Changes in plasma arginine vasopressin concentrations in cyclists participating in a 109-km cycle race

Objective:To evaluate the osmotic and non-osmotic regulation of arginine vasopressin (AVP) during endurance cycling. Design: Observational study. Setting: 109 km cycle race. Participants: 33 Cyclists. Interventions: None. Main outcome measurements: Plasma sodium concentration ([Na+]), plasma volume (PV) and plasma arginine vasopressin (AVP) concentration ([AVP] p). Results: A fourfold increase in [AVP]p occurred despite a 2-mmol l1 decrease in plasma [Na+] combined with only modest (5%) PV contraction. A significant inverse correlation was noted between [AVP]p δ and urine osmolality δ (r=20.41, p<0.05), whereas non-significant inverse correlations were noted between [AVP]p and both plasma [Na+] δ and % PV δ. Four cyclists finished the race with asymptomatic hyponatraemia. The only significant difference between the entire cohort with this subset of athletes was postrace plasma [Na+] (137.7 vs 133.5 mmol l-1, p<0.001) and plasma [Na+] δ (21.9 vs 25.1 mmol l -1, p<0.05). The mean prerace [AVP]p of these four cyclists was just below the minimum detectable limit (0.3 pg ml-1) and increased marginally (0.4 pg ml-1) despite the decline in plasma [Na+]. Conclusions: The osmotic regulation of [AVP]p during competitive cycling was overshadowed by non-osmotic AVP secretion. The modest decrease in PV was not the primary non-osmotic stimulus to AVP. Partial suppression of AVP occurred in four (12%) cyclists who developed hyponatraemia during 5 h of riding. Therefore, these results confirm that non-osmotic AVP secretion and exercise-associated hyponatraemia does, in fact, occur in cyclists participating in a 109 km cycle race. However, the stimuli to AVP is likely different between cycling and running.