Fluid volume and osmoregulation in humans after a week of head-down bed rest

The kidney, volume homeostasis and osmoregulation in space: current perspective and knowledge gaps

Although we have sent humans into space for more than 50 years crucial questions regarding kidney physiology, volume regulation and osmoregulation remain unanswered. The complex interactions between the renin-angiotensin-aldosterone system, the sympathetic nervous system, osmoregulatory responses, glomerular function, tubular function, and environmental factors such as sodium and water intake, motion sickness and ambient temperature make it difficult to establish the exact effect of microgravity and the subsequent fluid shifts and muscle mass loss on these parameters. Unfortunately, not all responses to actual microgravity can be reproduced with head-down tilt bed rest studies, which complicates research on Earth. Better understanding of the effects of microgravity on kidney function, volume regulation and osmoregulation are needed with the advent of long-term deep space missions and planetary surface explorations during which orthostatic intolerance complaints or kidney stone formation can be life-threatening for astronauts. Galactic cosmic radiation may be a new threat to kidney function. In this review, we summarise and highlight the current understandings of the effects of microgravity on kidney function, volume regulation and osmoregulation and discuss knowledge gaps that future studies should address.

Review A State of Natriuretic Peptide Deficiency

Measurement of natriuretic peptides (NPs) has proven its clinical value as biomarker, especially in the context of heart failure (HF). In contrast, a state partial NP deficiency appears integral to several conditions in which lower NP concentrations in plasma presage overt cardiometabolic disease. Here, obesity and type 2 diabetes have attracted considerable attention. Other factors - including age, sex, race, genetics, and diurnal regulation - affect the NP "armory" and may leave some individuals more prone to development of cardiovascular disease. The molecular maturation of NPs has also proven complex with highly variable O-glycosylation within the biosynthetic precursors. The relevance of this regulatory step in post-translational propeptide maturation has recently become recognized in biomarker measurement/interpretation and cardiovascular pathophysiology. An important proportion of people appear to have reduced effective net NP bioactivity in terms of receptor activation and physiological effects. The state of NP deficiency, then, both entails a potential for further biomarker development and could also offer novel pharmacological possibilities. Alleviating the state of NP deficiency before development of overt cardiometabolic disease in selected patients could be a future path for improving precision medicine.

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

An international collaboration studying the physiological and anatomical cerebral effects of carbon dioxide during head-down tilt bed rest: the SPACECOT study

Exposure to the microgravity environment results in various adaptive and maladaptive physiological changes in the human body, with notable ophthalmic abnormalities developing during 6-mo missions on the International Space Station (ISS). These findings have led to the hypothesis that the loss of gravity induces a cephalad fluid shift, decreased cerebral venous outflow, and increased intracranial pressure, which may be further exacerbated by increased ambient carbon dioxide (CO2) levels on the ISS. Here we describe the SPACECOT study (studying the physiological and anatomical cerebral effects of CO2 during head-down tilt), a randomized, double-blind crossover design study with two conditions: 29 h of 12° head-down tilt (HDT) with ambient air and 29 h of 12° HDT with 0.5% CO2. The internationally collaborative SPACECOT study utilized an innovative approach to study the effects of headward fluid shifting induced by 12° HDT and increased ambient CO2 as well as their interaction with a focus on cerebral and ocular anatomy and physiology. Here we provide an in-depth overview of this new approach including the subjects, study design, and implementation, as well as the standardization plan for nutritional intake, environmental parameters, and bed rest procedures.

NEW & NOTEWORTHY A new approach for investigating the combined effects of cephalad fluid shifting and increased ambient carbon dioxide (CO2) is presented. This may be useful for studying the neuroophthalmic and cerebral effects of spaceflight where cephalad fluid shifts occur in an elevated CO2 environment.

Effect of rowing ergometry and oral volume loading on cardiovascular structure and function during bed rest

