RENAL BLOOD-VOLUME REGULATION IN TRAINED AND UNTRAINED SUBJECTS DURING IMMERSION

To study the cause of the increased blood volume of endurance-trained athletes we assessed the renal blood volume regulating mechanisms in eight untrained (UT) and eight endurance-trained (TR) male subjects during a 4 h head-out immersion. In TR plasma volume remained constant whereas it decreased in UT by 2.4 ml/kg (p<0.025). Immersion diuresis of TR was only half as high as in UT (peak values: 3.22 ml/min in UT, 1.60 ml/min in TR). Free water clearance remained approximately constant in UT but temporarily decreased in TR (p<0.001). This points to poor or even absent inhibition of antidiuretic hormone secretion in the latter group. Osmolar clearance increased less in TR than in UT (p<0.02) which was partly due to a delayed increase of glomerular filtration rate. Plasma osmolality, creatinine, and protein concentrations as well as hematocrit values were reduced during immersion to a similar extent in both groups. The results indicate a reduced renal response of endurance-trained subjects to congestion of the low-pressure system resulting in an increase in blood volume. © 1979 Springer-Verlag.