Increased renal tubular sodium reabsorption during exercise-induced hypervolemia in humans

Physiological confounders of renal blood flow measurement

OBJECTIVES

Renal blood flow (RBF) is controlled by a number of physiological factors that can contribute to the variability of its measurement. The purpose of this review is to assess the changes in RBF in response to a wide range of physiological confounders and derive practical recommendations on patient preparation and interpretation of RBF measurements with MRI.

METHODS

A comprehensive search was conducted to include articles reporting on physiological variations of renal perfusion, blood and/or plasma flow in healthy humans.

RESULTS

A total of 24 potential confounders were identified from the literature search and categorized into non-modifiable and modifiable factors. The non-modifiable factors include variables related to the demographics of a population (e.g. age, sex, and race) which cannot be manipulated but should be considered when interpreting RBF values between subjects. The modifiable factors include different activities (e.g. food/fluid intake, exercise training and medication use) that can be standardized in the study design. For each of the modifiable factors, evidence-based recommendations are provided to control for them in an RBF-measurement.

CONCLUSION

Future studies aiming to measure RBF are encouraged to follow a rigorous study design, that takes into account these recommendations for controlling the factors that can influence RBF results.

Renal and Endocrine Responses to Arm Exercise in Persons with Cervical Spinal Cord Injury

The aim of this study was to assess renal functions and endocrine responses to arm exercise in persons with cervical spinal cord injury (CSCI) under euhydrated conditions (free drinking of water), and to determine the physiological effects of exercise on renal function in these subjects. Eleven CSCI individuals (spinal lesions between C6 and C8, American Spinal Injury Association impairment scale A) and nine able-bodied (AB) persons rested for 30 min before performing 30 min arm-crank ergometer exercises at 50% of their maximum oxygen consumption, followed by 60-min of rest/recovery. Urine and blood samples were collected before and immediately after the exercise and recovery period. The CSCI patients showed no increase in plasma adrenaline and plasma renin activity compared with the AB controls, but showed similar changes in plasma aldosterone and the plasma antidiuretic hormone in response to the exercise. Creatinine clearance, osmolal clearance, free water clearance, and the fractional excretion of Na⁺ did not change during exercise in both groups of subjects, however free water clearance in the CSCI group was higher than in the AB group throughout the study. These findings suggested that activated plasma aldosterone without an increase in adrenaline or renin activity during exercise in CSCI individuals may reflect an adaptation to the disturbance of the sympathetic nervous system to compensate for renal function. As a result, no adverse effects of exercise on renal function in CSCI patients were observed.

The influence of exercise volume and posture on exercise-induced plasma volume expansion

Acute high-intensity interval exercise is known to expand plasma volume 24 h after exercise. Upright exercise posture plays a role in expanding plasma volume by influencing lymphatic outflow and redistributing albumin while supine exercise does not. We examined if further upright and weight-bearing exercises could further promote plasma volume expansion. We also tested the volume of intervals needed to induce plasma volume expansion. To test the first hypothesis, 10 subjects performed intermittent high-intensity exercise (4 min at 85% V̇_O2max , 5 min at 40% V̇_O2max repeated 8 times) on separate days on the treadmill and cycle ergometer. For the second study, 10 subjects performed four, six, and eight intervals of the same interval protocol on separate days. Changes in plasma volume were calculated from changes in hematocrit and hemoglobin. Transthoracic impedance (Z₀ ) and plasma albumin were assessed while seated before and postexercise. Plasma volume increased 7.3% ± 4.4% and 6.3% ± 3.5% following treadmill and cycle ergometer exercise, respectively. For four, six, and eight intervals, plasma volume increased by 6.6% ± 4.0%, 4.7% ± 2.6%, and 4.2% ± 5.6%, respectively. The increases in plasma volume were similar for both exercise modes and all three exercise volumes. There were no differences in Z₀ or plasma albumin content between trials. In conclusion, rapid plasma volume expansion following eight bouts of high-intensity intervals appears to be independent of upright exercise posture (treadmill versus cycle ergometer). Meanwhile, plasma volume expansion was similar after four, six, and eight intervals of cycle ergometry.

Exercise Heat Acclimation With Dehydration Does Not Affect Vascular and Cardiac Volumes or Systemic Hemodynamics During Endurance Exercise

