The anemia of microgravity and recumbency: role of sympathetic neural control of erythropoietin production

PlanHab: Hypoxia counteracts the erythropoietin suppression, but seems to exaggerate the plasma volume reduction induced by 3 weeks of bed rest

The study examined the distinct and synergistic effects of hypoxia and bed rest on the erythropoietin (EPO) concentration and relative changes in plasma volume (PV). Eleven healthy male lowlanders underwent three 21‐day confinement periods, in a counterbalanced order: (1) normoxic bed rest (NBR; P_IO₂: 133.1 ± 0.3 mmHg); (2) hypoxic bed rest (HBR; P_IO₂: 90.0 ± 0.4 mmHg, ambient simulated altitude of ~4000 m); and (3) hypoxic ambulation (HAMB; P_IO₂: 90.0 ± 0.4 mmHg). Blood samples were collected before, during (days 2, 5, 14, and 21) and 2 days after each confinement to determine EPO concentration. Qualitative differences in PV changes were also estimated by changes in hematocrit and hemoglobin concentration along with concomitant changes in plasma renin concentration. NBR caused an initial reduction in EPO by ~39% (P = 0.04). By contrast, HBR enhanced EPO (P = 0.001), but the increase was less than that induced by HAMB (P < 0.01). All three confinements caused a significant reduction in PV (P < 0.05), with a substantially greater drop in HBR than in the other conditions (P < 0.001). Thus, present results suggest that hypoxia prevents the EPO suppression, whereas it seems to exaggerate the PV reduction induced by bed rest.

The role of catecholamines in mesenchymal stem cell fate

Mesenchymal stem cells (MSCs) are multipotent stem cells found in many adult tissues, especially bone marrow (BM) and are capable of differentiation into various lineage cells such as osteoblasts, adipocytes, chondrocytes and myocytes. Moreover, MSCs can be mobilized from connective tissue into circulation and from there to damaged sites to contribute to regeneration processes. MSCs commitment and differentiation are controlled by complex activities involving signal transduction through cytokines and catecholamines. There has been an increasing interest in recent years in the neural system, functioning in the support of stem cells like MSCs. Recent efforts have indicated that the catecholamine released from neural and not neural cells could be affected characteristics of MSCs. However, there have not been review studies of most aspects involved in catecholamines-mediated functions of MSCs. Thus, in this review paper, we will try to describe the current state of catecholamines in MSCs destination and discuss strategies being used for catecholamines for migration of these cells to damaged tissues. Then, the role of the nervous system in the induction of osteogenesis, adipogenesis, chondrogenesis and myogenesis from MSCs is discussed. Recent progress in studies of signaling transduction of catecholamines in determination of the final fate of MSCs is highlighted. Hence, the knowledge of interaction between MSCs with the neural system could be applied towards the development of new diagnostic and treatment alternatives for human diseases.

Renal sympathetic denervation: applications in hypertension and beyond

Renal afferent and efferent sympathetic nerves are involved in the regulation of blood pressure and have a pathophysiological role in hypertension. Renal sympathetic denervation is a novel therapeutic technique for the treatment of patients with resistant hypertension. Clinical trials of renal sympathetic denervation have shown significant reductions in blood pressure in these patients. Renal sympathetic denervation also reduces heart rate, which is a surrogate marker of cardiovascular risk. Conditions that are comorbid with hypertension, such as heart failure and myocardial hypertrophy, obstructive sleep apnoea, atrial fibrillation, renal dysfunction, and metabolic syndrome are closely associated with enhanced sympathetic activity. In experimental models and case-control studies, renal denervation has had beneficial effects on these conditions. Renal denervation could become a commonly used procedure to treat resistant hypertension and chronic diseases associated with enhanced sympathetic activation. Current work is focused on refining the techniques and interventional devices to provide safe and effective renal sympathetic denervation. Controlled studies in patients with mild-to-moderate, nonresistant hypertension and comorbid conditions such as heart failure, diabetes mellitus, sleep apnoea, and arrhythmias are needed to investigate the capability of renal sympathetic denervation to improve cardiovascular outcomes.

