Time Relation of Renal and Sweat Gland Adjustments to Salt Deficiency in Men

Abstract

NaCl deficiencies of 113-407 meq were produced gradually in men by having them sweat 0.8 to 1.3 kg/hour during work in heat (46[degree]C) up to 6 hours, and in other experiments by sweating 0.5-0.7 kg/hour during rest in more severe heat (51[degree]C) continuously up to 20 hours. In 25-hour salt depletion experiments with intermittent stress the men started by working in the heat (46[degree]C) for 3 or 4 hours and resting in a cool environment thereafter except for work periods in the heat in the 8th, 14th and 25th hours. The rate and amount of salt depletion of the men were determined in each experiment by controlling the subject''s hourly intake of salt, taking into account his current output as measured in sweat and urine. In response to gradual salt depletion in these experiments the men''s kidneys began to decrease their salt output in 1 to 2 hours and completed the adaptive salt conserving response in 5-14 hours. Work and continuous heat stress tended to facilitate salt retention by the kidneys. On the other hand the sweat glands responded much more slowly to salt deficiency, not beginning to decrease the Na and Cl concentrations in the sweat until the 8th hour, even under the most favorable conditions, encountered in the 25-hour experiments with intermittent stress. When salt depletion was combined with continuous stress (heat and work, or heat alone) the initial decrease in sweat concentration was delayed beyond 20 hours. Following the initial decrease in sweat concentration in salt deficiency further reduction was very slow as compared with the rapid reduction of salt in the urine. Previous studies have shown that several days are required for the sweat glands to complete their adaptation to a given salt deficiency. In alternate periods of rest and work in the heat, renal output of salt was lower in work than in rest, while both concentration and hourly salt output in the sweat were much higher in the work periods than in rest.