hypertonic saline produces an increase in thirst appreciation as well as increase in plasma Arginine Vasopressin (AVP) concentration, which may be estimated using thirst perception and/or plasma osmolality In this study, we examined the changes in thirst perception (TP), plasma osmolality (Posm) and plasma arginine vasopressin concentration (PAVP) as well as their relationships in dehydrated and oral saline loaded subjects. Forty (40) healthy subjects between the ages of 18 and 30 years were divided into 4 groups of 10 subjects (5 females and 5 males) each. Subjects in group A (dehydrated) were subjected to 18 hr fluid restriction, while those in B, C, and D orally ingested 7.1 ml/kg of 0.9%, 1.8% and 2.7% NaCl solutions respectively. TP was rated using the Visual Analogue Scale, VAS. Estimation of Posm was done using the standard formula and was also calculated from TP. PAVP was calculated from TP and Posm. The changes in TP in water restriction and salt loadings were similar in both males and females. However, the 2.7% NaCl sodium chloride solution intake induced a greater increase in TP than the 18 h fluid restriction. In females there was no significant difference between the measured and calculated Posm at 0 min, in groups A and D, whereas, values for group B (278.8±4.2 vs 292.0±2.9 mOsm/kgH2O) and C (282.6±4.1 vs 295.6±2.5 mOsm/kgH2O) differ significantly (p<0.05). At 90 min, the calculated Posm was significantly higher than the measured Posm in group D (283.6±2.1 vs 312.0±0.8 mOsm/kgH2O, p<0.05) only. In male subjects, there was no significant difference between the measured and calculated Posm in all the groups at 0 min, and at 90 min in groups A and B, while the calculated Posm was significantly higher (p<0.05) than the measured Posm in groups C (283.0±4.6 vs 302.4±3.3 mOsm/kgH2O) and D (275.6±4.8 vs 307.2±2.6 mOsm/kgH2O). In both male and female subjects, there was a significant (p<0.05) and positive correlation between TP and measured Posm at 0 min, but not at 90 min. PAVP values correlated significantly (p<0.05) with Posm and TP at 90 min only in group A. When group A was compared to group B, the changes in Posm were significant (p<0.05) in the males at 0 min (298.0±1.6 vs 281.0±4.4 mOsm/kgH2O), and at 90 min (303.0±1.8 vs 270.6±11.0 mOsm/kgH2O), whilst in the females the changes were only significant at 90 min (304.4±3.5 vs 285.0±4.3 mOsm/kgH2O). We can reasonably conclude the relationship between TP, Posm and PAVP may not be maintained during oral hypertonic saline loading, probably due to body fluid changes.