Permissive dehydration during exercise heat acclimation (HA) may enhance hematological and cardiovascular adaptations and thus acute responses to prolonged exercise. However, the independent role of permissive dehydration on vascular and cardiac volumes, ventricular-arterial (VA) coupling and systemic hemodynamics has not been systematically investigated. Seven males completed two 10-day exercise HA interventions with controlled heart rate (HR) where euhydration was maintained or permissive dehydration (-2.9 ± 0.5% body mass) occurred. Two experimental trials were conducted before and after each HA intervention where euhydration was maintained (-0.5 ± 0.4%) or dehydration was induced (-3.6 ± 0.6%) via prescribed fluid intakes. Rectal (T_re) and skin temperatures, HR, blood (BV) and left ventricular (LV) volumes, and systemic hemodynamics were measured at rest and during bouts of semi-recumbent cycling (55% V̇O_2peak) in 33°C at 20, 100, and 180 min. Throughout HA sweat rate (12 ± 9%) and power output (18 ± 7 W) increased (P < 0.05), whereas T_re was 38.4 ± 0.2°C during the 75 min of HR controlled exercise (P = 1.00). Neither HA intervention altered resting and euhydrated exercising T_re, BV, LV diastolic and systolic volumes, systemic hemodynamics, and VA coupling (P > 0.05). Furthermore, the thermal and cardiovascular strain during exercise with acute dehydration post-HA was not influenced by HA hydration strategy. Instead, elevations in T_re and HR and reductions in BV and cardiac output matched pre-HA levels (P > 0.05). These findings indicate that permissive dehydration during exercise HA with controlled HR and maintained thermal stimulus does not affect hematological or cardiovascular responses during acute endurance exercise under moderate heat stress with maintained euhydration or moderate dehydration.

Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies

A rise in body core temperature and loss of body water via sweating are natural consequences of prolonged exercise in the heat. This review provides a comprehensive and integrative overview of how the human body responds to exercise under heat stress and the countermeasures that can be adopted to enhance aerobic performance under such environmental conditions. The fundamental concepts and physiological processes associated with thermoregulation and fluid balance are initially described, followed by a summary of methods to determine thermal strain and hydration status. An outline is provided on how exercise-heat stress disrupts these homeostatic processes, leading to hyperthermia, hypohydration, sodium disturbances, and in some cases exertional heat illness. The impact of heat stress on human performance is also examined, including the underlying physiological mechanisms that mediate the impairment of exercise performance. Similarly, the influence of hydration status on performance in the heat and how systemic and peripheral hemodynamic adjustments contribute to fatigue development is elucidated. This review also discusses strategies to mitigate the effects of hyperthermia and hypohydration on exercise performance in the heat by examining the benefits of heat acclimation, cooling strategies, and hyperhydration. Finally, contemporary controversies are summarized and future research directions are provided.

Diurnal variation in red blood cell variables in athletes after single and repeated bouts of exercise

BACKGROUND

The aim of this study was to examine the diurnal and exercise-related changes in red blood cell variables which serve as indirect markers of doping in sports.

METHODS

Ten men and 7 women, all undoped highly trained endurance athletes aged 19-34 years, were included in the study. Before and on the day with single and repeated bouts of exercise, blood samples were collected at 07:00, 09:00, 13:00, 18:00, and 21:00, at least 2 hours after exercise. Hemoglobin (Hb), hematocrit (Hct), and the reticulocytes % (Ret%) were determined by flow cytometry. Changes in OFF-hr score were also calculated and repeated-measures ANOVA was used to compare diurnal differences.

RESULTS

In overall, the mean Hb decreased continuously by 4.5 and 3.3% (all P<0.001) over the day with single and repeated bouts of exercise, respectively. Corresponding values for the decline in Hct were 4.2 and 5.9% (all P<0.001). In contrast, the Ret% showed no apparent diurnal rhythm but single and repeated bouts of exercise increased the relative Ret% in the evening by 12.4 and 16.7% (P<0.01), respectively. Then the calculated OFF-hr score was reduced by 6.2 and 9.8% (all P<0.01) at the end of the day.

CONCLUSIONS

These results confirmed the normal diurnal pattern in the examined red blood cell variables in response to exercise in highly trained athletes. Furthermore, they showed noticeable between-subject variability and the possible risk of a false suspicion of blood doping in undoped athletes after repeated bouts of exercise.

Heat-induced hypervolemia: Does the mode of acclimation matter and what are the implications for performance at Tokyo 2020?

Tokyo 2020 will likely be the most heat stressful Olympics to date, so preparation to mitigate the effects of humid heat will be essential for performance in several of the 33 sports. One key consideration is heat acclimation (HA); the repeated exposure to heat to elicit physiological and psychophysical adaptations that improve tolerance and exercise performance in the heat. Heat can be imposed in various ways, including exercise in the heat, hot water immersion, or passive exposure to hot air (e.g., sauna). The physical requirements of each sport will determine the impact that the heat has on performance, and the adaptations required from HA to mitigate these effects. This review focuses on one key adaptation, plasma volume expansion (PVE), and how the mode of HA may affect the kinetics of adaptation. PVE constitutes a primary HA-mediated adaptation and contributes to functional adaptations (e.g., lower heart rate and increased heat loss capacity), which may be particularly important in athletes of "sub-elite" cardiorespiratory fitness (e.g., team sports), alongside athletes of prolonged endurance events. This review: i) highlights the ability of exercise in the heat, hot-water immersion, and passive hot air to expand PV, providing the first quantitative assessment of the efficacy of different heating modes; ii) discusses how this may apply to athletes at Tokyo 2020; and iii) provides recommendations regarding the protocol of HA and the prospect for achieving PVE (and the related outcomes).

