EFFECTS OF HYDRATION STATE ON HORMONAL AND RENAL RESPONSES DURING MODERATE EXERCISE IN THE HEAT

The effects of hydromineral hormones and catecholamines on renal conce ntrating ability at differ-ent hydration states were examined in five male volunteers while they performed three trials. Each of these trial s comprised a 60-min exercise bout on a treadmill (at 50% of maximal o xygen uptake) in a warm environment (dry bulb temperature, 35 degrees C; relative humidity, 20-30%). In one session, subjects were euhydrate d before exercise (C). In the two other sessions, after thermal dehydr ation (loss of 3% body mass) which markedly reduced plasma volume (PV) and increased plasma osmolality (osm(pl)), the subjects exercised eit her not rehydrated !Dh) or rehydrated (Rh) by drinking 600 mi of miner al water before and 40 min after the onset of exercise. During exercis e in the Dh compared to C state, plasma renin, aldosterone, arginine v asopressin (AVP)I noradrenaline and adrenaline concentrations were inc reased (P < 0.05). A reduction in creatinine clearance and urine flow was also observed (P < 0.05) together with a decrease in urine osmolal ity, osmolar clearance and sodium excretion, while free water clearanc e increased (P < 0.05). However, compared to Dh, Rh partially restored PV and osm(pl) and induced a marked reduction in the time courses of both the plasma AVP and catecholamine responses (P < 0.05). Values for renal water and electrolyte excretion were intermediate between those of Db and C. Plasma atrial natriuretic peptide presented similar chan ges whatever the hydration state, These results demonstrate that durin g moderate exercise in the heat, renal concentrating ability is parado xically reduced by prior dehydration in spite of high plasma AVP level s, and might be the result of marked activation of the sympatho-adrena l system. Rehydration, by reducing this activation, could partially re store the renal concentrating ability despite the lowered plasma AVP.