Renal denervation in moderate to severe CKD

Sympathetic activation contributes to the progression of CKD and is associated with adverse cardiovascular outcomes. Ablation of renal sympathetic nerves reduces sympathetic nerve activity and BP in patients with resistant hypertension and preserved renal function, but whether this approach is safe and effective in patients with an estimated GFR (eGFR) < 45 ml/min per 1.73 m(2) is unknown. We performed bilateral renal denervation in 15 patients with resistant hypertension and stage 3-4 CKD (mean eGFR, 31 ml/min per 1.73 m(2)). We used CO(2) angiography in six patients to minimize exposure to contrast agents. Estimated GFR remained unchanged after the procedure, irrespective of the use of CO(2) angiography. Mean baseline BP ± SD was 174 ± 22/91 ± 16 mmHg despite the use of 5.6 ± 1.3 antihypertensive drugs. Mean changes in office systolic and diastolic BP at 1, 3, 6, and 12 months were -34/-14, -25/-11, -32/-15, and -33/-19 mmHg, respectively. Night-time ambulatory BP significantly decreased (P<0.05), restoring a more physiologic dipping pattern. In conclusion, this study suggests a favorable short-term safety profile and beneficial BP effects of catheter-based renal nerve ablation in patients with stage 3-4 CKD and resistant hypertension.

Pathophysiology, diagnosis, and treatment of orthostatic hypotension and vasovagal syncope

Orthostatic hypotension (OH) occurs in 0.5% of individuals and as many as 7-17% of patients in acute care settings. Moreover, OH may be more prevalent in the elderly due to the increased use of vasoactive medications and the concomitant decrease in physiologic function, such as baroreceptor sensitivity. OH may result in the genesis of a presyncopal state or result in syncope. OH is defined as a reduction of systolic blood pressure (SBP) of at least 20 mm Hg or diastolic blood pressure (DBP) of at least 10 mm Hg within 3 minutes of standing. A review of symptoms, and measurement of supine and standing BP with appropriate clinical tests should narrow the differential diagnosis and the cause of OH. The fall in BP seen in OH results from the inability of the autonomic nervous system (ANS) to achieve adequate venous return and appropriate vasoconstriction sufficient to maintain BP. An evaluation of patients with OH should consider hypovolemia, removal of offending medications, primary autonomic disorders, secondary autonomic disorders, and vasovagal syncope, the most common cause of syncope. Although further research is necessary to rectify the disease process responsible for OH, patients suffering from this disorder can effectively be treated with a combination of nonpharmacologic treatment, pharmacologic treatment, and patient education. Agents such as fludrocortisone, midodrine, and selective serotonin reuptake inhibitors have shown promising results. Treatment for recurrent vasovagal syncope includes increased salt and water intake and various drug treatments, most of which are still under investigation.

Erythropoietin and the cardiorenal syndrome: cellular mechanisms on the cardiorenal connectors

We have recently proposed severe cardiorenal syndrome (SCRS), in which cardiac and renal failure mutually amplify progressive failure of both organs. This frequent pathophysiological condition has an extremely poor prognosis. Interactions between inflammation, the renin-angiotensin system, the balance between the nitric oxide and reactive oxygen species and the sympathetic nervous system form the cardiorenal connectors and are cornerstones in the pathophysiology of SCRS. An absolute deficit of erythropoietin (Epo) and decreased sensitivity to Epo in this syndrome both contribute to the development of anemia, which is more pronounced than renal anemia in the absence of heart failure. Besides expression on erythroid progenitor cells, Epo receptors are present in the heart, kidney, and vascular system, in which activation results in antiapoptosis, proliferation, and possibly antioxidation and anti-inflammation. Interestingly, Epo can improve cardiac and renal function. We have therefore reviewed the literature with respect to Epo and the cardiorenal connectors. Indeed, there are indications that Epo can diminish inflammation, reduce renin-angiotensin system activity, and shift the nitric oxide and reactive oxygen species balance toward nitric oxide. Information about Epo and the sympathetic nervous system is scarce. This analysis underscores the relevance of a further understanding of clinical and cellular mechanisms underlying protective effects of Epo, because this will support better treatment of SCRS.

