It is well known that estrogen deficiency induces a deterioration of bone strength in aged females. The aim of this study is to determine the effect of estrogen depletion on tibia bone strength in sexually mature mice that are still undergoing skeletal maturation. At 8 weeks of age, C57BL/6 female mice underwent an ovariectomy (OVX) or sham (SHAM) surgery. Mice were killed at 2, 4, or 8 weeks post-surgery. Tibia length and cross-sectional area continued to increase in both treatment groups until 4 weeks post-surgery. Compared to SHAM mice, OVX mice demonstrated a significant reduction in uterine weight and plasma estrogen levels. Three-point bending was used to quantify the mechanical properties (breaking point, stress, stiffness, and elasticity) of the tibia. The tibias from the SHAM mice had a higher breaking point than all the age-matched OVX mice. At 8 weeks post-surgery, the tibias from the SHAM mice demonstrated higher elasticity, stress, and stiffness than the younger SHAM mice and the age-matched OVX mice. Compared to the SHAM mice, our study suggests that (1) there is a reduction in the mechanical strength of tibias from young OVX mice, and (2) the greatest decline in tibia strength of the OVX mice was once they reached skeletal maturity.

The aim of this study was to determine the changes in elasticity and lattice structure in leg bone of rats which were: 1) under Hind-Limb Suspension (HLS) by tail for 2 weeks and 2) exposed to a total radiation of 10 Grays in 10 days. The animals were sacrificed at the end of 2 weeks and the leg bones were surgically removed, cleaned and fixed with a buffered solution. The mechanical strength of the bone (elastic modulus) was determined from measurement of bending of a bone when under an applied force. Two methodologies were used: i) a 3-point bending technique and ii) classical bending where bending is accomplished keeping one end fixed. Three point bending method used a captive actuator controlled by a programmable IDEA drive. This allowed incremental steps of 0.047mm for which the force is measured. The data is used to calculate the stress and the strain. In the second method a mirror attached to the free end of the bone allowed a reflected laser beam spot to be tracked. This provided the displacement measurement as stress levels changed. Analysis of stress vs. strain graph together with solution of Euler-Bernoulli equation for a cantilever beam allowed determination of the elastic modulus of the leg bone for (i) control samples, (ii) HLS samples and (iii) HLS samples with radiation effects. To ascertain changes in the bone lattice structure, the bones were cross-sectioned and imaged with a 20 keV beam of electrons in a Scanning Electron Microscope (SEM). A backscattered detector and a secondary electron detector in the SEM provided the images from well-defined parts of the leg bones. Elemental compositions in combination with mechanical properties (elastic modulus and lattice structure) changes indicated weakening of the bones under space-like conditions of microgravity and radiation. †Supported by a grant from the Arkansas Space Grant Consortium (ASGC)

The presence of estrogen in premenopausal women protects against the development of vascular dysfunctions, such as hypertension. Previous studies from our lab demonstrated a slight (p=0.12) increase in voltage‐gated, L‐type Ca2+ channel (CaL) expression in mesenteric arteries from ovariectomized (OVX) mice. Therefore, in this study we hypothesized that the in vivo loss of estrogen will increase Ca2+ influx in mesenteric arterial smooth muscle cells. Mice (C57BL/6) underwent an ovariectomy or sham surgery at 8 weeks of age. At 12 weeks, the mice were sacrificed, mesenteric arteries dissected, and smooth muscle cells isolated. Lack of gonadal estrogen production was verified by a reduction in uterine weights. No significant difference (p=0.496) in weight gain between mice groups was observed. Ratiometric fluorescent imagining was conducted on isolated cells loaded with the Ca2+ fluorescent dye, fura‐2AM. Cells isolated from the mesenteric arteries of OVX mice (n=23) demonstrated an increased (p<0.001) intracellular Ca2+response to the CaL agonist, FPL64176 (1×10−6), compared to the sham mice (n=15). We plan to conduct whole cell Ca2+ currents on these cells in the future. These results suggest that the loss of in vivo estrogen leads to the enhanced influx of Ca2+into arterial cells which may lead to a chronic elevation of vascular tone in postmenopausal women. Support: NIGMM of the NIH, Grant #P20 GM103429–11