AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 3 583-R589, Copyright © 1995 by American Physiological Society
Mechanism of thirst attenuation during head-out water immersion in men
F. Wada, S. Sagawa, K. Miki, K. Nagaya, S. Nakamitsu, K. Shiraki and J. E. Greenleaf
Department of Physiology, School of Medicine, University of Occupational and Environmental Health, Kitakyushu, Japan.
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 < 0.05) within 10 min of
immersion and continued thereafter. Mean plasma osmolality (288 +/- 1
mosmol/kgH2O) and sodium (140 +/- 1 meq/l) remained elevated, and plasma
volume increased by 4.2 +/- 1.0% (P < 0.05) throughout Hypo-H2O. A
sustained increase (P < 0.05) in stroke volume accompanied the prompt
and sustained decrease in plasma renin activity and sustained increase (P
< 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 <
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.