Anemia and erythropoietin in space flights

Since the very early manned missions in space, a state of anemia associated with reduced erythropoietin levels and reduced plasma volume was disclosed. The reduction in red blood cell mass is driven by a process of selective hemolysis, which has been named neocytolysis. This phenomenon also occurs in people living at a high altitude who descend rapidly to sea level. The origin of the signal leading to destruction of newly produced red blood cells probably is located in central circulation, but the operating mechanism is unknown. The importance of plasma cell volume reduction in the genesis of a lower red cell mass also is supported by the inverse correlation seen at moderate altitude. People arriving at moderate altitude have increased erythropoietin concentration that decreases after a few days and is in inverse correlation with central venous pressure. Studies under simulated microgravity conditions in human beings (bed rest, head-down tilt at -6 degrees , water immersion) and in rats provide further insight in unraveling the mechanism of astronauts' anemia, a problem difficult to study in space because of the limited availability of spaceflights.

Measurement of hemoglobin in long-term fixed erythroid cells: application development for cell science experiments in microgravity

Studying the effects of microgravity on cell differentiation will enhance our understanding of fundamental biology and is indispensable for a sustained space program. Rauscher murine erythroleukemic cells were chosen as a model system to study erythroid cell differentiation aboard the International Space Station because these cells undergo differentiation in response to the natural inducer, erythropoietin, as well as various chemical-inducers. We have now developed a method to quantify hemoglobin in Rauscher cells after weeks of fixation and storage required for such space biology experiments. By exploiting the pseudoperoxidase activity of hemoglobin and by using reagents that yield a soluble chromophore that freely passes out of fixed cells, we developed a highly specific and sensitive assay applicable to cells fixed as long as 4 months.

Impact of modeled microgravity on migration, differentiation, and cell cycle control of primitive human hematopoietic progenitor cells

OBJECTIVE

Migration, proliferation, and differentiation of bone marrow (BM) hematopoietic stem cells (HSC) are important factors in maintaining hematopoietic homeostasis. Homeostatic control of erythrocytes and lymphocytes is perturbed in humans exposed to microgravity (micro-g), resulting in space flight-induced anemia and immunosuppression. We sought to determine whether any of these anomalies can be explained by micro-g-induced changes in migration, proliferation, and differentiation of human BM CD34+ cells, and whether such changes can begin to explain any of the shifts in hematopoietic homeostasis observed in astronauts.

MATERIALS AND METHODS

BM CD34+ cells were cultured in modeled micro-g (mmicro-g) using NASA's rotating wall vessels (RWV), or in control cultures at earth gravity for 2 to 18 days. Cells were harvested at different times and CD34+ cells assessed for migration potential, cell-cycle kinetics and regulatory proteins, and maturation status.

RESULTS

Culture of BM CD34+ cells in RWV for 2 to 3 days resulted in a significant reduction of stromal cell-derived factor 1 (SDF-1alpha)-directed migration, which correlated with decreased expression of F-actin. Modeled micro-g induced alterations in cell-cycle kinetics that were characterized by prolonged S phase and reduced cyclin A expression. Differentiation of primitive CD34+ cells cultured for 14 to 18 days in RWV favored myeloid cell development at the expense of erythroid development, which was significantly reduced compared to controls.

CONCLUSIONS

These results illustrate that mmicro-g significantly inhibits the migration potential, cell-cycle progression, and differentiation patterns of primitive BM CD34+ cells, which may contribute to some of the hematologic abnormalities observed in humans during space flight.

Genetic models in applied physiology: selected contribution: effects of spaceflight on immunity in the C57BL/6 mouse. II. Activation, cytokines, erythrocytes, and platelets

This portion of the study quantified the effects of a 12-day space shuttle mission (Space Transport System-108/UF-1) on body and lymphoid organ masses, activation marker expression, cytokine secretion, and erythrocyte and thrombocyte characteristics in C57BL/6 mice. Animals in flight (Flt group) had 10-12% lower body mass compared with ground controls housed either in animal enclosure modules or under standard vivarium conditions (P < 0.001) and the smallest thymus and spleen masses. Percentages of CD25(+) lymphocytes, CD3(+)/CD25(+) T cells, and NK1.1(+)/CD25(+) natural killer cells from Flt mice were higher compared with both controls (P < 0.05). In contrast, CD71 expression was depressed in the Flt and animal enclosure module control mice compared with vivarium control animals (P < 0.001). Secretion of interferon-gamma, IL-2, and IL-4, but not tumor necrosis factor-alpha and IL-5, by splenocytes from Flt mice was decreased relative to either one or both ground controls (P < 0.05). Flt mice also had high red blood cell and thrombocyte counts compared with both sets of controls; low red blood cell volume and distribution width, percentage of reticulocytes, and platelet volume were also noted (P < 0.05) and were consistent with dehydration. These data indicate that relatively short exposure to the spaceflight environment can induce profound changes that may become significant during long-term space missions.

