Determination of the Time-course of Development of Insulin Resistance, and Associated Molecular and Muscular Adaptations, During Inactivity in 3 Days of Bed-rest

Bed rest is a widely accepted experimental model used to study physiological adaptations to space flight, including bone and muscle mass loss, fluid shift, and cardiovascular deconditioning. Generally, participants are asked to spend a period in bed, with all activities of daily living being performed in a horizontal or head-down tilt (HDT) position, thus minimising use of all muscles and bringing about significant physiological adaptations similar to those observed during space flight. A substantial number of human bed rest studies have been performed ranging from 3-370 days duration. However, there are currently gaps in our understanding of; the rate and magnitude of the physiological dysregulation that occurs during actual and simulated microgravity (as most studies to date have considered mainly pre and post-bed rest time-points); the sites and mechanisms controlling bed rest-induced dysregulation; whether the processes underlying acute physiological changes are the same as those seen in the longer term. Improving our understanding of these issues would enable interventions aimed at minimizing bed-rest induced physiological dysregulation to be most effectively focused on time periods when the rate of onset of physiological dysregulation is likely to be at its greatest. This would then contribute to future success being achieved in prolonged human space flight, e.g. a manned mission to Mars, because an understanding of the aetiology and time-course of the physiological dysregulation that accompanies prolonged exposure to microgravity would enable effective counter-measures to be implemented. Furthermore, bed rest models present a unique opportunity to study the consequences of bed rest on those who may sustain prolonged periods of inactivity during hospitalisation, arising from illness or injury.

Inactivity is associated with the development of insulin resistance and contributes to the development of many modern metabolic diseases including obesity, type 2 diabetes, dyslipidaemia and hypertension (1,2); with physical inactivity being cited as the principal cause of 27% of diabetes and 30% of ischaemic heart disease cases. However, the time course, relative tissue specificity (liver vs muscle) and mechanistic basis of inactivity induced insulin resistance, and its reversal by remobilisation, represent major gaps in our current understanding. Furthermore, insulin resistance is an observation with several possible aetiologies and the mechanisms involved in the development of this condition with short term bedrest/immobilisation may be different to those involved with longer term / chronic inactivity. Further research in this area is therefore warranted.

The current study is part of a larger project which includes a 60 day, long term, bed rest study conducted in collaboration with the European Space Agency, at the Institut Médecine Physiologie Spatiale facility in Toulouse, France, and will be compared to the measures being conducted in the long term bed rest experiment.

'Run-in' phase; Following a successful medical screening, participants will start a 'Run-in' phase during which their habitual activity levels will be assessed by accelerometry. Individualised energy requirements will be estimated using the modified Harris-Benedict resting metabolic rate equation and physical activity level factor and a standardised diet (macronutrient composition (expressed as a percentage of total dietary energy intake) being ~55% carbohydrates, ~30% fat and ~15% protein) will be provided for the 3-days preceding the first experimental session. The day before this session, participants will be asked to abstain from any strenuous exercise and to fast from midnight, consuming only water from that time. On arrival, a dual-energy X-ray absorptiometry (DEXA) scan will be carried out to characterise whole body and leg fat masses. The participant will then be asked to lie on a hospital bed in the supine position (one pillow) and muscle volume and architecture measurements will be determined by ultrasound imaging of the vastus lateralis muscle using a 100 mm linear array 13-4 megahertz probe. This will provide a global representation of quadriceps muscle anatomy in order to standardise calculations of leg glucose uptake and determine the contribution of muscle mass and intramyocellular lipid to leg insulin resistance. Muscle biopsy samples will be obtained from the vastus lateralis before and immediately after a 3hr hyperinsulinemic euglycemic clamp (3). Muscle biopsies will be obtained using the Bergström technique under sterile conditions, after injection of local anaesthetic. An anterograde femoral venous catheter will be inserted (using the Seldinger technique under ultrasound guidance) to enable venous blood draining from the leg to be analysed for glucose concentration and compared with the glucose concentration of arterialised-venous blood samples (Time points 0, 150, 160, 170 and 180 mins). At the same time as the femoral venous blood sample is taken, a femoral artery blood flow assessment (mmol/min) will be made using ultrasonography to enable leg glucose uptake to be calculated. Indirect calorimetry will be performed before and in the last 15 minutes of the 180min hyperinsulinaemic-euglycaemic clamp.

The day after the 'clamp' visit, a 3 Tesla magnetic resonance spectroscopy (MRS) scan will be undertaken to assess intramyocellular lipid (IMCL), extramyocellular lipid (EMCL), and hepatic triglyceride content. A magnetic resonance imaging (MRI) scan will also be performed to determine mid-thigh muscle cross-sectional area and whole body muscle mass. Following the magnetic resonance (MR) scans, participants will undergo a short lower limb proprioceptive assessment on a specially designed somatosensory apparatus called the Active Movement Extent Discrimination Assessment (AMEDA). Participants will stand on the apparatus and have each ankle moved through 5 different angles of inversion and are asked to rate the degree of ankle position. This method has been validated to determine proprioceptive discrimination and will be repeated after the MR performed on day 4 to assess differences before and after bed rest.