A Strategy for Research in Space Biology and Medicine in the New Century (1998) / Chapter Skim
Currently Skimming:
9 Endocrinology
9 Endocrinology
Pages 132-155
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 132... ...
The principal spaceflight responses with a significant endocrine contribution are fluid shifts, perturbation of the circadian rhythms, loss of red blood cell mass, possible changes in the immune system, loss of bone and muscle, and maintenance of energy balance. The last three are chronic responses, whereas fluid shifts occur only after entry into a microgravity environment and again after return to Earth.
|
From page 133... ...
These responses include activation of the hypothalamic-pituitary-adrenal (HPA) axis, increases in the wholebody protein turnover rate and acute phase protein synthesis, increased gluconeogenesis, substrate cycling, proinflammatory cytokine activity, basal energy expenditure, and loss of body protein.~0~~5 Collectively, these reactions limit the extent of the injury or stress, protect the rest of the organism against any further stresses by mobilizing host defense mechanisms, and initiate various processes aimed at restoring the organism's homeostatic balance.
|
From page 134... ...
During the first day or two of spaceflight, there is increased cortisol secretion, protein turnover, acute phase protein synthesis, and activation of the proinflammatory cytokine IL-6.27 Protein breakdown is increased more than synthesis, and so the net effect is a loss of body protein.28 29 The rise in protein turnover reflects the increased synthesis of proteins involved in host defenses. Thus, fibrinogen synthesis is increased even though there was no actual injury.30 All of these measurements indicate that entry into orbit is associated with a metabolic stress response, which is over after about 1 week in space.31 There is currently little interpretable in-flight data on the endocrine system beyond the initial response period (less than 1 week)
|
From page 135... ...
Although flight experiments should be the primary focus, there is a continuing need for groundbased studies with the appropriate models. Currently available models range from whole body (human bed rest, rodent hindlimb unloading, centrifugation)
|
From page 136... ...
Two in-flight energy balance studies have been done,48~50 the first on Skylab using a combination of dietary intake and body composition measurements,5~ and the second on the Life and Microgravity Sciences (LMS ~ shuttle mission using the doubly labeled water method.52 In both cases, astronauts were in negative energy balance. On the long-term Skylab mission, the deficit was about 3 kcal kg- days, with the deficit being greatest during the first month and tapering off toward the end of the mission (Table 9.1~.53 On the 16-day LMS mission, the average energy deficit for the four payload crew members was ~10 kcal kg-1 day-1.54 The ~;~;' ma She ~ A4~ ~;~;^~ ~7q~ ~ i~ i ~h~[q11 ;~ AT intake (24 kcal kg-1 day-l)
|
From page 137... ...
Dietary intake on the recent LMS mission in 1996 (24 kcal kg- Say-so was remarkably low, although exercise levels were much greater on this mission than on SLS-1 and SLS-2. The poor dietary intake raises questions as to whether enough time was allocated for eating on this busy mission, whether there were "taste" problems with diet, or whether something more serious occurs when there is a heavy exercise schedule.
|
From page 138... ...
not on SLS-1 and SLS-2.~°~ The postflight data on plasma aldosterone levels are conflicting. Plasma aldosterone levels were increased after Skylab,~02 shuttle missions,~03~~05 a Russian short-term mission,~06 and some of the longduration Russian flights.~07 ~08 But after other long-term Soviet Salyut missions, plasma aldosterone levels were lower than preflight.~09 In contrast, urinary aldosterone has consistently been elevated in the immediate postflight period.~0~~4 This increase has been ascribed to the need to retain sodium postflight.~5 The lack of significant changes during the SLS-1 and SLS-2 missions may have been because crew members were encouraged to maintain fluid intake.
|
From page 139... ...
Further study of the renal-endocrine relationships in microgravity is unlikely to contribute significantly to the study of disturbances in fluid and electrolyte equilibrium on Earth. Models Bed rest, particularly with 6° head-down tilt, has been the model of choice for investigating fluid shifts.
|
From page 140... ...
