To assess the relationship between atrial natriuretic peptide (ANP) and the reduction in plasma volume (PV) during exercise, we measured changes in PV and ANP in seven male volunteers during treadmill exercise in air (AE) and with water immersion (WE) together with time control studies of rest in air and in water. Blood samples were collected from a catheter in the antecubital vein at exercise intensities of 32, 49, 65, and 78% of peak oxygen consumption (V̇O2). Plasma ANP in AE increased significantly from the resting value [15 ± 1 (SE) pg/ml] only at 78% of peak V̇O2 (29 ± 5 pg/ml), whereas ANP in WE increased significantly at exercise levels of >49% of peak V̇O2 and reached 68 ± 9 pg/ml at 78% of peak V̇O2. Although PV in AE and WE decreased significantly with V̇O2 of >49% of peak V̇O2 (P < 0.01), the decrease from the resting value in WE was significantly greater than that in AE of >65% of peak V̇O2 (P < 0.01) and the decreases at 78% of peak V̇O2 were -9.7 ± 0.8 and -6.1 ± 1.7%, respectively. The difference in the decrease in PV between AE and WE at corresponding V̇O2 correlated strongly with that in the increase in ANP (r = -0.97; P < 0.01). These results are consistent with the hypothesis that ANP may be involved in the fluid shift from the intra- to extravascular space during exercise.
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