[Orthostatic circulatory instability: role of deconditioning of resistance vessels].

One of the effects of weightlessness on the circulatory system, i.e. reduction of the tension distending resistance vessels due to the loss of hydrostatic pressure, was simulated. For this purpose the abdominal aorta of rats was constricted by a wire spiral. As a result, the arterial pressure in the posterior body decreased by 30-50%. Beginning with postoperation day 7 the hydraulic resistance of resistance vessels of hindlimbs was decreased progressively and the myogenic regulation of their lumen was inhibited. The response to the stimulation of vasoconstrictive fibers of the sciatic nerve that was reduced 7-14 days after operation returned to normal 90 days later. An examination of the tail arteries demonstrated that the ability of their muscular layer to withstand the distending effect of the physiologically normal pressure was impaired. The magnitude of the constrictor reactions to norepinephrine (particularly to its moderate concentrations) was lowered. These changes may play an important role in the orthostatic disorders of circulation that occur after exposure to weightlessness.