Effect of hyperosmolality on control of blood flow and sweating

To study the effect of hyperosmolality on thermoregulatory responses, five men [average maximal O2 consumption (VO2 max) = 48 ml X kg-1 X min-1] cycled at 65-75% VO2max for up to 30 min in a 30 degrees C, 40% relative humidity environment under three conditions. First, control tests (C) were performed where preexercise plasma volume (PV) and osmolality (Osm) averaged 3,800 ml and 282 mosmol X kg-1, respectively. Second, exercise tests (D) were performed following dehydration induced by fluid restriction and mild exercise (30% VO2max) in hot (40 degrees C) ambient conditions. Each subject then rested in cool surroundings 1 h before performing the exercise test. Preexercise PV and Osm averaged 3,606 ml and 293 mosmol X kg-1, respectively. Third, exercise tests (I) were performed following dehydration, but during the 1-h rest interval, 3% saline was infused so that PV was restored to 3,826 ml and Osm averaged 294 mosmol X kg-1 prior to exercise. During D, esophageal temperatures (Tes) were significantly higher than C, an avg 0.56 degrees C after 20 min exercise due to a 0.22 degrees C increase in Tes threshold for vasodilation, a 39% reduction in slope of the forearm blood flow (BF)-Tes relationship, a 32% average reduction in maximal exercise BF, and a 0.22 degrees C increase in Tes sweating threshold. During I, responses were similar to D, except the BF-Tes slope and the maximum BF were not significantly different from C. Thus hyperosmolality modifies thermoregulation by elevating thresholds for both vasodilation and sweating even without decreases in PV.