Diabetic autonomic neuropathy

Diabetic autonomic neuropathy (DAN) is a serious and common complication of diabetes. Despite its relationship to an increased risk of cardiovascular mortality and its association with multiple symptoms and impairments, the significance of DAN has not been fully appreciated. The reported prevalence of DAN varies widely depending on the cohort studied and the methods of assessment. In randomly selected cohorts of asymptomatic individuals with diabetes, approximately 20% had abnormal cardiovascular autonomic function. DAN frequently coexists with other peripheral neuropathies and other diabetic complications, but DAN may be isolated, frequently preceding the detection of other complications. Major clinical manifestations of DAN include resting tachycardia, exercise intolerance, orthostatic hypotension, constipation, gastroparesis, erectile dysfunction, sudomotor dysfunction, impaired neurovascular function, "brittle diabetes," and hypoglycemic autonomic failure. DAN may affect many organ systems throughout the body (e.g., gastrointestinal [GI], genitourinary, and cardiovascular). GI disturbances (e.g., esophageal enteropathy, gastroparesis, constipation, diarrhea, and fecal incontinence) are common, and any section of the GI tract may be affected. Gastroparesis should be suspected in individuals with erratic glucose control. Upper-GI symptoms should lead to consideration of all possible causes, including autonomic dysfunction. Whereas a radiographic gastric emptying study can definitively establish the diagnosis of gastroparesis, a reasonable approach is to exclude autonomic dysfunction and other known causes of these upper-GI symptoms. Constipation is the most common lower-GI symptom but can alternate with episodes of diarrhea. Diagnostic approaches should rule out autonomic dysfunction and the well-known causes such as neoplasia. Occasionally, anorectal manometry and other specialized tests typically performed by the gastroenterologist may be helpful. DAN is also associated with genitourinary tract disturbances including bladder and/or sexual dysfunction. Evaluation of bladder dysfunction should be performed for individuals with diabetes who have recurrent urinary tract infections, pyelonephritis, incontinence, or a palpable bladder. Specialized assessment of bladder dysfunction will typically be performed by a urologist. In men, DAN may cause loss of penile erection and/or retrograde ejaculation. A complete workup for erectile dysfunction in men should include history (medical and sexual); psychological evaluation; hormone levels; measurement of nocturnal penile tumescence; tests to assess penile, pelvic, and spinal nerve function; cardiovascular autonomic function tests; and measurement of penile and brachial blood pressure. Neurovascular dysfunction resulting from DAN contributes to a wide spectrum of clinical disorders including erectile dysfunction, loss of skin integrity, and abnormal vascular reflexes. Disruption of microvascular skin blood flow and sudomotor function may be among the earliest manifestations of DAN and lead to dry skin, loss of sweating, and the development of fissures and cracks that allow microorganisms to enter. These changes ultimately contribute to the development of ulcers, gangrene, and limb loss. Various aspects of neurovascular function can be evaluated with specialized tests, but generally these have not been well standardized and have limited clinical utility. Cardiovascular autonomic neuropathy (CAN) is the most studied and clinically important form of DAN. Meta-analyses of published data demonstrate that reduced cardiovascular autonomic function as measured by heart rate variability (HRV) is strongly (i.e., relative risk is doubled) associated with an increased risk of silent myocardial ischemia and mortality. The determination of the presence of CAN is usually based on a battery of autonomic function tests rather than just on one test. Proceedings from a consensus conference in 1992 recommended that three tests (R-R variation, Valsalva maneuver, and postural blood pressure testing)or longitudinal testing of the cardiovascular autonomic system. Other forms of autonomic neuropathy can be evaluated with specialized tests, but these are less standardized and less available than commonly used tests of cardiovascular autonomic function, which quantify loss of HRV. Interpretability of serial HRV testing requires accurate, precise, and reproducible procedures that use established physiological maneuvers. The battery of three recommended tests for assessing CAN is readily performed in the average clinic, hospital, or diagnostic center with the use of available technology. Measurement of HRV at the time of diagnosis of type 2 diabetes and within 5 years after diagnosis of type 1 diabetes (unless an individual has symptoms suggestive of autonomic dysfunction earlier) serves to establish a baseline, with which 1-year interval tests can be compared. Regular HRV testing provides early detection and thereby promotes timely diagnostic and therapeutic interventions. HRV testing may also facilitate differential diagnosis and the attribution of symptoms (e.g., erectile dysfunction, dyspepsia, and dizziness) to autonomic dysfunction. Finally, knowledge of early autonomic dysfunction can encourage patient and physician to improve metabolic control and to use therapies such as ACE inhibitors and beta-blockers, proven to be effective for patients with CAN.