No Relation Between Zinc Status and Inflammatory Biomarkers in Adolescent Judokas

This study aimed to assess the relation between zinc status and inflammation biomarkers in adolescent judokas. This cross-sectional study included 52 male adolescents, aged between 14 and 19 years, who were subdivided into two groups: judoka group (n = 25) and control group (n = 27). Zinc intake was monitored using 3-day food records and the NutWin software version 1.5. The plasma and erythrocyte zinc concentrations were determined by flame atomic absorption spectrophotometry. Analysis of cytokines (IL-1β, IL-6, and TNF-α) was performed. The mean values of zinc concentration in the diet were found to be higher than those recommended (11.0±3.9 mg/day and 20.3±11.9 mg/day for control group and judokas, respectively) although there was no significant difference between the groups. The mean plasma concentrations of zinc were below the reference range (71.4±16.0 μg/dL and 71.9±13.8 μg/dL for control group and judokas, respectively), without a significant difference between the groups. The mean concentrations of zinc erythrocyte were within the reference range (41.2±8.6 μg/gHb and 42.6±11.3 μg/gHb for control group and judokas, respectively), without a significant difference between the groups. There was no significant difference in the inflammatory biomarkers between the judokas and controls. There was not a significant correlation between biochemical parameters of zinc and inflammation biomarkers in adolescent judokas. Regarding the data found in the study, it can be concluded that the athletes evaluated have low plasma zinc concentrations, normal erythrocyte values, and high dietary intake of zinc. Moreover, the study don't show a relationship between zinc parameters and inflammatory markers evaluated.

Heat and Dehydration Additively Enhance Cardiovascular Outcomes following Orthostatically-Stressful Calisthenics Exercise

Exercise and exogenous heat each stimulate multiple adaptations, but their roles are not well delineated, and that of the related stressor, dehydration, is largely unknown. While severe and prolonged hypohydration potentially “silences” the long-term heat acclimated phenotype, mild and transient dehydration may enhance cardiovascular and fluid-regulatory adaptations. We tested the hypothesis that exogenous heat stress and dehydration additively potentiate acute (24 h) cardiovascular and hematological outcomes following exercise. In a randomized crossover study, 10 physically-active volunteers (mean ± SD: 173 ± 11 cm; 72.1 ± 11.5 kg; 24 ± 3 year; 6 females) completed three trials of 90-min orthostatically-stressful calisthenics, in: (i) temperate conditions (22°C, 50% rh, no airflow; CON); (ii) heat (40°C, 60% rh) whilst euhydrated (HEAT), and (iii) heat with dehydration (no fluid ~16 h before and during exercise; HEAT+DEHY). Using linear mixed effects model analyses, core temperature (T_CORE) rose 0.7°C more in HEAT than CON (95% CL: [0.5, 0.9]; p < 0.001), and another 0.4°C in HEAT+DEHY ([0.2, 0.5]; p < 0.001, vs. HEAT). Skin temperature also rose 1.2°C more in HEAT than CON ([0.6, 1.8]; p < 0.001), and similarly to HEAT+DEHY (p = 0.922 vs. HEAT). Peak heart rate was 40 b·min⁻¹ higher in HEAT than in CON ([28, 51]; p < 0.001), and another 15 b·min⁻¹ higher in HEAT+DEHY ([3, 27]; p = 0.011, vs. HEAT). Mean arterial pressure at 24-h recovery was not consistently below baseline after CON or HEAT (p ≥ 0.452), but was reduced 4 ± 1 mm Hg after HEAT+DEHY ([0, 8]; p = 0.020 vs. baseline). Plasma volume at 24 h after exercise increased in all trials; the 7% increase in HEAT was not reliably more than in CON (5%; p = 0.335), but was an additional 4% larger after HEAT+DEHY ([1, 8]; p = 0.005 vs. HEAT). Pooled-trial correlational analysis showed the rise in T_CORE predicted the hypotension (r = −0.4) and plasma volume expansion (r = 0.6) at 24 h, with more hypotension reflecting more plasma expansion (r = −0.5). In conclusion, transient dehydration with heat potentiates short-term (24-h) hematological (hypervolemic) and cardiovascular (hypotensive) outcomes following calisthenics.

Heat stress and dehydration in adapting for performance: Good, bad, both, or neither?

