Regulation of body fluid volume and electrolyte concentrations in spaceflight.

Despite a number of difficulties in performing experiments during weightlessness, a great deal of information has been obtained concerning the effects of spaceflight on the regulation of body fluid and electrolytes. Many paradoxes and questions remain, however. Although body mass, extracellular fluid volume, and plasma volume are reduced during spaceflight and remain so at landing, the changes in total body water are comparatively small. Serum or plasma sodium and osmolality have generally been unchanged or reduced during the spaceflight, and fluid intake is substantially reduced, especially during the first of flight. The diuresis that was predicted to be caused by weightlessness, has only rarely been observed as an increased urine volume. What has been well established by now, is the occurrence of a relative diuresis, where fluid intake decreases more than urine volume does. Urinary excretion of electrolytes has been variable during spaceflight, but retention of fluid and electrolytes at landing has been consistently observed. The glomerular filtration rate was significantly elevated during the SLS missions, and water and electrolyte loading tests have indicated that renal function is altered during readaptation to Earth's gravity. Endocrine control of fluid volumes and electrolyte concentrations may be altered during weightlessness, but levels of hormones in body fluids do not conform to predictions based on early hypotheses. Antidiuretic hormone is not suppressed, though its level is highly variable and its secretion may be affected by space motion sickness and environmental factors. Plasma renin activity and aldosterone are generally elevated at landing, consistent with sodium retention, but inflight levels have been variable. Salt intake may be an important factor influencing the levels of these hormones. The circadian rhythm of cortisol has undoubtedly contributed to its variability, and little is known yet about the influence of spaceflight on circadian rhythms. Atrial natriuretic peptide does not seem to play an important role in the control of natriuresis during spaceflight. Inflight activity of the sympathetic nervous system, assessed by measuring catecholamines and their metabolites and precursors in body fluids, generally seems to be no greater than on Earth, but this system is usually activated at landing. Collaborative experiments on the Mir and the International Space Station should provide more of the data needed from long-term flights, and perhaps help to resolve some of the discrepancies between U.S. and Russian data. The use of alternative methods that are easier to execute during spaceflight, such as collection of saliva instead of blood and urine, should permit more thorough study of circadian rhythms and rapid hormone changes in weightlessness. More investigations of dietary intake of fluid and electrolytes must be performed to understand regulatory processes. Additional hormones that may participate in these processes, such as other natriuretic hormones, should be determined during and after spaceflight. Alterations in body fluid volume and blood electrolyte concentrations during spaceflight have important consequences for readaptation to the 1-G environment. The current assessment of fluid and electrolyte status during weightlessness and at landing and our still incomplete understanding of the processes of adaptation to weightlessness and readaptation to Earth's gravity have resulted in the development of countermeasures that are only partly successful in reducing the postflight orthostatic intolerance experienced by astronauts and cosmonauts. More complete knowledge of these processes can be expected to produce countermeasures that are even more successful, as well as expand our comprehension of the range of adaptability of human physiologic processes.

Regulation of body fluid compartments during short-term spaceflight.

The fluid and electrolyte regulation experiment with seven subjects was designed to describe body fluid, renal, and fluid regulatory hormone responses during the Spacelab Life Sciences-1 (9 days) and -2 (14 days) missions. Total body water did not change significantly. Plasma volume (PV; P < 0.05) and extracellular fluid volume (ECFV; P < 0.10) decreased 21 h after launch, remaining below preflight levels until after landing. Fluid intake decreased during weightlessness, and glomerular filtration rate (GFR) increased in the first 2 days and on day 8 (P < 0.05). Urinary antidiuretic hormone (ADH) excretion increased (P < 0.05) and fluid excretion decreased early in flight (P < 0.10). Plasma renin activity (PRA; P < 0.10) and aldosterone (P < 0.05) decreased in the first few hours after launch; PRA increased 1 wk later (P < 0.05). During flight, plasma atrial natriuretic peptide concentrations were consistently lower than preflight means, and urinary cortisol excretion was usually greater than preflight levels. Acceleration at launch and landing probably caused increases in ADH and cortisol excretion, and a shift of fluid from the extracellular to the intracellular compartment would account for reductions in ECFV. Increased permeability of capillary membranes may be the most important mechanism causing spaceflight-induced PV reduction, which is probably maintained by increased GFR and other mechanisms. If the Gauer-Henry reflex operates during spaceflight, it must be completed within the first 21 h of flight and be succeeded by establishment of a reduced PV set point.

