Mechanisms of osteoporosis development during prolonged restriction of motor activity in dog

The pattern in the mechanisms of osteoporosis development during prolonged motor activity restriction (hypokinesia) in animals has been studied. Twenty-four male dogs with initial body weights of 6.8 to 8.9 kg were divided into two equal groups: the 1st group were placed under ordinary vivarium conditions and served as control and the 2nd group were subjected to pure hypokinesia (HK) without any form of physical exercise and served as experimental animals. They were kept for 364 days in small individual wooden cages which restricted their movements without hindering food and water intake. Animals of each group were decapitated on the 120th, 240th and 360th day of the hypokinetic period and bones were x-rayed, histological specimens were examined, mineralization or organic bone was measured, microroentgenographic analysis was performed and calculation was made of calcium, phosphorus, potassium and sodium. By the 120th and 240th day of the hypokinetic period bone resorption increased significantly, while by the 360th day it decreased significantly. The mature bone microstructures manifested a higher degree of mineralization, whereas the young bone microstructures exhibited a lower degree of mineralization. In bone, calcium content decreased, that of potassium increased, while sodium and phosphorus content remained unchanged. It was concluded that the development of osteoporosis in osseous tissues during prolonged restriction of motor activity of animals is associated not only to quantitative changes, which consist of a reduction of bone mass, but qualitative changes as well.