Physiological systems respond acutely to stress to minimize homeostatic disturbance, and typically adapt to chronic stress to enhance tolerance to that or a related stressor. It is legitimate to ask whether dehydration is a valuable stressor in stimulating adaptation per se. While hypoxia has had long-standing interest by athletes and researchers as an ergogenic aid, heat and nutritional stressors have had little interest until the past decade. Heat and dehydration are highly interlinked in their causation and the physiological strain they induce, so their individual roles in adaptation are difficult to delineate. The effectiveness of heat acclimation as an ergogenic aid remains unclear for team sport and endurance athletes despite several recent studies on this topic. Very few studies have examined the potential ergogenic (or ergolytic) adaptations to ecologically-valid dehydration as a stressor in its own right, despite longstanding evidence of relevant fluid-regulatory adaptations from short-term hypohydration. Transient and self-limiting dehydration (e.g., as constrained by thirst), as with most forms of stress, might have a time and a place in physiological or behavioral adaptations independently or by exacerbating other stressors (esp. heat); it cannot be dismissed without the appropriate evidence. The present review did not identify such evidence. Future research should identify how the magnitude and timing of dehydration might augment or interfere with the adaptive processes in behaviorally constrained versus unconstrained humans.

Characterization of the effects of the vasopressin V2 receptor on sweating, fluid balance, and performance during exercise

A regulatory effect of arginine vasopressin (AVP) on sweat water conservation has been hypothesized but not definitively evaluated. AVP-mediated insertion of sweat and salivary gland aquaporin-5 (AQP5) water channels through activation of the vasopressin type 2 receptor (V2R) remains an attractive, yet unexplored, mechanism that could result in a more concentrated sweat with resultant decreased water loss. Ten runners participated in a double-blind randomized control treadmill trial under three separate pharmacological conditions: a placebo, V2R agonist (0.2 mg desmopressin), or V2R antagonist (30 mg tolvaptan). After a familiarization trial, runners ran for 60 min at 60% of peak speed followed by a performance trial to volitional exhaustion. Outcome variables were collected at three exercise time points: baseline, after the steady-state run, and after the performance run. Body weight losses were <2% across all three trials. Significant pharmacological condition effects were noted for urine osmolality [F = 84.98; P < 0.0001] and urine sodium concentration ([Na(+)]) [F = 38.9; P < 0.0001], which verified both pharmacological activation and inhibition of the V2R at the kidney collecting duct. Plasma osmolality and [Na(+)] demonstrated significant exercise (F = 26.0 and F = 11.1; P < 0.0001) and condition (F = 5.1 and F = 3.8; P < 0.05) effects (osmolality and [Na(+)], respectively). No significant exercise or condition effects were noted for either sweat or salivary [Na(+)]. Significant exercise effects were noted for plasma [AVP] (F = 22.3; P < 0.0001), peak core temperature (F = 103.3; P < 0.0001), percent body weight change (F = 6.3; P = 0.02), plasma volume change (F = 21.8; P < 0.0001), and thirst rating (F = 78.2; P < 0.0001). Performance time was not altered between conditions (P = 0.80). In summary, AVP acting at V2R does not appear to regulate water losses from body fluids other than renal excretion during exercise.

The prevalence of exercise-associated hyponatremia in 24-hour ultra-mountain bikers, 24-hour ultra-runners and multi-stage ultra-mountain bikers in the Czech Republic

BACKGROUND

To assess the prevalence of exercise-associated hyponatremia (EAH) in two 24-hour mountain bike (MTB) (R1,R2), one 24-hour running (R3) and one multi-stage MTB (R4) races held in the Czech Republic in a cluster of four cross-sectional studies.

METHODS

In 27 ultra-mountain bikers (ultra-MTBers), 12 ultra-runners, and 14 multi-stage MTBers, fluid intake, changes (Δ) in body mass, hematocrit, plasma volume, plasma [Na+], plasma [K+], plasma osmolality, urine [Na+], urine [K+], urine specific gravity, urine osmolality, K+/Na+ ratio in urine, transtubular potassium gradient and glomerular filtration rate were measured and calculated. The use of non-steroidal anti-inflammatory drugs and symptoms of EAH were recorded using post-race questionnaires.

RESULTS

Of the 53 finishers, three (5.7%) developed post-race EAH, thereof one (3.7%) ultra-MTBer, one (8.3%) ultra-runner and one (7.1%) multi-stage MTBer. Plasma [Na+] decreased significantly (p < 0.001) only in R4. Urine osmolality (R1, R3, R4 p < 0.001; R2 p < 0.05) and glomerular filtration rate (p < 0.001) increased, and body mass decreased in all races (p < 0.05). Δ body mass was inversely related to the number of kilometers achieved (p < 0.001) in R2 where better ultra-MTBers tended to lose more weight. Δ body mass (p < 0.001) and %Δ body mass (p = 0.05) were positively related to lower post-race plasma [Na+] in R3 that was associated with increased loss in body mass. Fluid intake was positively related to race performance in R1 and R2 (R1: p = 0.04; R2: p = 0.01) where ultra-MTBers in R1 and R2 who drank more finished ahead of those who drank less. Post-race plasma [Na+] was negatively associated with race performance in ultra-MTBers in R2 (p < 0.05), similarly ultra-runners in R3 (p < 0.05) where finishers with more kilometres had lower post-race plasma [Na+].