Drug treatment of orthostatic hypotension and vasovagal syncope

Orthostatic hypotension is a common problem, estimated to occur in 5 out of every 1000 individuals and in as many as 7% to 17% of patients in an acute care setting. Moreover, orthostatic hypotension may be more prominent in elderly patients due to the increased intake of vasoactive medications and concomitant decrease in physiologic function, such as baroreceptor sensitivity, often seen with aging. Orthostatic hypotension is a fall in blood pressure on assuming an upright position. Absolute cutoffs for the drop in blood pressure are often difficult to determine because different patients exhibit varying degrees of tolerance to falls in blood pressure. Therefore, strict numerical criteria may lead to underdiagnosis and improper intervention. A thorough review of patient symptomatology combined with appropriate clinical tests should be employed to narrow the vast differential diagnosis and pinpoint the etiology. The fall in blood pressure seen in orthostatic hypotension results from the inability of the autonomic nervous system to adequately compensate for the 500 mL blood that is estimated to pool in the lower extremities on assuming an upright posture. The decrease in venous return results in a concomitant decrease in cardiac output and thus hypoperfusion of the cerebral circulation, possibly resulting in syncope or various other symptoms. A complete investigation should consider hypovolemia, removal of offending medications, primary autonomic disorders, secondary autonomic disorders and, of course, vasovagal syncope, the most common cause of syncope. Although further research is still necessary to rectify the disease process responsible for orthostatic hypotension, patients suffering from this disorder can effectively be treated through a combination of nonpharmacologic treatment, pharmacologic treatment and patient education. Agents such as fludrocortisone, midodrine and erythropoietin show promising results as therapeutic adjuncts. Treatment for recurrent vasovagal syncope includes increased salt intake, and various drug treatments, most of which are still under investigation.

Rhythmicity of engraftment and altered cell cycle kinetics of cytokine-cultured murine marrow in simulated microgravity compared with static cultures

Space flight with associated microgravity is complicated by "astronaut's anemia" and other hematologic abnormalities. Altered erythroid differentiation, red cell survival, plasma volume, and progenitor numbers have been reported. We studied the impact of microgravity on engraftable stem cells, culturing marrow cells in rotary wall vessel (RWV) culture chambers mimicking microgravity and in normal gravity nonadherent Teflon bottles. A quantitative competitive engraftment technique was assessed under both conditions in lethally irradiated hosts. We assessed 8-wk engraftable stem cells over a period spanning at least one cell cycle for cytokine (FLT-3 ligand, thrombopoietin [TPO], steel factor)-activated marrow stem cells. Engraftable stem cells were supported out to 56 h under microgravity conditions, and this support was superior to that seen in normal-gravity Teflon bottle cultures out to 40 h, with Teflon bottle culture support superior to RWV from 40 to 56 h. A nadir of stem cell number was seen at 40 h in Teflon and 48 h in RWV, suggesting altered marrow stem cell cycle kinetics under microgravity. This is the first study of engraftable stem cells under microgravity conditions, and the differences between microgravity and normal gravity cultures may present opportunities for unique future stem cell expansion strategies.

Orthostatic intolerance. A historical introduction to the pathophysiological mechanisms

Several of the pathophysiological mechanisms resulting in orthostatic intolerance (ie, tachycardia) have been recognized individually over the course of the past 100 years or more. More recent definitions of the normal ranges of orthostatic blood pressure and heart rate changes have facilitated the recognition of pathogenetic disorders that are probably shared in various proportions between orthostatic intolerance and various types of orthostatic hypotension. These include autonomic dysfunction of (1) the leg veins almost invariably causing excessive gravitational blood pooling, usually associated with (2) hypovolemia of circulating erythrocytes and plasma that is probably attributable to impaired autonomic stimulation of erythropoietin production, renin release, and (less consistently) aldosterone secretion. Improved understanding of these apparent results of lower body dysautonomia should facilitate more effective therapy in the future.