Reactivity of the arterial system to orthostasis

: Reported are data of experiments with unconscious tilted rats (30 degrees, 45 degrees and 60 degrees) in which a dependence of orthostatic hypotensive reactions of systolic (APs) and diastolic (APd) pressure on spontaneous level of baseline mean arterial pressure (APb.mean) was established. Rats tilted at 30 degrees and 45 degrees, APs and APd were in inverse linear dependence with APb.mean above 95 mm Hg and in direct dependence with APb.mean below 95 mm Hg. Pressor effects of phenylephrine correlated directly and to the same extent with APb.mean only below 95 mm Hg in the 30 degrees and 45 degrees tilt and both above and below 95 mm Hg in the 60 degrees tilt. Mean values of the pressor effect before and after experiment were different in animals tilted at 60 degrees. No dependence of cardiac output on APb.mean was determined. Therefore, the trend and depth of changes in the arterial system reactivity to tilt are dependent on the degree of inclination and the APb.mean range. Reactivity to the adrenergic agent alters only when inclination is greater than 45 degrees.