Osteocyte Apoptosis Contributes to Cold Exposure-induced Bone Loss

Bone mineral metabolism and different indices of skeletal health of Ladakhi women living at high altitude

Objectives

High altitude possesses a great challenge for human survival owing to low oxygen tension and has been reported to cause bone deterioration among sojourns of high altitude. The bone health of Ladakhi women is investigated for the first time in this study.

Methods

Dual energy X-ray absorptiometry of Ladakhi women and sea level women was done at the radius and calcaneus using EXA-3000 (Osteosys, Korea), followed by colorimetric and Enzyme Linked Immunosorbent Assay analysis of parameters regulating bone health.

Results

There was no statistically significant difference between bone mineral density of Ladakhi women and sea level women at radius (P = 0.287) or calcaneus (P = 0.839). Almost similar cases of osteopenia were reported at both sites measured in the study among both groups. Two post-menopausal Ladakhi women however, had osteoporosis at the radius while 4 had osteoporosis at calcaneus. Significant increase in calcium levels with a decrease in intact parathyroid hormone and an increase in calcitonin levels were observed in Ladakhi women as compared to sea level women. Though there was no significant difference in 25-hydroxy vitamin D levels of both groups, a higher percentage of 25-hydroxy vitamin D deficiency (77% vs 23%) was observed in Ladakhi women as compared to sea level women. Estradiol levels were similar in both groups.

Conclusions

The present study suggest that there is no significant relationship between high altitude living and bone mineral density among Ladakhi women.

Temperature, heat shock proteins and growth regulation of the bone tissue

Ambient heat modulates the elongation of bones in mammals, and the mechanism of such a plasticity has not been studied completely. The influence of heat on growth and development of bone depends on its values. Five zones of temperature influence on the bone tissue with different biological effects have been distinguished : a) under-threshold thermal zone < 36.6 ºС, insufficient amount of heat is a limiting factor for osteogenesis; b) normal temperature zone 36.6‒37.5 ºС, the processes of breakdown and development of bone in this temperature range is balanced; b) zone of mild thermal shock 39‒41 ºС, the processes of functioning of osteoblasts, osteocytes and formation of the bone tissue intensify; d) the zone of sublethal thermal shock > 42 ºС, growth of bone slows; e) zone of non-critical shock > 50 ºС, bone tissue cells die. We propose a model of the mechanism of influence of heat shock on bone growth. Mild heat shock is a type of stress to which membrane enzymes adenylyl cyclase and cAMP-protein kinase react. Protein kinase A phosphorylates the gene factors of thermal shock proteins, stress proteins and enzymes of energy-generating processes – glycolysis and lipolysis. Heat shock protein HSP70 activates alkaline phosphatase and promotes the process of mineralization of the bone tissue. In the cells, there is intensification in syntheses of insulin-like growth factor-I, factors of mitogenic action, signals of intensification of blood circulation (NO) and synthesis of somatotropin. The affinity between insulin-like growth factor I and its acid-labile subunit decreases, leading to increased free and active insulin-like growth factor I. Against the background of acceleration of the capillarization process, energy generation and the level of stimulators of growth of bone tissue, mitotic and functional activities of producer cells of the bone – osteoblasts and osteocytes – activate. The generally known Allen’s rule has been developed and expanded: “Warm-blooded animals of different species have longer distal body parts (tails) if after birth the young have developed in the conditions of higher temperature”. The indicated tendency is realized through increased biosynthesis of heat shock proteins and other stimulators of growth processes in the bone tissue.