Hormonal changes after supine posture, immersion, and swimming

This study was undertaken to evaluate the effects of the supine posture, immersion, and swimming on hormones involved in the regulation of hydrosaline equilibrium. Plasma levels of atrial natriuretic peptide (ANP), arginine vasopressin (AVP), plasma renin activity (PRA), and aldosterone (ALDO) were measured by radioimmunoassay in eight untrained young subjects (five males and three females). Blood samples were collected on different days: control morning samples in a relaxed standing posture before each test; after 20 min in a supine position; after 20 min of horizontal immersion in water at 28 degrees C; after 20 min of backstroke swimming (speed about 1 m/s). No changes occurred in AVP levels after each test. ALDO and PRA increased significantly only after swimming and were directly correlated. ANP increased significantly after immersion, but no further increase was observed after swimming. The hematocrit, which increased after swimming, was inversely correlated with ANP levels in the post-exercise samples. These data show that while ALDO and PRA increase only in response to swimming, even at moderate intensity, ANP probably requires more prolonged and intense exercise to reach a significantly higher level than in immersion.