CONCLUSIONS

The prevalence of EAH in the Czech Republic was no higher compared to existing reports on ultra-endurance athletes in other countries. Lower plasma [Na+] and development of EAH may be attributed to overdrinking, a pituitary secretion of vasopressin, an impaired mobilization of osmotically inactive sodium stores, and/or an inappropriate inactivation of osmotically active sodium.

Enhanced renal Na+ reabsorption by carbohydrate in beverages during restitution from thermal and exercise-induced dehydration in men

We examined whether carbohydrate in beverages accelerated fluid retention during recovery from thermal and exercise-induced dehydration and whether it was caused in part by an enhanced renal Na+ reabsorption rate due to insulin secretion. After dehydrating by ∼2.3% body weight by exercise in a hot environment, seven young men underwent high-carbohydrate, low-carbohydrate, or control rehydration trials by drinking one of three beverages with 3.4 g glucose + 3.1 g fructose, 1.7 g glucose + 1.6 g fructose, or 0.0 g glucose + 0.0 g fructose per deciliter, respectively, in a common composition of electrolyte solution: 21 meq/l [Na+], 5 meq/l [K+], 16.5 meq/l [Cl-], 10 meq/l [citrate(-3)]. They drank the same amount of beverage as total body weight loss within 30 min. During the 60 min before the start of drinking and the following 180 min, we measured plasma volume (PV), plasma glucose ([Glc]p), serum insulin ([Ins]s), plasma Na+ concentrations, and the renal clearances of inulin, lithium, and Na+ with plasma vasopressin ([AVP]p) and aldosterone concentrations ([Ald]p) every 30 min. After dehydration, PV decreased by ∼5% and plasma osmolality increased by ∼6 mosmol/kg H2O in all trials with no significant differences among them. We found in the high-carbohydrate trial that 1) PV increased faster than in the control trial and remained at the higher level than other trials for the last 60 min (P < 0.05); 2) accumulated urine volume was smallest after 90 min (P < 0.05); 3) the renal Na+ reabsorption rate was greatest for the first 120 min (P < 0.05); 4) during which period [AVP]p and [Ald](p) were not significantly different from other trials (both, P > 0.9); and 5) [Glc](p) and [Ins]s were highest from 45 to 105 min (P < 0.05) during rehydration. Thus carbohydrate in beverages enhances renal Na+ reabsorption, and insulin is possibly involved in this enhancement.

Prevalence of exercise-associated hyponatremia in male ultraendurance athletes

OBJECTIVE

The prevalence of exercise-associated hyponatremia (EAH) has mainly been investigated in marathoners and Ironman triathletes. The aim of this study was to investigate the prevalence of EAH in male ultraendurance athletes in other disciplines, such as ultraswimming, ultracycling, and ultramarathon running.

DESIGN

Observational field study.

SETTING

"Marathon Swim" in Lake Zurich, the "Swiss Cycling Marathon," the "Swiss Bike Masters," the "100-km Lauf Biel," and the "Swiss Jura Marathon."

PARTICIPANTS

Fifteen ultraswimmers, 28 ultra-road cyclists, 37 ultra-mountain bikers, 95 ultramarathoners, and 25 mountain ultramarathoners.

MAIN OUTCOME MEASURES

Changes in body mass, plasma sodium, urinary specific gravity, and hematocrit were measured. The athletes recorded their intake of fluids.

RESULTS

Two swimmers (13%), 3 road cyclists (10.7%), no mountain bikers (0%), 5 ultramarathoners (5%), and 2 mountain ultramarathoners (8%) developed EAH. In the mountain bikers (r = -0.41) and the 100-km ultramarathoners (r = -0.52), fluid intake was significantly and negatively related to race time. In the mountain ultramarathoners, fluid consumption increased during the race.

CONCLUSIONS

The prevalence of EAH was no higher in ultraendurance athletes compared with existing reports on marathoners and Ironman triathletes. Of the 200 investigated ultraendurance athletes, 12 finishers (6%) developed EAH.

The effect of heavy muscle activity on renal cytoresistance in rats

Cytoresistance is the term used to describe the response of the proximal tubule cells to various stress inducers via cholesterol accumulation. However, the role of extensive exercise as a renal insult has not been examined. In this study, the effect of heavy muscle activity on proximal tubule cytoresistance was investigated. Results obtained from rats subjected to running a treadmill for five days were compared to those of controls. Extensive muscle activity-induced soleus citrate synthase and blood lactate elevation were associated with normal MAP, RBF, and GFR. Blood electrolytes and cholesterol levels remained unchanged, whereas the total and free cholesterol accumulations in the proximal tubule cells of the exercised group were higher than controls. Cholesterol-loaded tubules were more resistant (as proved by LDH release) to an ATP-depleted/calcium overloaded second stress. These data clearly demonstrate that heavy muscle activity induces cholesterol accumulation in the proximal tubules of kidney, without influencing ATP generation.

