Lower body negative pressure vs. lower body positive pressure to prevent cardiac atrophy after bed rest and spaceflight. What caused the controversy?

Comprehensive analysis of transcriptomics and metabolomics to understand tail-suspension-induced myocardial injury in rat

Background/Aims

The effect and underlying mechanism of microgravity on myocardium still poorly understood. The present study aims to reveal the effect and underlying mechanism of tail-suspension-induced microgravity on myocardium of rats.

Methods

Tail-suspension was conducted to simulate microgravity in rats. Echocardiography assay was used to detect cardiac function. The cardiac weight index was measured. Hematoxylin and eosin (HE) staining and transmission electron microscopy assay were conducted to observe the structure of the tissues. RNA sequencing and non-targeted metabolomics was employed to obtain transcriptome and metabolic signatures of heart from tail-suspension-induced microgravity and control rats.

Results

Microgravity induced myocardial atrophy and decreased cardiac function in rats. Structure and ultrastructure changes were observed in myocardium of rats stimulated with microgravity. RNA sequencing for protein coding genes was performed and identified a total of 605 genes were differentially expressed in myocardium of rats with tail suspension, with 250 upregulated and 355 downregulated (P < 0.05 and | log2fold change| > 1). A total of 55 differentially expressed metabolites were identified between the two groups (VIP > 1 and P < 0.05) by the metabolic profiles of heart tissues from microgravity groups and control. Several major pathways altered aberrantly at both transcriptional and metabolic levels, including FoxO signaling pathway, Amyotrophic lateral sclerosis, Histidine metabolism, Arginine and proline metabolism.

Conclusion

Microgravity can induce myocardial atrophy and decreases cardiac function in rats and the molecular alterations at the metabolic and transcriptomic levels was observed, which indicated major altered pathways in rats with tail suspension. The differentially expressed genes and metabolites-involved in the pathways maybe potential biomarkers for microgravity-induced myocardial atrophy.

MNX (Medium Duration Nutrition and Resistance-Vibration Exercise) Bed-Rest: Effect of Resistance Vibration Exercise Alone or Combined With Whey Protein Supplementation on Cardiovascular System in 21-Day Head-Down Bed Rest

Current inflight countermeasures do not completely prevent bone and cardiovascular changes induced by microgravity. High load Resistance Exercise combined with whole body Vibration (RVE) demonstrated benefits on bone and cardiovascular system during previous Head-Down Bed Rest (HDBR) studies. We examined the effectiveness of RVE alone or combined with a nutritional supplementation of Whey protein (NeX) on cardiovascular deconditioning. Eight male subjects (age 34 ± 8 years) in a crossover design completed three 21-day HDBR campaigns (Control-CON, RVE, and NeX). Pre and post HDBR Orthostatic Tolerance (OT) was evaluated by a 15-min head-up tilt test followed by increasing levels of Lower Body Negative Pressure (LBNP). Heart rate (HR), blood pressure (BP), and Sympathetic Index (ΣI) through spectral analysis were measured during OT test. Plasma Volume (PV), and Maximal Oxygen Uptake (VO2max) were measured before and after each campaign. Left ventricular mass, left ventricular end diastolic (LVEDV), end systolic (LVESV), stroke (SV) volumes, and circumferential deformation at rest and during an orthostatic stress simulated by a 30 mmHg LBNP were measured by cardiac MRI. RVE failed to prevent any change in these variables and NeX did not have any additional effect over exercise alone. In the 3 groups, (1) OT time dropped similarly (bed rest p < 0.001), (2) HR and ΣI were increased at rest at the end of HDBR and HR increased markedly during LBNP-tilt test, with inability to increase further the ΣI, (3) PV dropped (bed rest p < 0.001), along with LVEDV, LVESV and SV (p = 0.08, p < 0.001, and p = 0.045, respectively), (4) Left ventricle mass did not change significantly, (5) Deformation of the heart assessed by global circumferential strain was preserved and early diastolic circumferential strain rate was increased during orthostatic stress at the end of HDBR, illustrating preserved systolic and diastolic function respectively, without any difference between groups. Despite the drop in PV and LV volumes, RVE and NeX tended to alleviate the decrease in VO2max. In conclusion, RVE and NeX failed to prevent the cardiovascular deconditioning induced by a 21 day-HDBR.

Supine cycling plus volume loading prevent cardiovascular deconditioning during bed rest

There are two possible mechanisms contributing to the excessive fall of stroke volume (and its contribution to orthostatic intolerance) in the upright position after bed rest or spaceflight: reduced cardiac filling due to hypovolemia and/or a less distensible heart due to cardiac atrophy. We hypothesized that preservation of cardiac mechanical function by exercise training, plus normalization of cardiac filling with volume infusion, would prevent orthostatic intolerance after bed rest. Eighteen men and three women were assigned to 1) exercise countermeasure (n=14) and 2) no exercise countermeasure (n=7) groups during bed rest. Bed rest occurred in the 6 degrees head-down tilt position for 18 days. The exercise regimen was prescribed to compensate for the estimated cardiac work reduction between bed rest and ambulatory periods. At the end of bed rest, the subjects were further divided into two additional groups for post-bed rest testing: 1) volume loading with intravenous dextran to normalize cardiac filling pressure and 2) no volume loading. Dextran infusion was given to half of the exercise group and all of the sedentary group after bed rest, leading ultimately to three groups: 1) exercise plus volume infusion; 2) exercise alone; and 3) volume infusion alone. Exercise training alone preserved left ventricular mass and distensibility as well as upright exercise capacity, but lower body negative pressure (LBNP) tolerance was still depressed. LBNP tolerance was maintained only when exercise training was accompanied by dextran infusion. Dextran infusion alone following bed rest without exercise maintained neither orthostatic tolerance nor upright exercise capacity. We conclude that daily supine cycle exercise sufficient to prevent cardiac atrophy can prevent orthostatic intolerance after bed rest only when combined with plasma volume restoration. This maintenance of orthostatic tolerance was achieved by neither exercise nor dextran infusion alone. Cardiac atrophy and hypovolemia are likely to contribute independently to orthostatic intolerance after bed rest.