Short-term space flight on nitrogenous compounds, lipoproteins, and serum proteins.

Biochemical variables in blood were measured in venous blood samples from 38 to 72 Space Shuttle astronauts before and immediately after flights of 2 to 11 days. Mean pre- and postflight values were compared using the paired t-test or the Wilcoxon signed-rank test. The largest change in serum enzymes was a 21% increase (P = .0014) in gamma-glutamyl-transpeptidase, which may have been related to stress. The median value of apolipoprotein (apo) A-I decreased from 152 to 127 mg/dL (P < .0001), but the change in apo B (77 to 73 mg/dL) was not statistically significant, and the mean apo A-I/apo B ratio remained well above 1.5. A decrease in dietary fat and cholesterol intake during shuttle missions may have been a cause of the change in apo A-I. Twelve of the 16 nonenzyme serum proteins measured were significantly elevated (P < .05), possibly because of hemoconcentration and increased protein catabolism. The 56% increase in haptoglobin may be related to release of suppressed erythropoiesis at landing.

Metabolic changes observed in astronauts.

Study of metabolic alterations that occur during space flight can provide insight into mechanisms of physiologic regulation. Results of medical experiments with astronauts reveal rapid loss of volume (2 L) from the legs and a transient early increase in left ventricular volume index. These findings indicate that, during space flight, fluid is redistributed from the legs toward the head. In about 2 days, total body water decreases 2 to 3%. Increased levels of plasma renin activity and antidiuretic hormone while blood sodium and plasma volume are reduced suggest that space flight-associated factors are influencing the regulatory systems. In addition to fluid and electrolyte loss. Skylab astronauts lost an estimated 0.3 kg of protein. Endocrine factors, including increased cortisol and thyroxine and decreased insulin, are favorable for protein catabolism. The body appears to adapt to weightlessness at some physiologic cost. Readaptation to Earth's gravity at landing becomes another physiologic challenge.

Cholesterol in serum lipoprotein fractions after spaceflight.

Cholesterol, triglycerides, very low density lipoprotein cholesterol (VLDL-C), low density lipoprotein cholesterol (LDL-C), and high density lipoprotein cholesterol (HDL-C) were measured in blood samples from 125 crewmembers on the first 24 space shuttle flights. Samples were obtained before, immediately after, and 3-23 days after spaceflight. On landing day, only HDL-C was significantly changed from its preflight level; it had decreased 12.8%. Later in the postflight period, total cholesterol and LDL-C as well as HDL-C decreased significantly. Possible causes of these decreases in estimated cholesterol content of lipoprotein fractions include increased levels of thyroxine during flight and reduced physical activity. The postflight decrease in HDL-C is not considered to have clinical significance for shuttle astronauts, but lipoproteins and apolipoproteins should be measured in blood drawn during longer missions.

Direct action of norepinephrine on aortic baroreceptors of rat adventitia.

The adventitia of the rat aortic arch, with the depressor nerve intact, was separated from the underlying media to obtain a preparation in which the direct effects of norepinephrine on baroreceptor discharge could be examined free of smooth muscle effects. Cyclic stretch was applied to the adventitia to evoke discharge in single aortic baroreceptor fibers. Norepinephrine and phenylephrine in concentrations of 10(-7) to 10(-5) M increased baroreceptor discharge. No effect on discharge was seen with isoproterenol. Phentolamine but not propranolol blocked the increase in discharge produced by norepinephrine. An alpha-adrenergic modulation of baroreceptor discharge that is not dependent on smooth muscle tension has been demonstrated.

Morphology of presumptive slowly adapting receptors in dog trachea.

