ALBERT

Description: The purpose was to determine whether extracellular volume or osmolality was the major contributing factor for reduction of thirst in air and head-out water immersion in hypohydrated subjects. Eight males (19 - 25 yr) were subjected to thermoneutral immersion and thermoneutral air under two hydration conditions without further drinking: euhydration in water (Eu-H2O) and euhydration in air, and hypohydration in water (Hypo-H2O) and hypohydration in air (3.7% wt loss after exercise in heat). The increased thirst sensation with Hypo-H2O decreased (P less than 0.05) within 10 min of immersion and continued thereafter. Mean plasma osmolality (288 +/- 1 mosmol/kg H2O) and sodium (140 +/- 1 meq/1) remained elevated, and plasma volume increased by 4.2 +/- 1.0% (P less than 0.05) throughout Hypo-H2O. A sustained increase (P less than 0.05) in stroke volume accompanied the prompt and sustained decrease in plasma renin activity and sustained increase (P less than 0.05) in plasma atrial natriuretic peptide during Eu-H2O and Hypo-H2O. Plasma vasopressin decreased from 5.3 +/- 0.7 to 2.9 +/- 0.5 pg/ml (P less than 0.05) during Hypo-H2O but was unchanged in Eu-H2O. These findings suggest a sustained stimulation of the atrial baroreceptors and reduction of a dipsogenic stimulus without major alterations of extracellular osmolality in Hypo-H2O. Thus it appears that vascular volume induced stimuli of cardiopulmonary baroreceptors play a more important role than extracellular osmolality in reducing thirst sensations during immersion in hypohydrated subjects. Thus the purpose for this study was to determine the relative importance of volume and osmotic stimuli, and associated hormonal interaction, for attenuation of thirst during immersion.

Description: The effect of water immersion on voluntary water intake, subjective evaluations of thirst and gastrointestinal state, and associated fluid-electrolyte and hormonal interaction were investigated. Eight men (19-25 yrs of age) were immersed to the neck while sitting for three hours at 34.5 C or in air at 28 C when euhydrated and hypohydrated by 3.6 percent body weight loss. Within the first ten minutes of immersion the significant reduction in drinking in the hypo-H2O experiment was associated with unchanged plasma Na(+), plasma osmolality, heart rates, and mean arterial pressures. Different responses increased cardiac output, plasma volume, and atrial natriuretic peptides and decreased plasma renin activity and arginine vasopressin. It is concluded that the extracellular pathway, as opposed to the osmotic pathway, appears to be the major mechanism for immersion-induced suppression of drinking.