Calvarial and limb bone cells in organ and monolayer culture do not show the same early responses to dynamic mechanical strain

High bone mass and altered relationships between bone mass, muscle strength, and body constitution in adolescent boys on a high level of physical activity

We have recently demonstrated a general relationship between bone mass, muscle strength, and several body constitutional parameters in adolescent boys with a moderate exercise level. The present study was conducted to evaluate these previously described relationships in adolescent boys subjected to high physical activity and also to compare the bone mass of the same group with that of adolescents on a moderate level of physical activity. The reference group consisted of 24 boys, age 15.9 +/- 0.3 years, not training for more than 3 h per week. The ice hockey players consisted of 20 boys, age 15.9 +/- 0.3 years, from an ice hockey junior team training for about 10 h per week. The groups were matched according to age, pubertal stage, and weight. Areal bone mineral density (BMD) was measured in total body, head, humerus, spine, femur, and proximal femur using dual energy X-ray absorptiometry. BMD was significantly higher in the humerus (p < 0.01), femur (p < 0.05), and proximal femur (p < 0.05) in the high activity group. Furthermore, physical activity (h/week) was an independent predictor of humerus BMD (p < 0.01) and proximal femur BMD (p < 0.05), among all subjects investigated (n = 44). Isokinetic muscle strength of the quadriceps and hamstrings muscles was significantly higher in the high activity group (p < 0.05). In the reference group, there was a general strong independent relationship between BMD, muscle strength, and different body constitutional parameters. In the high activity group, muscle strength of the thigh independently predicted BMD of humerus and spine. Furthermore, in the same group, weight, BMI, and fat mass independently predicted only spine BMD. In conclusion, the higher BMD demonstrated in the ice hockey players seems to be site-specific and may well be associated with the type and magnitude of loading from predominantly ice hockey. High physical activity seems to weaken the relationship between BMD, muscle strength, and body constitution demonstrated in adolescent boys on a low or moderate level of physical activity.

Microgravity induces prostaglandin E2 and interleukin-6 production in normal rat osteoblasts: role in bone demineralization

It has been suggested that microgravity alters bone metabolism. Evidence for this phenomenon includes the negative calcium balance and decreased bone density in astronauts, as well as, inhibition of bone formation in rats flown for 2 to 3 weeks. However, the specific mechanisms that modulate these changes in microgravity are unknown. The purpose of this study was to clarify the mechanism of microgravity-induced bone demineralization using normal rat osteoblasts obtained from femur marrow cultures. The osteoblasts were cultured for 5 days during a Shuttle-Spacelab flight (STS-65). After collection of the culture medium, the cellular DNA and RNA were fixed on board. Enzyme-immunoassay of the culture medium for prostaglandin E2 (PGE2) indicated that microgravity induced a 4.5- to 136-fold increase in flight samples as compared to the ground control cultures. This increase of PGE2 production was consistent with a 3.3- to 9.5-fold elevation of inducible prostaglandin G/H synthase-2 (PGHS-2) mRNA, quantitated by reverse transcription-polymerase chain reaction (RT-PCR). The mRNA induction for the constitutive isozyme PGHS-1 was less than that for PGHS-2. The interleukin-6 (IL-6) mRNA was also increased (6.4- to 9.3-fold) in microgravity as compared to the ground controls. Since PGE2 and IL-6 are both known to play a role in osteoclast formation and bone resorption, these data provide molecular mechanisms that contribute to our understanding of microgravity-induced alterations in the bone resorption process.

Mechanical loading and sex hormone interactions in organ cultures of rat ulna

The separate and combined effects of loading and 17 beta-estradiol (E2) or 5 alpha-dihydrotestosterone (DHT) on [3H]thymidine and [3H]proline incorporation were investigated in cultured ulna shafts from male and female rats. Ulnae were cultured and loaded to produce physiological strains in the presence or absence of 10(-8) M E2 or DHT. Loading engendered similar increases in incorporation of [3H]thymidine and [3H]proline in male and female bones. E2 engendered greater increases in incorporation in females than in males, and DHT greater increases in males than in females. In males E2 with loading produced increases in both [3H]thymidine and [3H]proline incorporation, which approximated to the arithmetic addition of the increases due to E2 and loading separately. In females E2 with loading produced increases greater than those in males, and substantially greater than the addition of the effects of E2 and loading separately. Loading with DHT in males also showed additional [3H]thymidine and [3H]proline incorporation. In females there was additional incorporation of [3H]proline, but not [3H]thymidine. The location of incorporation of [3H]thymidine and [3H] proline was consistent with their level of incorporation reflecting periosteal osteogenesis, in which case the early osteogenic effects of sex hormones are gender-specific when acting alone and in combination with loading. In males the effects of estrogen and testosterone add to, but do not enhance, the osteogenic responses to loading. In females testosterone with loading produces an additional effect on [3H]proline incorporation but no greater effect than loading alone on that of [3H]thymidine. In contrast, estrogen and loading together produce a greater effect than the sum of the two influences separately. Because premenopausal bone mass will have been achieved under the influence of loading and estrogen acting together, these findings suggest that the bone loss which follows estrogen withdrawal may result, at least in part, from reduction in the effectiveness of the loading-related stimulus on bone cell activity. This stimulus is normally responsible for maintaining bone mass and architecture.