This study examined the effectiveness of a short-duration but high-intensity exercise countermeasure in combination with a novel oral volume load in preventing bed rest deconditioning and orthostatic intolerance. Bed rest reduces work capacity and orthostatic tolerance due in part to cardiac atrophy and decreased stroke volume. Twenty seven healthy subjects completed 5 wk of -6 degree head down bed rest. Eighteen were randomized to daily rowing ergometry and biweekly strength training while nine remained sedentary. Measurements included cardiac mass, invasive pressure-volume relations, maximal upright exercise capacity, and orthostatic tolerance. Before post-bed rest orthostatic tolerance and exercise testing, nine exercise subjects were given 2 days of fludrocortisone and increased salt. Sedentary bed rest led to cardiac atrophy (125 ± 23 vs. 115 ± 20 g; P < 0.001); however, exercise preserved cardiac mass (128 ± 38 vs. 137 ± 34 g; P = 0.002). Exercise training preserved left ventricular chamber compliance, whereas sedentary bed rest increased stiffness (180 ± 170%, P = 0.032). Orthostatic tolerance was preserved only when exercise was combined with volume loading (-10 ± 22%, P = 0.169) but not with exercise (-14 ± 43%, P = 0.047) or sedentary bed rest (-24 ± 26%, P = 0.035) alone. Rowing and supplemental strength training prevent cardiovascular deconditioning during prolonged bed rest. When combined with an oral volume load, orthostatic tolerance is also preserved. This combined countermeasure may be an ideal strategy for prolonged spaceflight, or patients with orthostatic intolerance.

Blood vessel remodeling and physical inactivity in humans

Physical inactivity is associated with an increase in cardiovascular risk that cannot be fully explained by traditional or novel risk factors. Inactivity is also associated with changes in hemodynamic stimuli, which exert direct effects on the vasculature leading to remodeling and a proatherogenic phenotype. In this review, we synthesize and summarize in vivo evidence relating to the impact of local and systemic models of physical inactivity on conduit arteries, resistance vessels, and the microcirculation in humans. Taken together, the literature suggests that a rapid inward structural remodeling of vessels occurs in response to physical inactivity. The magnitude of this response is dependent on the "dose" of inactivity. Moreover, changes in vascular function are found at resistance and microvessel levels in humans. In conduit arteries, a strong interaction between vascular function and structure is present, which results in conflicting data regarding the impact of inactivity on conduit artery function. While much of the cardioprotective effect of exercise is related to the nitric oxide pathway, deconditioning may primarily be associated with activation of vasoconstrictor pathways. The effects of deconditioning on the vasculature are therefore not simply the opposite of those in response to exercise training. Given the importance of sedentary behavior, future studies should provide further insight into the impact of inactivity on the vasculature and other (novel) markers of vascular health. Moreover, studies should examine the role of (hemodynamic) stimuli that underlie the characteristic vascular adaptations during deconditioning. Our review concludes with some suggestions for future research directions.

Physical (in)activity and endothelium-derived constricting factors: overlooked adaptations

The inner surrounding of arterial vessels, the endothelium, is optimally located to detect changes in blood characteristics or blood flow that may result from changes in physical activity or from diseases. In response to physical stimuli, the endothelium varies its release of circulating vasoactive substances and serves as a source of local and systemic endothelium-derived dilator and vasoconstrictor factors. Endothelial dysfunction is one of the earliest markers of vascular abnormalities observed in cardiovascular disease and ageing. Exercise training is an efficient therapeutic strategy to improve endothelial function. Traditionally, studies on endothelial dysfunction and physical (in)activity-related effects on vascular adaptations are primarily focused on vasodilator substances (i.e. nitric oxide). One may suggest that augmentation of vasoconstrictor pathways (such as endothelin-1 and angiotensin II) contributes to the endothelial dysfunction observed after physical inactivity. Moreover, these pathways may also explain the exercise-induced beneficial cardiovascular adaptations. This review summarizes the current knowledge on the effects of physical (in)activity on several endothelium-derived vasoconstrictor substances.

From space to Earth: advances in human physiology from 20 years of bed rest studies (1986-2006)

Bed rest studies of the past 20 years are reviewed. Head-down bed rest (HDBR) has proved its usefulness as a reliable simulation model for the most physiological effects of spaceflight. As well as continuing to search for better understanding of the physiological changes induced, these studies focused mostly on identifying effective countermeasures with encouraging but limited success. HDBR is characterised by immobilization, inactivity, confinement and elimination of Gz gravitational stimuli, such as posture change and direction, which affect body sensors and responses. These induce upward fluid shift, unloading the body's upright weight, absence of work against gravity, reduced energy requirements and reduction in overall sensory stimulation. The upward fluid shift by acting on central volume receptors induces a 10-15% reduction in plasma volume which leads to a now well-documented set of cardiovascular changes including changes in cardiac performance and baroreflex sensitivity that are identical to those in space. Calcium excretion is increased from the beginning of bed rest leading to a sustained negative calcium balance. Calcium absorption is reduced. Body weight, muscle mass, muscle strength is reduced, as is the resistance of muscle to insulin. Bone density, stiffness of bones of the lower limbs and spinal cord and bone architecture are altered. Circadian rhythms may shift and are dampened. Ways to improve the process of evaluating countermeasures--exercise (aerobic, resistive, vibration), nutritional and pharmacological--are proposed. Artificial gravity requires systematic evaluation. This review points to clinical applications of BR research revealing the crucial role of gravity to health.