The three-dimensional branching pattern and ultrastructure of afferent myelinated fibers and their terminals located in the trachealis muscle of the dog are described. The afferent endings are believed to be those of the slowly adapting stretch receptors of the trachea. They have structural features typical of mechanoreceptors: distal to the loss of myelin, their shape becomes more irregular and the cytoplasm is filled with mitochondria, glycogen, and osmiophilic bodies. In some places the cell membrane is attached directly to basal lamina without interposition of a Schwann cell. A bundle of unmyelinated fibers accompanies each myelinated fiber and continues for an undetermined distance beyond (luminal to) terminations of the myelinated fiber. The unmyelinated fibers contain many round, clear vesicles and a few dense-cored vesicles and are also attached directly to basal lamina in places. Three-dimensional reconstruction of three receptors revealed three quite different branching patterns, but all included apparent rings as part of more or less contorted terminal regions (some neurons apparently having more than one terminal region). No obvious structural basis for the activation of receptors by transverse but not longitudinal stretch was found.

Microfibrils in the aorta.

Ruthenium red was used to stain microfibrils in rat aorta after incubation of the tissues with or without one of the enzymes trypsin, collagenase, phospholipase C, chondroitinase ABC, hyaluronidase or neuraminidase, or the reducing agent dithiothreitol. Microfibrils exhibiting periodicity of ruthenium red binding were associated with elastic laminae and collagen fibrils and appeared to attach these structures to each other as well as to basal lamina. Microfibrils in rat and human aorta demonstrated fibronectinlike immunoreactivity, therefore fibronectin may be a component of aorta microfibrils and important in the architecture of blood vessels.

Ultrastructural study of unencapsulated vertebrate mechanoreceptor terminals facilitated by double staining and resectioning of thick plastic sections.

A set of techniques for localization of unencapsulated sensory receptor terminals in plastic-embedded tissue is described. The tissue is pinned flat in fixative and flat-embedded in a small amount of medium-soft Epon. Pieces of the specimen are remounted so that tangential thick sections (2-3 micron) can be cut. These sections are stained with a mixture of 15 ml of 0.5% toluidine blue, 1% sodium borate; 10 ml of 2% p-phenylenediamine, and 5 ml of acetone. Stained sections are examined with a light microscope and brown-stained sensory receptor regions are located. Sections to be examined in the electron microscope are remounted by inverting a capsule of medium-hard Epon over them, and polymerized blocks are removed from the slide after heating it. Thin sections cut from remounted thick sections are stained with uranyl acetate and lead citrate. This procedure has been used successfully to locate aortic baroreceptors and very small apparent nerve endings in the atrium of the rat as well as stretch receptors in dog trachealis muscle. All of these receptor endings stain brown and are surrounded by light blue collagen and darker blue Schwann cells.

Ultrastructure of Aplysia neurons having different degrees of light sensitivity.

The relationship between ultrastructure and photosensitivity of pigmented neurons of the abdominal ganglion of Aplysia californica was investigated using electron microscopy and electrophysiological methods. Four identified neurons of similar light microscopic appearance were examined; two are photoresponsive and two are not. Illumination hyperpolarizes both responsive neurons. One of them, R2, requires roughly 100 times greater light intensities than does the other, the ventral photoresponsive neuron (VPN), for similar responses. Two neurons lying adjacent to VPN and similar in appearance to VPN do not have measurable electrophysiological responses to even the highest light intensities. All four neurons contained lipochondria, pigmented organelles associated with the light response. Therefore the presence of these organelles is not the only requirement for light sensitivity in these neurons. Illumination appeared to increase the number of membranous lipochondria in both R2 and the ventral neurons, but only in R2 was this increase significant. Factors such as the concentration of lipochondria near the plasma membrane may affect quantitative aspects of the light response, but in the insensitive cells the lipochondria are apparently uncoupled from other factors required for the light response.

Structure of rat aortic baroreceptors and their relationship to connective tissue.

