Impact of 14 Days of Bed Rest in Older Adults and an Exercise Countermeasure on Body Composition, Muscle Strength, and Cardiovascular Function: Canadian Space Agency Standard Measures

INTRODUCTION

Head-down bed rest (HDBR) has long been used as an analog to microgravity, and it also enables studying the changes occurring with aging. Exercise is the most effective countermeasure for the deleterious effects of inactivity. The aim of this study was to investigate the efficacy of an exercise countermeasure in healthy older participants on attenuating musculoskeletal deconditioning, cardiovascular fitness level, and muscle strength during 14 days of HDBR as part of the standard measures of the Canadian Space Agency.

METHODS

Twenty-three participants (12 males and 11 females), aged 55-65 years, were admitted for a 26-day inpatient stay at the McGill University Health Centre. After 5 days of baseline assessment tests, they underwent 14 days of continuous HDBR followed by 7 days of recovery with repeated tests. Participants were randomized to passive physiotherapy or an exercise countermeasure during the HDBR period consisting of 3 sessions per day of either high-intensity interval training (HIIT) or low-intensity cycling or strength exercises for the lower and upper body. Peak aerobic power (V̇O2peak) was determined using indirect calorimetry. Body composition was assessed by dual-energy X-ray absorptiometry, and several muscle group strengths were evaluated using an adjustable chair dynamometer. A vertical jump was used to assess whole-body power output, and a tilt test was used to measure cardiovascular and orthostatic challenges. Additionally, changes in various blood parameters were measured as well as the effects of exercise countermeasure on these measurements.

RESULTS

There were no differences at baseline in main characteristics between the control and exercise groups. The exercise group maintained V̇O2peak levels similar to baseline, whereas it decreased in the control group following 14 days of HDBR. Body weight significantly decreased in both groups. Total and leg lean masses decreased in both groups. However, total body fat mass decreased only in the exercise group. Isometric and isokinetic knee extension muscle strength were significantly reduced in both groups. Peak velocity, flight height, and flight time were significantly reduced in both groups with HDBR.

CONCLUSION

In this first Canadian HDBR study in older adults, an exercise countermeasure helped maintain aerobic fitness and lean body mass without affecting the reduction of knee extension strength. However, it was ineffective in protecting against orthostatic intolerance. These results support HIIT as a promising approach to preserve astronaut health and functioning during space missions, and to prevent deconditioning as a result of hospitalization in older adults.

Hindlimb unloading in C57BL/6J mice induces bone loss at thermoneutrality without change in osteocyte and lacuno-canalicular network

Impaired mechanical stimuli during hindlimb unloading (HLU) are believed to exacerbate osteocyte paracrine regulation of osteoclasts. We hypothesized that bone loss and deterioration of the osteocyte lacuno-canalicular network are attenuated in HLU mice housed at thermoneutrality (28 °C) compared with those housed at ambient temperature (22 °C). Following acclimatization, 20-week-old male C57BL/6J mice were submitted to HLU or kept in pair-fed control cages (CONT), for 5 days (5d) or 14d, at 22 °C or 28 °C. In the femur distal metaphysis, thermoneutral CONT mice had higher bone volume (p = 0.0007, BV/TV, in vivo μCT, vs. 14dCONT22) whilst osteoclastic surfaces of CONT and HLU were greater at 22 °C (5dCONT22 + 53 %, 5dHLU22 + 50 %, 14dCONT22 + 186 %, 14dHLU22 + 104 %, vs matching 28 °C group). In the femur diaphysis and at both temperatures, 14dHLU exhibited thinner cortices distally or proximally compared to controls; the mid-diaphysis being thicker at 28 °C than at 22 °C in all groups. Expression of cortical genes for proteolytic enzyme (Mmp13), markers for osteoclastogenic differentiation (MCSF, RANKL), and activity (TRAP, Ctsk) were increased following 22 °C HLU, whereas only Ctsk expression was increased following 28 °C HLU. Expression of cortical genes for apoptosis, senescence, and autophagy were not elevated following HLU at any temperature. Osteocyte density at the posterior mid-diaphysis was similar between groups, as was the proportion of empty lacunae (<0.5 %). However, analysis of the lacuno-canalicular network (LCN, fluorescein staining) revealed unstained areas in the 14dHLU22 group only, suggesting disrupted LCN flow in this group alone. In conclusion, 28 °C housing influences the HLU bone response but does not prevent bone loss. Furthermore, our results do not show osteocyte senescence or death, and at thermoneutrality, HLU-induced bone resorption is not triggered by osteoclastic activators RANKL and MCSF.