Renal, Endocrine, and Cardiovascular Responses to Bed Rest in Male Subjects on a Constant Diet

Background

Exposure to actual and simulated microgravity induces cardiovascular deconditioning through a variety of factors. Although the mechanisms involved remain uncertain, one involves alterations in volume-regulating systems—the hypothesis being tested in this study. To maximize our ability to detect subtle changes in the volume-regulating systems, subjects were studied on a high-average salt intake to maximally suppress these systems basally.

Methods

Fourteen healthy male subjects underwent 14-day head-down tilt bed rest (HDTB) during which a constant 200 mEq sodium, 100 mEq potassium diet was maintained. Daily 24-hour urine collection was performed; plasma renin activity, serum aldosterone, plethysmography, and cardiovascular system identification were performed during a control period (pre-HDTB) and at the end of HDTB (end HDTB).

Results

Sodium excretion increased initially (pre-HDTB = 182.8 ± 10.4 mEq/total volume; early HDTB = 236.4 ± 13.0; p = .002) and then returned to baseline values. Potassium excretion increased 4 days after the initiation of HDTB and remained elevated thereafter (pre-HDTB = 82.2 ± 2.4/total volume; mid- to late HDTB = 89.4 ± 2.1; p = .02). Plasma renin activity increased significantly with HDTB (pre-HDTB = 1.28 ± 0.21 ng/mL/h; end HDTB = 1.69 ± 0.18; p = .01), but serum aldosterone did not change. A significant decrease in autonomic responsiveness and an increase in leg compliance were observed.

Conclusions

We conclude that even in the presence of a high-average salt intake diet, simulated microgravity leads to renal, cardioendocrine, and cardiovascular system alterations that likely contribute to cardiovascular deconditioning.

Enhanced sympathoinhibitory response to volume expansion in conscious hindlimb-unloaded rats

Prolonged exposure to microgravity or bed rest produces cardiovascular deconditioning, which is characterized by reductions in plasma volume, alterations in autonomic function, and a predisposition toward orthostatic intolerance. Although the precise mechanisms have not been fully elucidated, it is possible that augmented cardiopulmonary reflexes contribute to some of these effects. The purpose of the present study was to test the hypothesis that sympathoinhibitory responses to volume expansion are enhanced in the hindlimb-unloaded (HU) rat, a model of cardiovascular deconditioning. Mean arterial blood pressure, heart rate, and renal sympathetic nerve activity (RSNA) responses to isotonic volume expansion (0.9% saline iv, 15% of plasma volume over 5 min) were examined in conscious HU (14 days) and control animals. Volume expansion produced decreases in RSNA in both groups; however, this effect was significantly greater in HU rats (-46 +/- 7 vs. -25 +/- 4% in controls). Animals instrumented for central venous pressure (CVP) did not exhibit differences in CVP responses to volume expansion. These data suggest that enhanced cardiopulmonary reflexes may be involved in the maintenance of reduced plasma volume and contribute to attenuated baroreflex-mediated sympathoexcitation after spaceflight or bed rest.

Effects of supine, prone, and lateral positions on cardiovascular and renal variables in humans