The ultrastructure of fibres and sensory terminals of the aortic nerve innervating the aorta between the left common carotid and left subclavian arteries was investigated in the rat. This is the region from which most baroreceptor responses are recorded electrophysiologically. The fibres of the aortic nerve enter the adventitia and separate into bundles generally containing one myelinated fibre and four or five unmyelinated fibres of various sizes. The bundles pursue a roughly helical course through the adventitia; when they are close to the aortic media, the myelinated fibre loses its myelin sheath. A complex sensory terminal region is formed, as both the unmyelinated and 'premyelinated' axons become irregularly varicose. The concentration of mitochondria becomes very dense and cytoplasmic deposits of glycogen are observed. Both unmyelinated and premyelinated axons branch, and the unmyelinated axons wind irregularly around the premyelinated axon. The latter may have several loops and small holes. The terminal regions of both types of axon contain clusters of clear 40 nm vesicles. Part of the surface of each terminal region is ensheathed by Schwann cells, but the rest of the axolemma is directly exposed to extracellular connective tissue. There are often several layers of basal lamina around the sensory terminals and parts of the axolemma and Schwann cell membranes are attached to it by fine fibrillar material. The basal laminae are also attached to fibroblasts, fibroblast-like perineurial cells and elastic laminae, and the whole cellular and extracellular system appears to be tightly bound together. No differences between baroreceptors of spontaneously hypertensive and normal rats were found.

Relationship of aortic wall and baroreceptor properties during development in normotensive and spontaneously hypertensive rats.

We studied the relationship between aortic baroreceptor function and aortic wall properties in normotensive (NTR) and spontaneously hypertensive (SHR) rats 10-20 weeks old. Baroreceptor discharge, static pressure-volume (P-V), and pressure-radius relationships were measured in excised aortic segments. Histological studies of wall thickness and receptor numbers also were made. Circumferential wall stress and strain were calculated, as was the incremental elastic modulus (EINC). EINC in NTR's at 100 mm Hg was similar to values reported for in vivo human, dog, and rat aortas. At 10 weeks, SHR's had significantly elevated blood pressure, but SHR and NTR aortas had similar relationships among pressures, volumes, strains, and EINC's. Differences arose subsequently and, at 20 weeks, NTR aortas had larger volumes, larger strains, and smaller EINC's at equivalent pressures, whereas SHR aortas were unchanged. Thus the reduced distensibility of SHR relative to NTR aortas, rather than being due to retrogressive changes from normal, appeared to result from a failure to pass through a phase of increased distensibility. At 10 weeks, SHR baroreceptors showed resetting in both pressure-response and strain-response curves, and it was concluded that early hypertensive baroreceptor resetting was due to primary changes in the receptors. At 20 weeks, the order of the strain-response curves for NTR and SHR baroreceptors was reversed due to a reduction in strain sensitivity of NTR baroreceptors. Resetting of NTR baroreceptors during development may have important implications as a mechanism of blood pressure control in development.

Isolation of pigmented granules involved in extra-retinal photoreception in Aplysia californica neurons.

Many neurons in the ganglia of Aplysia californica contain pigmented, membrane-bound granules (lipochondria), which are thought to mediate the light response of some of the neurons, including the giant cell of the abdominal ganglion. A method of isolating the lipochondria by centrifugation of ganglia homogenates has now been developed. Electron microscopy was used to demonstrate that most of the lipochondria remain morphologically intact. As shown by X-ray microanalysis, isolated lipochondria contain the same elements, including calcium, as do lipochondria in intact giant cells. The calcium can be released into the medium by treatment of the organelles with the Ca2+ ionophore A23187. It appears that the lipochondria of Aplysia ganglia are similar in their morphology, elemental content and susceptibility to the ionophore. Two pigments were isolated from the lipochondria, and chromatography and spectrophotometric studies indicated that they are beta-carotene and a "retinol-like" compound.

Morphological changes associated with stretch in a mechano-receptor.

The S neurons of decapod crustacean coxal receptors respond to a constant stimulus with a steady-state response lasting for at least 20 min. This property makes it possible to fix neurons in the stretched or relaxed condition by rotating and pinning the coxa up or down respectively. In specimens perfused with 5 mM lanthanum chloride before fixation, it was found that the extracellular space around the nerve terminals, particularly the naked tubular endings, is open to ions. The cross-sections of the tubular processes (dendritic fingers) are about 25% smaller in stretched specimens, and the secondary dendritic branches connecting the fingers with the remainder of the dendrite appear shorter and thicker. The presence of mitochondria at the periphery of the main part of the dendrite and in the secondary branches, but not in the fingers, is interpreted as evidence for an electrical model similar to that proposed for vertebrate photoreceptors. The observed morphological effects of stretch are discussed in terms of this model.