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.
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 l−1 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 = −0.41, 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 (−1.9 vs −5.1 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.
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Exercise-associated hyponatraemia (EAH) is a lifethreatening disorder of fluid homeostasis that has claimed the lives of five otherwise healthy marathon runners over the past 15 years.1 Although the incidence of hyponatraemia has been reported to be as high as 13% in marathon runners2 and 27% of Ironman triathletes,3 the incidence of EAH in cyclists is seemingly rare. Only one published case of symptomatic EAH has been reported after an endurance cycle race4, whereas one additional case has been documented after a laboratory cycling trial in the heat.5 Both of these cases of symptomatic EAH involved older (57 and 65 years, respectively), petite (67 and 46 kg) women who gained 2.4 kg by the end of the exercise bout despite modest fluid intake (735 ml h−1) in the former and excessive fluid intake (2.8 litres over 60 min of total exercise) in the latter. Plasma concentrations of arginine vasopressin (AVP) were not measured in either report.
Non-osmotic stimulation of AVP secretion during endurance exercise has recently been verified in 82 runners participating in a 56 km running race.1 The proposed stimuli for non-osmotic AVP secretion during ultraendurance running was significant (∼9%) plasma volume (PV) contraction combined with concurrent stimulation by other endocrine factors such as oxytocin, brain natriuretic peptide and corticosterone.
The aim of this study was to evaluate the osmotic and non-osmotic regulation of AVP during endurance cycling. Because cycling involves concentric rather than eccentric muscle contractions (less muscle damage)6 and requires a sitting posture (less PV contraction from decreased hydrostatic forces associated with an upright posture),7 we hypothesised that non-osmotic stimulation of AVP during cycling may have had less of an overriding effect on osmotic AVP secretion than during running. This hypothesised decrease in non-osmotic AVP secretion during cycling exercise could then explain why the incidence of EAH is markedly less in endurance cyclists than is currently reported in runners and Ironman triathletes (where running is the final leg).
Subjects and sample collection
Informed written consent was obtained in 34 cyclists competing in a 109-km cycle race in Cape Town, South Africa, in 2005. This study was approved by both the Ethics Committee of the University of Cape Town and the Georgetown University Institutional Review Board.
Baseline body weight, blood and urine samples were obtained within 60 min of the start of the race. Postrace body weight, blood and urine samples were obtained within 10 min of the race completion. All prerace and postrace blood samples were immediately placed on ice and centrifuged at 3000 rpm. Separated plasma was stored on dry ice until the samples were frozen to −80°C. All samples remained frozen until further analysis was performed. Body weight was measured with athletes in racing attire without shoes on calibrated Adamlab JPS electronic scales placed on a hard flat surface (Scales, Brackenfell, South Africa). Food and fluid intake were allowed ad libitum during the race, and estimated total fluid intake was self-reported immediately after the race via a questionnaire.
Changes in PV were estimated by comparing prerace and postrace measurements of plasma protein using a clinical refractometer (Schuco Clinical Refractometer 5711–2020; Tokyo, Japan). Plasma and urine sodium ([Na+]) were measured using ion-selective electrodes (Beckman Synchron EI-ISE, Fullerton, California, USA). Urine osmolality (Uosm) was measured using a vapour pressure osmometer (VAPRO 5520; WESCOR, Logan, Utah, USA).
Plasma levels of arginine vasopressin ([AVP]p) was measured by specific radioimmunoassay following acetone–ether extraction as described previously.8 The standard curve for AVP is linear between 0.5- and 10.0-pg tube with the use of a synthetic AVP standard (PerkinElmer Life Sciences, Boston, Massachusetts, USA). The minimum detectable concentration of AVP in extracted plasma was 0.5 pg ml−1. The AVP antiserum (R-4) displayed <1% cross-reactivity with OT.
Differences (Δ) were calculated as postrace values minus prerace values and presented as mean (SE). Paired t tests were used to assess significant differences between prerace and postrace. Statistical significance was accepted when p<0.05. In addition to the original cohort, four cyclists who participated in this trial also participated in a 56 km running race held in Cape Town 2 weeks later.1 Data collected from these two separate events were compared in these four individuals.
Thirty-three (26 men and 7 women) of 34 subjects successfully completed the cycle race and presented to the finish line for postrace testing. The average age of the cyclists was 42.9 (2.1) years with an average finishing time of 4:56 (h:min). The only significant differences between men and women cyclists were the expected anthropometric differences in height (1.8 vs 1.7 m, p<0.05) and weight (86.0 vs 68.5 kg, p<0.05). Thus, all data were analysed as a single cohort.
