Suppression of plasma aldosterone by partial immersion

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).

Water immersion induced alterations of plasma vasopressin levels and activity of the renin-angiotensin-aldosterone system in noninflammatory acute renal failure and end-stage renal failure

Water immersion (WI) induced alterations of plasma renin activity (PRA), plasma aldosterone (Aldo), vasopressin (AVP), sodium and potassium concentrations and plasma volume (PV) were examined in 12 patients with noninflammatory acute renal failure (NARF) in the anuric/oliguric phase, in 20 patients with chronic renal failure (CRF) and in 15 healthy subjects. In all examined groups a significant increase of PV and decrease of PRA, plasma Aldo, AVP after WI were observed. Significantly more marked decreases of PRA were observed in patients with NARF than in those with CRF and in healthy subjects. In patients with NARF the increase of PV was significantly greater than in CRF and in healthy subjects. In patients with NARF a significantly more marked decrease of plasma sodium concentration was also noted after WI than in subjects with CRF and in normals. Only in healthy subjects was a significant negative correlation noted between the WI increase of PV and decrease of plasma AVP level. In healthy subjects, significantly positive correlations were noted between WI induced decrease of PRA and Aldo and between PRA and plasma AVP, respectively. Such correlations were absent in uraemic patients.

IN CONCLUSION

(1) patients with CRF and NARF show similar, but quantitatively different responses of PRA, Aldo and AVP secretion to WI as compared with normals; (2) patients with NARF and CRF are characterized by an inappropriate volumetric and osmotic regulation of AVP secretion; (3) in uraemic patients the physiological relationship between the renin-angiotensin system and AVP secretion seems to be dissociated.

Early fluid and protein shifts in men during water immersion

High precision blood and plasma densitometry was used to measure transvascular fluid shifts during water immersion to the neck. Six men (28-49 years) undertook 30 min of standing immersion in water at 35.0 +/- 0.2 degrees C; immersion was preceded by 30 min control standing in air at 28 +/- 1 degrees C. Blood was sampled from an antecubital catheter for determination of blood density (BD), plasma density (PD), haematocrit (Ht), total plasma protein concentration (PPC), and plasma albumin concentration (PAC). Compared to control, significant decreases (p less than 0.01) in all these measures were observed after 20 min immersion. At 30 min, plasma volume had increased by 11.0 +/- 2.8%; the average density of the fluid shifted from extravascular fluid into the vascular compartment was 1006.3 g.l-1; albumin moved with the fluid and its albumin concentration was about one-third of the plasma protein concentration during early immersion. These calculations are based on the assumption that the F-cell ratio remained unchanged. No changes in erythrocyte water content during immersion were found. Thus, immersion-induced haemodilution is probably accompanied by protein (mainly albumin) augmentation which accompanies the intravascular fluid shift.

Body fluid homeostasis in man. A contemporary overview

In the steady state, urinary excretion of sodium is closely matched to dietary salt intake. Given rigorous defense of extracellular fluid osmolality, it is the quantity of sodium in the extracellular fluid that determines the volume of this compartment. Changes in extracellular fluid volume are detected by volume sensors located in the intrathoracic vascular bed, kidney and other organs. These mechanoreceptors gauge the adequacy of intravascular volume, relative to capacitance, at various sites within the circulation. The perception of a change in the normal relationship between intravascular volume and circulatory capacity evokes a host of renal effector mechanisms that lead ultimately to physiologically appropriate changes in urinary sodium excretion. These effector mechanisms involve physical adjustments in the glomerular filtration rate, renal microvascular hemodynamics and peritubular capillary Starling forces, tubule fluid composition, flow rate and transtubular ion gradients. Neural and humoral pathways are also involved and, among the latter, angiotensin II, aldosterone, prostaglandins and kinins have been studied extensively. The continuous interaction between these sensor and effector mechanisms serves to ensure near-constancy of the extracellular fluid volume, a condition essential for optimal circulatory performance.

Reduced aldosterone and sodium excretion in endurance-trained athletes before and during immersion

Aldosterone excretion (AE) and plasma renin activity (PRA) were measured in eight untrained (UT) and eight endurance-trained (TR) male subjects before and during 4 h head-out immersion to study the mechanism of reduced renal sodium excretion in athletes. AE was significantly lower before immersion, and decreased less during immersion, in TR than in UT. Fractional sodium excretion, too, was lower and increased less during immersion in TR than in UT. PRA decreased in the water bath in all subjects (p less than 0.001) with no significant difference between the groups. During immersion, plasma sodium concentration oscillated whereas potassium concentration showed a temporary rise in TR (p less than 0.001). The attenuated response of AE in TR may be due partly to this increase of plasma potassium concentration. The generally reduced aldosterone release in TR might be caused by a training induced adaptation of the adrenals to corticotropin. The lowered renal sodium excretion of TR in spite of the decreased AE suggests an intensified aldosterone effect in these subjects, diminishing the salt loss during exercise.

Renal blood volume regulation in trained and untrained subjects during immersion

To study the cause of the increased blood volume of endurance-trained athletes we assessed the renal blood volume regulating mechanisms in eight untrained (UT) and eight endurance-trained (TR) male subjects during a 4 h head-out immersion. In TR plasma volume remained constant whereas it decreased in UT by 2.4 ml/kg (p less than 0.025). Immersion diuresis of TR was only half as high as in UT (peak values: 3.22 ml/min in UT, 1.60 ml/min in TR). Free water clearance remained approximately constant in UT but temporarily decreased in TR (p less than 0.001). This points to poor or even absent inhibition of antidiuretic hormone secretion in the latter group. Osmolar clearance increased less in TR than in UT (p less than 0.02) which was partly due to a delayed increase of glomerular filtration rate. Plasma osmolality, creatinine, and protein concentrations as well as hematocrit values were reduced during immersion to a similar extent in both groups. The results indicate a reduced renal response of endurance-trained subjects to congestion of the low-pressure system resulting in an increase in blood volume.

Some evidence for aldosterone action on 2,3-diphosphoglycerate level in human red cells

In healthy male subjects aldosterone excretion and plasma renin activity were reduced by a 4-6 hr head-out immersion in thermoindifferent water baths (35.5 +/- 0.1 degrees C). The red cell 2,3-diphosphoglycerate (DPG) concentration before and throughout immersion period was positively correlated both with aldosterone excretion in 2 hr pooled urine (r = +0.69; 2 p less than 0.001) and with renin activity (r = + 0.54; 2p less than 0.001) despite a concomitant increase of cubital venous pH and inorganic phosphate concentration. These findings furnish evidence for a regulatory role of aldosterone in DPG metabolism, possibily by a direct influence on red cell glycolysis.