Experimental Orthostasis Elicits Sustained Hypertension, Which Can Be Prevented by Sympathetic Blockade in the Rat

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Abstract
Incidence of orthostatic hypertension is estimated at 5% but is even more prevalent in borderline hypertension and autonomic neuropathies. The aim of this study was to develop a potential model to investigate orthostatic hypertension. We used normotensive and hypertensive Wistar rats to analyze responses and diurnal variations of arterial blood pressure, heart rate, temperature, and locomotor activity by telemetry. Orthostatic tests were carried out during 45 degrees head-up tilt (R, repeated 3 times for 5 minutes; or S, sustained for 120 minutes). Hypertension was induced by blockade of nitric oxide synthesis. In normotensives, horizontal control blood pressure was R115.4 +/- 1.4/S113.7 +/- 1.6 mm Hg and heart rate R386.4 +/- 7.0/S377.9 +/- 8.8 bpm. Head-up tilt increased blood pressure by R4.5/S8.4 mm Hg, including a 3.8 mm Hg hydrostatic component. The sustained hypertensive response was prevented by prazosin (10 mg/kgbw) and augmented by a subanesthetic dose of chloralose (26 mg/kgbw). In NO-deprived hypertension, horizontal control blood pressure and heart rate were R138.4 +/- 2.6/S140.3 +/- 2.7 mm Hg and R342.1 +/- 12.0/S346.0 +/- 8.3 bpm, respectively. Tilt increased blood pressure further by R4.2/S9.4 mm Hg. In both normo- and hypertensives, variables exhibited similar diurnal rhythms except for nighttime locomotor activity, reduced from 3.7 +/- 0.4 to 2.8 +/- 0.3 counts/s. These data demonstrate that conscious rats respond to sustained orthostasis with hypertension, probably as a result of increased sympathetic output. Decreasing stress using a subanesthetic dose of chloralose increased this response, reducing the inhibitory effect on hypertensive responses.