A significant increase in [AVP]p (prerace value, 1.2 pg ml−1) was documented despite a significant decrease in plasma [Na+] (prerace, 139.5 mmol l−1) (table 1). Significant decreases in PV, body weight and urine [Na+] were also noted with an expected but non-significant 639.4 mOsmol kg−1 increase in urine osmolality (prerace, H2O). Positive linear correlations were noted between urine osmolality versus urine [Na+] in both the postrace (r = 0.40, p<0.05) and Δ (r = 0.44, p<0.05) conditions. Significant inverse correlations were noted between total fluid intake and % body weight Δ (r = −0.40, p<0.05) and between [AVP]p Δ and urine osmolality Δ (fig 1A). Nonsignificant inverse correlations were noted between [AVP]p and both plasma [Na+] Δ and % PV Δ (fig 1B,C). These correlations with [AVP] Δ were performed when one outlier—who was two SDs outside of the mean value for both prerace (11.8 pg ml−1) and postrace (41.3 pg ml−1) [AVP]p—was removed from the analysis. This enabled the mathematical spread of the cohort to be viewed and analysed as a normalised group not heavily skewed by a single outlier. The mean values for the entire cohort, however, did not significantly change with or without this outlier (table 1).
Four cyclists (12%; three men and one woman) finished the cycle race with asymptomatic EAH (table 2). The only significant difference between the entire cohort with this subset of hyponatraemic athletes was postrace plasma [Na+] (137.7 vs 133.5 mmol l−1, p<0.001) and plasma [Na+] Δ (−1.9 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 despite the significant (∼5 mmol l−1) decline in postrace plasma [Na+]. Two of the cyclists gained weight (0.2 and 0.8%), whereas the remaining two cyclists lost weight (−1% and −3%). None of these subjects reported nausea and/or vomiting during the race, and only one athlete reported urinating along the course. The only athlete who was able to urinate was the lightest (64 kg) and slowest (405 min) cyclist and the only athlete in this group with a negative [AVP]p Δ (prerace: 1.3 pg ml−1, postrace: 0.6 pg ml−1). Fluid intake was not significantly different in this group of hyponatraemic athletes compared with the entire cohort when expressed as ml kg−1 (28.6 vs 26.5), ml kg h−1 (5.4 vs 5.3) or ml h−1 (427.0 vs 439.9), respectively.
Additionally, four separate cyclists (three men and one woman) participated in a 56-km running race held 2 weeks later where the experimental protocol was repeated.1 No statistically significant differences existed between the two exercise conditions with regard to any fluid balance parameter or biochemical measurement despite significant differences in finishing time and total fluid intake (table 3). Similar prerace, postrace and Δ values for [AVP]p and plasma [Na+] were noted despite a threefold increase in PV contraction combined with a 50% less body weight loss after the cycle race. The fluid requirements necessary to maintain plasma [Na+] in the cycling versus running conditions were twofold to threefold larger when expressed as: ml kg−1 (14.9 vs 41.3), ml kg h−1 (3.6 vs 6.9) or ml h−1 (272.9 vs 538.4).
This study documents the overriding presence of non-osmotic AVP secretion in cyclists participating in a 109 km cycle race. The approximately fourfold increase in [AVP]p occurred despite an ∼2-mmol l−1 decrease in plasma [Na+] combined with only modest (∼5%) PV contraction. These integrated findings suggest that other non-osmotic stimuli, such as stimulation by other endocrine factors,1 nausea/vomiting,9 hypoglycaemia,10 cytokines11 and/or elevated body temperature12, may have alternatively stimulated AVP secretion during prolonged endurance cycling. The non-significant inverse correlations between [AVP]p Δ with both plasma [Na+]D (fig 1B) and % PVD (fig 1C) further support this premise.
The prerace to postrace increase in [AVP]p combined with a decrease in plasma [Na+] was similarly documented in a large cohort of marathon runners participating in a 56 km running race.1 The primary non-osmotic stimulus proposed in those runners was a more robust PV contraction (∼9%) as verified by a significant increase in plasma aldosterone concentration that was linearly related with % PV decrease. The degree of PV contraction that was documented in this cohort of endurance cyclists was more modest (<8%) and therefore less likely to be the primary non-osmotic stimulus to AVP secretion during endurance cycling. This conclusion is supported by a laboratory investigation involving 13 well-trained men cycling at 55% and 75% of maximal aerobic power, whereas a −4% and −6% respective decrease in PV did not correlate with changes in [AVP]p.13 In that study, AVP changed in relationship to plasma osmolality.