DAILY EXERCISE DOES NOT ALTER CEREBRAL BLOOD FLOW REGULATION DURING TILT IN OLDER ADULTS AFTER TWO WEEKS OF BEDREST

Bedrest is associated with cardiovascular and cerebrovascular changes that are linked to a greater prevalence of orthostatic intolerance; however, much of what we know from bedrest trials comes from young adults. How older adults respond to extended periods of activity restriction is not well understood. The current work tested the hypothesis that impaired control of cerebral blood flow in older adults following bedrest could be attenuated by regular exercise. Twenty-two older adults (55-65 years, 11 women) participated in a randomized controlled trial involving 14 days of continuous 6-degree head-down bedrest. The cohort was randomized to an exercise intervention (EX), involving daily aerobic, high-intensity interval, and resistance training, or a control group (CON), involving passive manual therapy. Cerebral blood flow velocity, cardiac output, and arterial blood pressure were measured continuously during a passive 80-degree head-up tilt protocol for up to 15 minutes. Responses in the cerebral and peripheral vascular beds were quantified by vascular resistance (cerebral: resistance area product (RAP); peripheral: total peripheral resistance (TPR)). Following bedrest, both supine RAP and TPR were elevated (p&lt; 0.01), with no effect of exercise training. In addition, the number of participants who could not complete 15 minutes of tilt increased from 3 to 15 (8 EX, 7 CON). In these non-finishers, RAP dropped ~20% and TPR dropped ~15% during the final 2 minutes of tilt, with no difference between groups. These data suggest that multimodal exercise training is insufficient to prevent changes in cardiovascular and cerebrovascular regulation in older adults observed during 14-day bedrest.

TILT TEST TOLERANCE AFTER TWO-WEEKS BED REST IN OLDER ADULTS IS NOT PROTECTED BY DAILY AEROBIC AND RESISTANCE EXERCISE

Bed rest is associated with increased risk of orthostatic intolerance on return to upright posture and, at least in young adults, regular exercise during bed rest is proposed as protective. 22 older adults (55-65 years, half women) participated in a randomized trial with 14-days of continuous 6-degree head-down bed rest. Half completed 60 min/day of aerobic and high-intensity interval cycling plus resistance exercises (Ex) while the other half were control (Con). A passive 80-degree head-up tilt (HUT) to a maximum of 15-min assessed orthostatic tolerance while cardio- and cerebrovascular responses were monitored to a pre-syncopal level of systolic blood pressure < 90 mmHg. In pre-bed rest baseline, all but 3 women (Ex) completed 15-min HUT. After 14-days bed rest, only 3 men (1 Ex) and 1 woman (Ex) completed 15-min with no differences in tolerance between men and women or Ex and Con. Vasovagal syncope presented in 5 participants (2 M-Con, 1 M-Ex, 1 F-Con, 1 F-Ex), most other non-finishers had progressive reduction in stroke volume without adequate vasoconstriction. Reductions in cerebral blood velocity measured during HUT by Doppler ultrasound revealed strong linear relationships to both end-tidal PCO2 and mean arterial pressure across all participants. These data indicate that neither older men nor older women were protected from orthostatic intolerance by 60-min per day exercise including high-intensity interval training. These data contrast with previous findings in younger adults in controlled trials of bed rest pointing to the need for greater understanding of mechanisms regulating blood pressure and brain blood flow.