Serum iron levels in spaceflight and on Earth did not differ, but in-flight incorporation of 59Fe was 66 percent of preflight levels when measured after 22 hours in orbit. Although RBC production decreased in flight, the survival rate of labeled RBCs was not changed by spaceflight.~39 ~40 Spaceflight anemia appears to be a self-limiting and appropriate response to fluid shifts associated with microgravity.
|
From page 141... ...
1 AL 1 TO atrophy response, lea an energy dellclt,l-/~l-~ a metabolic stress response,l50 l5l altered activity of the HPA axis,l52~l54 tension-induced remodeling,l55 156 and altered neuroendocrine signals.l57 Each of these proceeds via different mechanisms, and there are different ground-based models for each. For atrophy, there is bed rest and hindlimb suspension; for an energy deficit, starvation; for altered HPA axis
|
From page 142... ...
There is a good correlation between the increased insulin secretion and the protein loss. Progressively increasing insulin secretion and decreased nitrogen retention are consistent with the development of insulin resistance and with a role for the insulin resistance in the etiology of the poorer nitrogen balance found toward the end of the mission.~67 Postflight measurements by the Skylab investigators and Soviet scientists showed that postflight plasma insulin levels were increased, an increase that persisted for as long as 2 weeks after landing.~68 ~69 This increase was probably associated with an increase in dietary intake and so was part of an anabolic response.
|
From page 143... ...
· ~ · ~ ~ . ~ ~ T T muscle lOSS 1n nlnclllmo-suspenclecl rats.^V/ ~vo ~ut tne comomatlon o~ exercise ancl supplemental un or IGF-1209 was more effective in attenuating muscle protein loss than either parameter alone.2l0 It is apparent that GH has major effects on muscle mass and energy metabolism in the rodent hypogravity model of hindlimb unloading.
|
From page 144... ...
Summary Comments The 1987 Goldberg report pointed out that "the identification of the cellular and molecular mechanisms that link active muscle contraction and chances in contractile Protein synthesis remains a fundamental question of muscle biology." · .
|
From page 145... ...
This enabled the animals to maintain normal heart rate cycling and motor activity, even though actual heart rate was decreased, presumably because of the lower rate of energy expenditure.223 224 In contrast, several studies have now documented delays in the phasing of body temperature rhythms in monkeys and rats.225~227 This discrepancy probably reflects the presence of more than one pacemaker in the body.228 Perturbed circadian rhythms are not unique to monkeys; the free-running activity rhythm of beetles was decreased,229 as was spore formation in the fungus Neurospora crassa.230 Circadian rhythms are important and should be assigned a high priority. Quite apart from performance and psychosocial effects (which are discussed in Chapter 12)
|
From page 146... ...
1993. Acute response of human muscle protein to catabolic hormones.
|
From page 147... ...
1996. Measurement of energy expenditure in rhesus monkeys during spaceflight using doubly labeled water (2H2180~.
|
From page 148... ...
1992. Effects of lower limb unloading on skeletal muscle mass and function in humans.
|
From page 149... ...
1987. Space shuttle inflight and post flight fluid shifts measured by leg volume changes.
|
From page 150... ...
1987. Space shuttle inflight and post flight fluid shifts measured by leg volume changes.
|
From page 151... ...
1995. Decreased production of red blood cells in human subjects exposed to microgravity.
|
From page 152... ...
1996. Experimental modification of rat pituitary growth hormone cell function during and after spaceflight.
|
From page 153... ...
1996. Prolonged bed rest decreases skeletal muscle and whole body protein synthesis.
|
From page 154... ...
1994. Resistance exercise and growth hormone as countermeasures for skeletal muscle atrophy in hindlimb suspended rats.
|
From page 155... ...
1993. Gender differences in endocrine responses to posture and 7 days of 6 degrees head-down bed rest.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.