A comparison of biochemical and fluid balance parameters in the four athletes who participated in both the 109 km cycle and the 56 km footrace1 also did not support the hypothesis that a significant decrease in PV during prolonged endurance exercise acted as the primary stimulus for non-osmotic AVP secretion (table 3). In this subset of athletes, the degree of PV contraction was actually greater after cycling (∼9%) than after running (∼3%) despite near identical prerace, postrace and Δ values for both [AVP]p and plasma [Na+]. A greater degree of PV contraction in the same individual exercising at the same intensity during cycling (∼8%) compared with running (∼4%)14 or a similar degree of PV contraction between exercise conditions (7–10%)15 16 has been confirmed previously, although changes in [AVP]p were never measured. Interestingly, all four athletes in our comparative analysis were able to urinate during the ∼6 h marathon footrace, whereas only one of four athletes was able to urinate during the ∼4 h cycle event. This finding may allude to the dynamic changes in [AVP]p during exercise which highlights the limitations of measuring plasma concentrations of AVP, volume and [Na+] only prerace and postrace. Alternatively, this finding may reflect a decrease in the renal sensitivity of AVP V2 receptors during the running condition only, as reflected by the paradoxical decrease in urine osmolality despite increased [AVP]p. An increase in aldosterone secretion during running compared with cycling may have also contributed to the decrease in urine osmolality seen after the running condition only (from increased sodium reabsorption), although aldosterone was not measured in these cyclists. Increased free water excretion despite a dehydration-induced increase in [AVP]p has been previously documented in five men exercising on a treadmill which would support this seemingly paradoxical finding.17
The significant linear relationship between [AVP]p Δ versus urine osmolality Δ (fig 1A) in this cohort of cyclists suggests that the renal response to AVP was appropriate during endurance cycling. The inverse nature of this correlation was curious, however, and most likely representative of a delay in the stimulus–response relationship of urine osmolality (response) to the more acute changes in [AVP]p (stimulus). Changes in [AVP]p in relationship to changes in both plasma [Na+] (fig 1B) and volume (fig 1C) were not linearly related from a statistical standpoint. However, osmotic and volaemic regulation of [AVP]p should not be considered to be ineffective during endurance cycling but simultaneous stimuli occurring alternatively overridden by unexpectedly in the field scenario. The osmotic regulation of [AVP]p during cycling has been well documented in controlled laboratory environments lasting more than 2 h18 below maximal exercise intensities.19 20 Therefore, a safe conclusion from these combined data would be to assume that osmotic regulation of [AVP]p occurs during endurance cycling (as documented in laboratory settings) but acknowledge that multiple non-osmotic stimuli can often supersede the osmotic suppression of AVP secretion during competitive and prolonged cycling events.
This conclusion is further supported in a separate analysis of four cyclists who developed asymptomatic EAH during the 109 km cycle race (table 2). Both prerace and postrace [AVP]p hovered around the minimal detectable limit for this hormone. This would suggest that appropriate osmotic suppression of AVP secretion partially athletes who presented 134 mmol l−1 and a corresponding [AVP]p of 0.7 pg ml−1. The large documented decrease in plasma [Na+](−5.1 mmol l−1) associated with the small increase in [AVP]p (0.4 pg ml−1), however, suggests that AVP secretion was still inappropriate and classifies these cases of EAH as a variant of the syndrome of inappropriate antidiuretic hormone secretion. PV contraction was still evident in these subjects although PV was relatively “expanded” in relation to the value obtained for the entire cohort (−4% vs −5%, respectively). Body weight loss was negligible (−0.8%) which suggests that the hyponatraemia was largely dilutional. The decrease in urine osmolality from prerace to postrace implied an increase in free water excretion, which would further support appropriate AVP suppression in these athletes. Conversely, the postrace urine osmolality value (549 mOsmol kg−1 H2O) could be considered “inappropriately high” in relationship to the degree of hypo-osmolality present in the plasma and thereby inappropriate antidiuretic should be emphasised that 75% of these cyclists did not report emptying their bladder during 5 h of riding. Thus, these urine measurements represent an aggregated sample which may not accurately reflect simultaneous plasma measurement of postrace AVP concentration.
In summary, non-osmotic AVP secretion during a 109 km cycle race overshadowed the osmotic regulation of pituitary AVP secretion during competitive cycling exercise. It is unclear whether or not the modest (5%) decrease in PV from prerace to postrace was the primary non-osmotic stimulus to AVP. Partial suppression of AVP was documented in four (12%) athletes who developed asymptomatic EAH during 5 h of cycling. The decreased influence of PV on AVP secretion or enhanced ability occurred in these hyponatraemic with a postrace plasma [Na+]of diagnostic of the hormone secretion.21 syndrome of However, it to osmotically suppress AVP secretion when hypo-osmolality does develop during prolonged cycling exercise may thereby explain the unequal incidence of morbidity and mortality from EAH in cyclists compared with runners who participate in prolonged endurance competitive events.
This work was supported by a research grant from Astellas Pharma US, Inc., Deerfield, Illinois, USA and the MRC/UCT Research Unit for Exercise Science and Sports Medicine for financial support. The authors wish to thank Yoshihisa Sugimura and Ying Tian for their kind assistance with biochemical analyses; Lara Dugas, Ross Tucker, Julia Goedecke, Liane Berretta, George Mokone, Courtney Jennings and Bill Butler for their technical assistance; and special thanks to the 33 enthusiastic cyclists who selflessly participated in this investigation to help other athletes.
Competing interests None.
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