Effect of Water Temperature on Diuresis-Natriuresis: AVP, ANP, and Urodilatin During Immersion in Men

Abstract

Effects of water temperature on diuresis, natriuresis, and associated endocrine responses during head-out immersion were studied in 8 men (23.4 +/- 0.3 yr) during four 5-h experimental conditions: air con- trol at 28C and immersion at 34.5C thermoneutral (Tnt), 36C above Tnt (aTnt) and 32C below Tnt (bTnt). Esophageal tempera- ture decreased by -0.4C in bTnt and increased by 0.5C in aTnt. Cardiac output increased by 80% in aTnt and 40% in bTnt while thoracic impedance, an index of central blood pooling, decreased by 7.5 ohm in bTnt (NS vs. Tnt) and 8.8 ohm in aTnt (P<0.05 vs. Tnt and bTnt). Total peripheral resistance decreased al all temperatures (50% in aTnt, 20% in bTnt). Urine flow and Na+ excretion increased by sixfold in bTnt and Tnt but by only threefold at aTnt. Creatini- ne clearance was unchanged while osmolal clearance (but not free wa- ter clearance) increased twofold with all immersions. Plasma atrial natriuretic peptide (ANP), urinary urodilatin, and urinary guanosine 3',5'-cyclic monophosphate increased while plasma renin activity, aldosterone, and arginine vasopressin (AVP) decreased similarly at all temperatures. bTnt did not potentiate diuresis by selective attenuation of AVP. The overal natriuretic response exhibited a higher correlation with urodilatin (r&#610.45, P<0.001) than with ANP (r&#610.26, P<0.01). Because diuresis and natriuresis were significant- ly attenuated in aTnt where central blood pooling was greater, we conclude that mechanims other than the atrial stretch receptor reflex, i.e., urodilatin and effective arterial blood volume, may play morepredominant roles in the mechanism of immersion-induced diuresis and natriuresis.