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 less than 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 less than 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 less than 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.