Adaptations to high-intensity intermittent exercise in rodents

In humans, exercise-induced plasma volume (PV) expansion is typically associated with an increase in plasma albumin content, due in part to an increase in hepatic albumin synthesis. We tested the ability of a 12-day high-intensity intermittent exercise protocol to induce an increase in PV in rodents. Since albumin synthesis is transcriptionally regulated, we tested the hypothesis that exercise training would induce an increase in hepatic albumin gene expression. Fifty adult male Sprague-Dawley rats weighing between 245 and 350 g were randomly assigned to one of five groups: cage control (CC), sham exercise (sham), continuous moderate-intensity exercise training (MI), high-intensity intermittent exercise training (HI), or a single day of HI training (1-HI). Twenty-four hours after the last training session, rats were anesthetized. PV was determined, and the liver was removed, flash frozen, and stored for later analysis. Citrate synthase (CS) activity of the red quadriceps muscle, a marker of aerobic adaptation, increased with training (MI and HI) and in response to 1-HI (P < 0.05). We did not see a significant exercise-induced PV expansion as PV averaged 23.6 +/- 2.7 ml/kg body wt in the CC group and 26.6 +/- 1.3 ml/kg body wt in the HI group (P > 0.05). However, hepatic albumin mRNA expression, as determined by real-time PCR, increased 2.9 +/- 0.4- and 4.1 +/- 0.4-fold after MI and HI, respectively, compared with CC. A single bout of HI (1-HI) did not alter hepatic albumin mRNA expression. These data demonstrate an increase in both CS activity and hepatic albumin gene expression with 12 days of aerobic exercise training in the rodent with a rapid (within 24 h) adaptation in the skeletal muscle to high-intensity intermittent exercise.

Exercise-associated hyponatremia

Exercise-associated hyponatremia has been described after sustained physical exertion during marathons, triathlons, and other endurance athletic events. As these events have become more popular, the incidence of serious hyponatremia has increased and associated fatalities have occurred. The pathogenesis of this condition remains incompletely understood but largely depends on excessive water intake. Furthermore, hormonal (especially abnormalities in arginine vasopressin secretion) and renal abnormalities in water handling that predispose individuals to the development of severe, life-threatening hyponatremia may be present. This review focuses on the epidemiology, pathogenesis, and therapy of exercise-associated hyponatremia.

The median preoptic nucleus is involved in the facilitation of heat-escape/cold-seeking behavior during systemic salt loading in rats

Systemic salt loading has been reported to facilitate operant heat-escape/cold-seeking behavior. In the present study, we hypothesized that the median preoptic nucleus (MnPO) would be involved in this mechanism. Rats were divided into two groups (n = 6 each): one group had the MnPO lesion with ibotenic acid (4.0 mug) and the other was the vehicle control. After subcutaneous injection (10 ml/kg) of either isotonic- (154 mM) or hypertonic-saline (2,500 mM), each rat was placed in a behavior box, where the ambient temperature was changed to 26 degrees C, 35 degrees C, and 40 degrees C every 1 h. The position of a rat in the box and the body core temperature (T(core)) were monitored. A rat could trigger 0 degrees C air for 45 s in the 35 degrees C and 40 degrees C heat when moved in a specific area in the box (operant behavior). In the control group, counts of the operant behavior were greater (P < 0.05) in the hypertonic- than in the isotonic-saline injection (17 +/- 2 and 10 +/- 2 at 35 degrees C, 24 +/- 2 and 18 +/- 1 at 40 degrees C). T(core) remained unchanged throughout the exposure, although the level was lower (P < 0.05) in the hypertonic- than in the isotonic-saline trial (36.6 +/- 0.2 degrees C and 37.4 +/- 0.1 degrees C at 26 degrees C and 36.9 +/- 0.2 degrees C and 37.4 +/- 0.1 degrees C at 40 degrees C, respectively). However, in the MnPO-lesion group, counts of the behavior were similar between the hypertonic- and isotonic-saline injection trials (10 +/- 2 and 8 +/- 1 at 35 degrees C, and 17 +/- 1 and 16 +/- 1 at 40 degrees C, respectively). T(core) increased (P < 0.05) in the heat in both trials (36.8 +/- 0.1 degrees C and 37.4 +/- 0.1 degrees C at 26 degrees C and 37.4 +/- 0.2 degrees C and 37.8 +/- 0.2 degrees C at 40 degrees C in the hypertonic- and isotonic-saline injection trials, respectively). These results may suggest that, at least in part, the MnPO is involved in the facilitation of heat-escape/cold-seeking behavior during osmotic stimulation.

