EFFECTS OF ALTITUDE ACCLIMATIZATION ON FLUID REGULATORY HORMONE RESPONSE TO SUBMAXIMAL EXERCISE

To determine the effect of altitude acclimatization on plasma levels o f atrial natriuretic peptide (ANP) during submaximal exercise and its relationship with renin and aldosterone, seven male volunteers aged 17 -23 yr exercised to exhaustion on a cycle ergometer at 80-85% of their maximum O2 uptake at sea level (SL; 50 m), during 1 h in a hypobaric chamber [acute altitude (AA); 4,300 m], and after 14 or 16 days of res idence on the summit of Pikes Peak, CO [chronic altitude (CA); 4,300 m ]. Plasma samples taken before exercise, 10 min after the start of exe rcise, and 5 min postexercise were analyzed for ANP, plasma renin acti vity (PRA), and aldosterone (ALDO). ANP showed a progressive increase from rest to postexercise [7.49 +/-1.63 to 11.32 +/- 1.80 (SE) pmol/ml and 6.05 +/- 2.55 to 10.38 +/-7.20 pmol/ml; P = 0.049, exercise] at S L and AA, respectively, but not at CA (P = 0.039, altitude). Similarly , PRA and ALDO rose from rest to postexercise (P < 0.001, exercise), b ut the rise in ALDO with exercise was less during AA than during SL an d CA (P = 0.002, phase). The decreased ANP levels during exercise afte r altitude acclimatization, with no change in PRA and ALDO, suggest th at ANP has little effect on PRA and ALDO under these conditions.