Fluid regulating factors: Responses to oral and intravenous saline rehydration and subsequent exercise in the heat

Date of Completion

January 1995


Biology, Animal Physiology|Education, Physical|Health Sciences, Recreation




The purpose of investigation was to examine the responses of plasma arginine vasopressin (AVP), renin activity (PRA), and aldosterone (ALD) to oral or intravenous (IV) saline rehydration, following dehydration, and before and after an exercise tolerance test (ETT), in a hot environment. We hypothesized that the responses of these factors would not differ between the rehydration treatments. On four occasions, 8 males (22.1 $\pm$ 0.8 yr, 179.6 $\pm$ 1.5 cm, 73.6 $\pm$ 2.5 kg, 57.9 $\pm$ 1.6 ml $\cdot$ kg$\sp{-1} \cdot$ min$\sp{-1}$, 7.7 $\pm$ 0.9% fat) performed an dehydration protocol (34$\sp\circ$C), to establish a 4% reduction in body weight. Following dehydration, subjects underwent one of four randomly assigned treatments (0.9% IV saline, 0.45% IV saline, 0.45% oral saline, and no rehydration (Control)), during the first 45 minutes of a 100 minute rest period. Subjects then performed the ETT consisting of treadmill walking at 50% VO$\sb2$max, in 36$\sp\circ$C. Blood and urine samples were obtained pre- and post-dehydration, and further blood samples were drawn at 15, 35 and 55 min post-rehydration, and pre- and immediately post-ETT. Plasma AVP and ALD concentrations were not different between each treatment prior to dehydration, but were elevated (p $<$ 0.05) post-dehydration. In general, 15, 35, 55 min post-rehydration, and pre- and post-ETT measures of AVP, ALD, osmolality, and plasma volume shifts did not differ between the rehydration treatments, but did differ from Control measures. Both the Control and 0.9% IV plasma (Na$\sp+$) values were greater (p $<$ 0.05) at each post-rehydration time point and pre- and post-ETT, compared to the other rehydration treatments. No changes in urine volume, (Na$\sp+$) or (K$\sp+$) were observed between the rehydration treatments or the Control trial. Control measures of pre-ETT heart rate and rectal temperature were significantly elevated (p $<$ 0.05) above those in the rehydration treatments. Exercise time in the Control trial was also significantly lower (p $<$ 0.05) compared to rehydration treatment exercise times. These results demonstrate that method or composition of rehydration did not influence the responses of fluid regulatory factors following rehydration or during exercise in a hot environment. Nor does the method of rehydration appear to influence pre- and post-exercise heart rate, core temperature, and performance time. ^