Regional hemodynamics during postexercise hypotension. I. Splanchnic and renal circulations

Moderate exercise elicits a relative postexercise hypotension that is caused by an increase in systemic vascular conductance. Previous studies have shown that skeletal muscle vascular conductance is increased postexercise. It is unclear whether these hemodynamic changes are limited to skeletal muscle vascular beds. The aim of this study was to determine whether the splanchnic and/or renal vascular beds also contribute to the rise in systemic vascular conductance during postexercise hypotension. A companion study aims to determine whether the cutaneous vascular bed is involved in postexercise hypotension (Wilkins BW, Minson CT, and Halliwill JR. J Appl Physiol 97: 2071-2076, 2004). Heart rate, arterial pressure, cardiac output, leg blood flow, splanchnic blood flow, and renal blood flow were measured in 13 men and 3 women before and through 120 min after a 60-min bout of exercise at 60% of peak oxygen uptake. Vascular conductances of leg, splanchnic, and renal vascular beds were calculated. One hour postexercise, mean arterial pressure was reduced (79.1 +/- 1.7 vs. 83.4 +/- 1.8 mmHg; P < 0.05), systemic vascular conductance was increased by approximately 10%, leg vascular conductance was increased by approximately 65%, whereas splanchnic (16.0 +/- 1.8 vs. 18.5 +/- 2.4 ml.min(-1).mmHg(-1); P = 0.13) and renal (20.4 +/- 3.3 vs. 17.6 +/- 2.6 ml.min(-1).mmHg(-1); P = 0.14) vascular conductances were unchanged compared with preexercise. This suggests there is neither vasoconstriction nor vasodilation in the splanchnic and renal vasculature during postexercise hypotension. Thus the splanchnic and renal vascular beds neither directly contribute to nor attenuate postexercise hypotension.

Baroreflex-mediated heart rate and vascular resistance responses 24 h after maximal exercise

INTRODUCTION

Plasma volume, heart rate (HR) variability, and stimulus-response relationships for baroreflex control of forearm vascular resistance (FVR) and HR were studied in eight healthy men after and without performing a bout of maximal exercise to test the hypotheses that acute expansion of plasma volume is associated with 1) reduction in baroreflex-mediated HR response, and 2) altered operational range for central venous pressure (CVP).

METHODS

The relationship between stimulus (DeltaCVP) and vasoconstrictive reflex response (DeltaFVR) during unloading of cardiopulmonary baroreceptors was assessed with lower-body negative pressure (LBNP, 0, -5, -10, -15, -20 mm Hg). The relationship between stimulus (Deltamean arterial pressure (MAP)) and cardiac reflex response (DeltaHR) during loading of arterial baroreceptors was assessed with steady-state infusion of phenylephrine (PE) designed to increase MAP by 15 mm Hg alone and during application of LBNP (PE+LBNP) and neck pressure (PE+LBNP+NP). Measurements of vascular volume and autonomic baroreflex responses were conducted on two different test days, each separated by at least 1 wk. On one day, baroreflex response was tested 24 h after graded cycle exercise to volitional exhaustion. On another day, measurement of baroreflex response was repeated with no exercise (control). The order of exercise and control treatments was counterbalanced.

RESULTS

Baseline CVP was elevated (P = 0.04) from a control value of 10.5 +/- 0.4 to 12.3 +/- 0.4 mm Hg 24 h after exercise. Average DeltaFVR/DeltaCVP during LBNP was not different (P = 0.942) between the exercise (-1.35 +/- 0.32 pru x mm Hg-1) and control (-1.32 +/- 0.36 pru x mm Hg-1) conditions. However, maximal exercise caused a shift along the reflex response relationship to a higher CVP and lower FVR. HR baroreflex response (DeltaHR/DeltaMAP) to PE+LBNP+NP was lower (P = 0.015) after maximal exercise (-0.43 +/- 0.15 beats x min-1 x mm Hg-1) compared with the control condition (-0.83 +/- 0.14 beats x min-1 x mm Hg-1).

CONCLUSION

Expansion of vascular volume after acute exercise is associated with altered operational range for CVP and reduced HR response to arterial baroreceptor stimulation.

