Femur-bending properties as influenced by gravity: V. Strength vs. calcium and gravity in rats exposed for 2 weeks.

Growing bone material strength (S) can increase with gravitational intensity (g). That would be consistent with demineralization reported during space flight and reports that strength increases with mineral content. This study, however, shows an increase in material strength independent of calcium content (C). Male, Sprague-Dawley rats were exposed to chronic simulations of altered gravity from the 28th to 42nd d of age. Zero G was stimulated for 13 animals by harness suspension and 3 G for 30 animals by centrifugation. For fresh femurs, S as determined by bending and C as determined by AA spectrometry were compared with results for 11 harnessed, control animals and 13 normal, control animals. Multiple regression shows significant dependence of S (10(6) N.m-2) upon g (multiples of Earth's gravity, G) as independent from C (% by mass) for which there is no significant coefficient of partial regression: S = (62 +/- 1) + (7 +/- 1 g) + (0 +/- C).

Knee-ligament loading properties as influenced by gravity: I. Junction with bone of 3-G rodents.

After chronic 3-G centrifugation of rats, their bone-to-ligament junctions exhibited 95 +/- 12% of the control junctions' force-sustaining capacity (F). F was actually 29 +/- 5% greater for centrifuged rats than for control rats of comparable size, as experimental animals grew to smaller body mass. This suggests that gravity determines part of F's magnitude. These junctions are, therefore, hypothesized to be weaker after development in a weightless environment. The effect was less measurable for mice. F was measured in situ as load needed to separate the knee's medial collateral ligament from the tibia of 34 male rats (Sprague-Dawley, 27-320 d of age, exposed 4-65 d), 30 control rats, 22 male mice (Swiss Webster, 35-166 d, exposed 9-56 d), and 15 control mice.

Femur-bending properties as influenced by gravity: IV. Limits after high and low weight-bearing.

Gravity enhances femur growth as measured in terms of strength sigma u, but shows little or even a growth-retarding effecting in terms of "relative brittleness," defined as the inverse 1/epsilon u of ultimate or tolerable strain. Chronic weightlessness was simulated by harness suspension or by extrapolation of results from 3-G centrifugation. Experimental results from 45 male, white rats (34-520 d old) were compared to 72 control or baseline rats (28-520 d old) white correction for age and size differences. After suspension, the youngest rats showed subnormal epsilon u. Combined results, however, although predicting 19 +/- 1% below normal sigma u, after a week of weightlessness, predicted less effect (1 +/- 4%) for epsilon u.

Femur-bending properties as influenced by gravity: III. Sex-related weakness after 4-G mouse growth.

Optimum gravitational intensity for growth stimulation depends upon the type of growth measurement. Although the 4-G field resulted in relatively weaker male bones, relative bone size increased. The more moderate 3-G field is known to stimulate relative bone-strength as well. Femurs from 36 male white mice demonstrated no growth of load-supporting ability Fu after 1 to 8 weeks of chronic centrifugation at 4-G from the fifth week of age. Although measurable with 35 female mice, this growth intended to fall below the control rate. When compared to 82 younger control femurs of the same cross-sectional geometry, experimental female femurs could sustain comparable moments Mu while experimental male femurs could not.

Femur-bending properties as influenced by gravity: II. Ultimate load, moment, and stress for 3-G mice.

Material strength as approximated from bending studies of fresh femurs grew during 1 to 8 weeks of hypergravity at rates which were not measurably affected, even though the animal's body growth was measurably slower. As a result, femurs could support greater ultimate loads, moments, and stresses when compared to control bones of comparable rotational moments of their cross-sectional areas. Chronic centrifugation simulated 3.1 G for 45 male. Swiss Webster mice compared to 37 control animals. Effects were most noticeable after the first week of exposure and for younger animals. Effect were only about 50% that noted in comparable treatment of rats. This suggests that four times more mice than rats may be necessary for space-experiments designed to test the effects of weightlessness.

Femur-bending properties as influenced by gravity: I. Ultimate load and moment for 3-G rats.

Fresh experimental bones can withstand greater bending forces and moments after 1.0 to 2.5 weeks of 3-G exposure. This appears more attributable to a 50% greater strength of bone material than to effects upon size or shape, and is most measurable for animals of 5 to 8 weeks of age. Experimental bone material seems to grow to its mature level at a younger age rather than there being so marked an effect upon the mature level itself. We simulated 3.1 G by chronic centrifugation of 66 albino rats and compared them to 63 1-G controls. Extrapolation of the simplest mathematical description of the present results to weaker, zero-G bones could be tested by a total of 60 space-based control and experimental animals. A flight of only 15 animals would be necessary for comparison to ground-based control animals. This is consistent with reports of bone demineralization during space-flight. In light of the differences in bone histology, however, extrapolation of these results to humans would be premature and, if at all applicable, are most likely to be so for children rather than adults.

Food and oxygen requirements for growing mice and turtles after hypergravitational development.

Although development of mice and turtles, both low-payload animals, can be influenced by ambient gravitational intensity, little is known regarding the influence of either their growth or their gravitational history upon life-support requirements. A 25-g male Box Turtle (Terrapene carolina, optimally growing at 30 degrees C and 1 G to double its size in 7 weeks) exhibits daily requirements of oxygen (100 ml) and food (200 mg, dry) which are only 3% of the requirement for a mouse (Swiss Webster, centrifuged at 22 degrees) of comparable size and growth rate. These requirements grow in proportion to the turtle's (body mass)0.9 and to the mouse's (body mass)0.5. After development at hypergravity g (of from 1.5 to 5 G expressed in multiples of the earth's gravity) there is a one-day delay for turtles but an immediate change for mice in these requirements upon return to 1 G. Although both show a decreased oxygen intake, the drop is greater for turtles (equalling g -0.4 of the baseline value, as corrected for body size, after return from 75 days exposure against g-0.08 for mice after 1-13 days exposure); the food intake drops for turtles (varies as g-1) while it rises for mice (varies as g0.6).