Reactivity of arterial vessels during antiorthostasis and systemic hypotension

In experiments on rats, changes in blood pressure and cardiac output in response to antiorthostatic (head-down) tilt 15° and 45° were examined under conditions of decreased arterial tone induced by sodium nitroprusside. The decrease in initial systemic blood pressure to 90 mm Hg attenuated the pressor responses to antiorthostasis, while further drop in blood pressure to 70-60 mm Hg provoked depressor responses in half cases. No blood pressure responses to antiorthostasis were observed when blood pressure dropped to 50 mm Hg. Correlation analysis confirmed the existence of a direct dependence of pressor responses to orthostasis on the initial blood pressure.