The hypothesis was tested that changing the direction of the transverse gravitational stress in horizontal humans modulates cardiovascular and renal variables. On different study days, 14 healthy males were placed for 6 h in either the horizontal supine or prone position following 3 h of being supine. Eight of the subjects were in addition investigated in the horizontal left lateral position. Compared with supine, the prone position slightly increased free water clearance (349 +/- 38 vs. 447 +/- 39 ml/6 h, P = 0.05) and urine output (1,387 +/- 55 vs. 1,533 +/- 52 ml/6 h, P = 0.06) with no statistically significant effect on renal sodium excretion (69 +/- 3 vs. 76 +/- 5 mmol/6 h, P = 0.21). Mean arterial pressure and left atrial diameter were similar comparing effects of supine with prone. The prone position induced an increase in heart rate (54 +/- 2 to 58 +/- 2 beats/min, P < 0.05), total peripheral vascular resistance (13 +/- 1 to 16 +/- 1 mmHg. min(-1). l(-1), P < 0.05), forearm venous plasma concentration of norepinephrine (97 +/- 9 to 123 +/- 16 pg/ml, P < 0.05), and atrial natriuretic peptide (49 +/- 4 to 79 +/- 12 pg/ml, P < 0.05), whereas stroke volume decreased (122 +/- 5 to 102 +/- 3 ml, P < 0.05, n = 6). The left lateral position had no effect on renal variables, whereas left atrial diameter increased (32 +/- 1 to 35 +/- 1 mm, P < 0.05) and mean arterial pressure decreased (90 +/- 2 to mean value of 85 +/- 2 mmHg, P < 0.05). In conclusion, the prone position reduced stroke volume and increased sympathetic nervous activity, possibly because of mechanical compression of the thorax with slight impediment of arterial filling. The mechanisms of the slightly augmented urine output in prone position require further experimentation.

Natriuresis induced by mild hypernatremia in humans

The hypothesis that increases in plasma sodium induce natriuresis independently of changes in body fluid volume was tested in six slightly dehydrated seated subjects on controlled sodium intake (150 mmol/day). NaCl (3.85 mmol/kg) was infused intravenously over 90 min as isotonic (Iso) or as hypertonic saline (Hyper, 855 mmol/l). After Hyper, plasma sodium increased by 3% (142.0 +/- 0.6 to 146.2 +/- 0.5 mmol/l). During Iso a small decrease occurred (142.3 +/- 0.6 to 140.3 +/- 0.7 mmol/l). Iso increased estimates of plasma volume significantly more than Hyper. However, renal sodium excretion increased significantly more with Hyper (291 +/- 25 vs. 199 +/- 24 micromol/min). This excess was not mediated by arterial pressure, which actually decreased slightly. Creatinine clearance did not change measurably. Plasma renin activity, ANG II, and aldosterone decreased very similarly in Iso and Hyper. Plasma atrial natriuretic peptide remained unchanged, whereas plasma vasopressin increased with Hyper (1.4 +/- 0.4 to 3.1 +/- 0.5 pg/ml) and decreased (1.3 +/- 0.4 to 0.6 +/- 0.1 pg/ml) after Iso. In conclusion, the natriuretic response to Hyper was 50% larger than to Iso, indicating that renal sodium excretion may be determined partly by plasma sodium concentration. The mechanism is uncertain but appears independent of changes in blood pressure, glomerular filtration rate, the renin system, and atrial natriuretic peptide.

Prolonged hypobaric hypoxemia attenuates vasopressin secretion and renal response to osmostimulation in men

Effects of hypobaric hypoxemia on endocrine and renal parameters of body fluid homeostasis were investigated in eight normal men during a sojourn of 8 days at an altitude of 4,559 m. Endocrine and renal responses to an osmotic stimulus (5% hypertonic saline, 3.6 ml/kg over 1 h) were investigated at sea level and on day 6 at altitude. Several days of hypobaric hypoxemia reduced body weight (-2.1 +/- 0.4 kg), increased plasma osmolality (+5.3 +/- 1.4 mosmol/kgH(2)O), elevated blood pressure (+12 +/- 1 mmHg), reduced creatinine clearance (122 +/- 6 to 96 +/- 10 ml/min), inhibited the renin system (19.5 +/- 2.0 to 10.9 +/- 0.9 mU/l) and plasma vasopressin (1.14 +/- 0.16 to 0.38 +/- 0.06 pg/ml), and doubled circulating levels of norepinephrine (103 +/- 16 to 191 +/- 35 pg/ml) and endothelin-1 (3.0 +/- 0.2 to 6.3 +/- 0.6 pg/ml), whereas urodilatin excretion rate decreased from day 2 (all changes P < 0.05 compared with sea level). Plasma arginine vasopressin response and the antidiuretic response to hypertonic saline loading were unchanged, but the natriuretic response was attenuated. In conclusion, chronic hypobaric hypoxemia 1) elevates the set point of plasma osmolality-to-plasma vasopressin relationship, possibly because of concurrent hypertension, thereby causing hypovolemia and hyperosmolality, and 2) blunts the natriuretic response to hypertonic volume expansion, possibly because of elevated circulating levels of norepinephrine and endothelin, reduced urodilatin synthesis, or attenuated inhibition of the renin system.