Why is erythropoietin made in the kidney? The kidney functions as a 'critmeter' to regulate the hematocrit

The normal hematocrit is not a random number, but one that maximizes oxygen delivery. While the feedback loop wherein tissue oxygen pressure determines the production of erythropoietin, which further drives the production of red blood cells in the bone marrow, explains how the hematocrit is generated, it does not speak to how the hematocrit is regulated. The regulation of the hematocrit requires the coordination of the plasma volume and the red cell mass. By controlling red cell mass via erythropoietin and plasma volume through excretion of salt and water, the kidney is able to generate the hematocrit. It is hypothesized that the kidney functions as a critmeter by sensing the relative volumes of each component of the blood through the common signal of tissue oxygen tension. The kidney's unique ability to sense ECF volume through tissue oxygen signal allows it to coordinate these two volumes to produce the normal hematocrit. Hence, it may be the kidneys ability to report a measure of ECF volume as a tissue oxygen signal and thus to regulate the hematocrit that establishes it as the logical site of erythropoietin production. The critmeter is proposed to be a functional unit located at the tip of the cortical labyrinth at the juxta-medullary region of the kidney where erythropoietin is made physiologically. Renal vasculature and nephron segment heterogeneity in sodium reabsorption likely provides the anatomical construct to generate the marginal tissue oxygen pressure required to trigger the production of erythropoietin. The balance of oxygen consumption for sodium reabsorption and oxygen delivery is reflected by the tissue oxygen pressure. This balance hence determines RBC mass adjusted to plasma volume. Factors that affect blood supply and sodium reabsorption in a discordant manner may modulate the critmeter, e.g. angiotensin II. The objective of this work is to describe the hypothesis of the kidney's function as a critmeter, including the anatomical and physiological components, and the role of the renin-angiotensin system in modulating erythropoietin. Clinical examples of the dysregulation of the critmeter may be found in the anemia of renal failure and in sports anemia.

Human albumin synthesis is increased by an ultra-endurance trial

PURPOSE

The purpose of this study was to determine whether an ultra-endurance event is a strong stimulus to increase albumin synthesis involved in the process of intravascular albumin mass increase associated with transient hypervolemia.

METHODS

The albumin synthetic rate was measured in six young men, 3 d before (C) and on the 1st (R1) and 8th (R8) days of the recovery from an ultra-endurance trial (5 h daily for 4 d). Albumin fractional (FSR) and absolute (ASR) synthetic rate were determined using a primed-constant infusion of [1(-13) C] leucine. Plasma volume (PV) using Evans Blue dye dilution and total body water (TBW) using bioelectrical impedance analysis were measured on C, R1, and R8.

RESULTS

On R1 as compared with C: 1). PV (+23.3 +/- 3.2%; P<or= 0.001) and TBW (+4.2 +/- 0.8%; P<or= 0.01) expanded; 2). FSR and ASR both increased from 5.36 +/- 0.46 to 6.86 +/- 0.62%.24 h (+ 29.0 +/- 7.1%; P<or= 0.01) and from 103 +/- 10 to 153 +/- 19 mg.kg body weight (-1).24 h (-1) (+47.5 +/- 6.8%; P<or= 0.01); and 3). plasma albumin and protein masses increased (P<or= 0.05) from 1.92 +/- 0.08 to 2.22 +/- 0.14 g.kg body weight (-1) (i.e., +15.3 +/- 4.9%) and from 3.15 +/- 0.18 to 3.56 +/- 0.21 g.kg body weight (-1) (+13.3 +/- 3.9%), respectively. By R8, all these parameters had returned to their control levels except for albumin mass (1.73 +/- 0.08 g.kg body weight(-1), which was lower than C ( P<or= 0.05).

CONCLUSION

The increase in the albumin synthetic rate the day after an ultra-endurance trial is a major event associated with a greater circulating protein mass. The impact of exercise on albumin synthesis has disappeared 8 d later, as did PV and total protein mass expansion.

Effect of lymphatic outflow pressure on lymphatic albumin transport in humans

The effects of posture on the lymphatic outflow pressure and lymphatic return of albumin were examined in 10 volunteers. Lymph flow was stimulated with a bolus infusion of isotonic saline (0.9%, 12.6 ml/kg body wt) under four separate conditions: upright rest (Up), upright rest with lower body positive pressure (LBPP), supine rest (Sup), and supine rest with lower body negative pressure (LBNP). The increase in plasma albumin content (Delta Alb) during the 2 h after bolus saline infusion was greater in Up than in LBPP: 82.9 +/- 18.5 vs. -28.4 mg/kg body wt. Delta Alb was greater in LBNP than in Sup: 92.6 vs. -22.5 +/- 18.9 mg/kg body wt (P < 0.05). The greater Delta Alb in Up and Sup with LBNP were associated with a lower estimated lymphatic outflow pressure on the basis of the difference in central venous pressure (Delta CVP). During LBPP, CVP was increased compared with Up: 3.8 +/- 1.4 vs. -1.2 +/- 1.2 mmHg. During LBNP, CVP was reduced compared with Sup: -3.0 +/- 2.2 vs. 1.7 +/- 1.0 mmHg. The translocation of protein into the vascular space after bolus saline infusion reflects lymph return of protein and is higher in Up than in Sup. Modulation of CVP with LBPP or LBNP in Up and Sup, respectively, reversed the impact of posture on lymphatic outflow pressure. Thus posture-dependent changes in lymphatic protein transport are modulated by changes in CVP through its mechanical impact on lymphatic outflow pressure.