Unloading-Induced Cortical Bone Loss is Exacerbated by Low-Dose Irradiation During a Simulated Deep Space Exploration Mission

Spaceflight-induced bone losses have been reliably reproduced in Hind-Limb-Unloading (HLU) rodent models. However, a considerable knowledge gap exists regarding the effects of low-dose radiation and microgravity together. Ten-week-old male C57BL/6J mice, randomly allocated to Control (CONT), Hind-Limb Unloading (HLU), and Hind-Limb Unloading plus Irradiation (HLUIR), were acclimatized at 28 °C, close to thermoneutral temperature, for 28 days prior to the 14-day HLU protocol. HLUIR mice received a 25 mGy dose of X-ray irradiation, simulating 14 days of exposure to the deep space radiation environment, on day 7 of the HLU protocol. Trabecular bone mass was similarly reduced in HLU and HLUIR mice when compared to CONT, with losses driven by osteoclastic bone resorption rather than changes to osteoblastic bone formation. Femoral cortical thickness was reduced only in the HLUIR mice (102 μm, 97.5-107) as compared to CONT (108.5 μm, 102.5-120.5). Bone surface area was also reduced only in the HLUIR group, with no difference between HLU and CONT. Cortical losses were driven by osteoclastic resorption on the posterior endosteal surface of the distal femoral diaphysis, with no increase in the numbers of dead osteocytes. In conclusion, we show that low-dose radiation exposure negatively influences bone physiology beyond that induced by microgravity alone.

Human Secretory IgM: An Elusive Player in Mucosal Immunity

Secretory IgMs (SIgMs) were amongst the first identified immunoglobulins. However, their importance was not fully understood and recent advances have shown they play a key role in establishing and promoting commensal gut tolerance in mice and humans. The true interactions between SIgMs and the microbiota remain controversial and we aim to consolidate current knowledge in this review. Through comprehensive examination of SIgMs and their corresponding B cell secretors in several different pathological immunological contexts, we review the presumed role of these molecules in gut tolerance, inflammatory bowel diseases, and lung immunity. As SIgMs harbor a mostly tolerogenic function, we posit that their inclusion in further immunological research is paramount.

The upper limit of normal CSF opening pressure is related to bilateral transverse sinus stenosis in headache sufferers

There are limited data on the relationship between normal cerebrospinal fluid (CSF) opening pressure and bilateral transverse sinus stenosis (BTSS); there are also several conflicting reports about the upper limit of normal CSF opening pressure. To evaluate the influence of BTSS on the upper limit of normal CSF opening pressure, we prospectively recorded lumbar CSF opening pressures in 217 adult patients with neurological symptoms who underwent cerebral magnetic resonance venography (MRV). The CSF opening pressures ranged between 65 and 286 mmH2O (mean = 149.3, s.d. = 47.5). The upper limit of opening pressure in patients with both normal appearance of transverse sinuses and unilateral transverse sinus stenosis on MRV ( n = 167) was 195 mmH2O with a range of 65–195 mmH2O. All patients with BTSS were headache sufferers, and the upper limit of opening pressure in patients with BTSS ( n = 50) was 286 mmH2O with a range of 91–286 mmH2O. All patients with opening pressures > 200 mmH2O displayed BTSS, whereas only 13% of patients with a pressure < 200 mmH2O displayed BTSS. Our findings demonstrate that the upper limit of normal CSF opening pressure is related to BTSS, and they also highlight that headache sufferers with opening pressures > 200 mmH2O should be tested for BTSS by MRV.

Transverse sinus stenoses persist after normalization of the CSF pressure in IIH

BACKGROUND

Bilateral transverse sinus (TS) stenosis has been found in more than 90% of patients with idiopathic intracranial hypertension (IIH).

OBJECTIVE

To evaluate whether TS stenosis changed after normalization of CSF pressure in patients with IIH during medical treatment.

METHODS

Fourteen consecutive patients with IIH with bilateral TS stenosis on cerebral MR venography (MRV) during the medical treatment were studied. Patients were followed for over a 6-year period. During the follow-up, patients underwent repeated lumbar punctures (LPs) and cerebral MRV. MRV was always performed before each LP.

RESULTS

TS stenosis persisted in all the patients during the follow-up. In 9 of 14 (64%) patients with IIH, CSF pressure normalized during medical treatment.

CONCLUSIONS

Transverse sinus (TS) stenoses, as revealed by MR venography, persist in patients with idiopathic intracranial hypertension after normalization of CSF pressure, suggesting the lack of a direct relationship between the caliber of TS and CSF pressure.