Inverse-orthostasis may induce elevation of blood pressure due to sympathetic activation.
Microgravity and simulated microgravity may cause cardiovascular deconditioning, but mechanisms of instantaneous responses to inverse-orthostasis are not studied. Hence, we investigated transient and steady state cardiovascular changes by combining the tilt technique with cardiovascular telemetry. Normotensive and NO-deprived hypertensive Wistar rats were used to analyze responses of mean arterial blood pressure, heart rate, contractility, spontaneous baroreflex sensitivity (sBRS), and autonomic balance. Inverse-orthostasis tests were carried out by 45 degrees head-down tilting (repeated 3 x 5 mins "R", or sustained for 120 mins "S"). In normotensive rats, horizontal control blood pressure was R111.3 +/- 1.7/S110.4 +/- 2.3 mm Hg and heart rate was R385.2 +/- 5.9/S371.1 +/- 6.1 BPM. Head-down tilt induced an increase in blood pressure by R5.9/S10.6 mm Hg, while heart rate, contractility, sBRS, and autonomic balance did not change. The hypertensive response was sustained, could be prevented by prazosin (10 mg/kgbw), and augmented by subanesthetic doses of chloralose (26 and 43 mg/kgbw). In NO-suppressed hypertension, control blood pressure and heart rate were R132.4 +/- 2.9/S130.0 +/- 4.1 mm Hg and R339.2 +/- 7.9/S307.2 +/- 23.6 BPM, respectively. Head-down tilt further increased blood pressure by R5.1/S10.5 mm Hg. These data demonstrate that conscious rats respond to inverse-orthostasis by sustained elevation of blood pressure independent of NO synthesis. This response is neither due to increased contractility and altered sBRS, nor due to non-specific stress, but probably due to sympathetic activation elicited by gravity-related reflexes, which increase peripheral resistance.
Raffai
Gábor
G
Institute of Human Physiology and Clinical Experimental Research, Semmelweis University, Faculty of Medicine, Budapest, Hungary. raffai@elet2.sote.hu
Kocsis
László
L
Mészáros
Márta
M
Monos
Emil
E
Dézsi
László
L
D018686:Anesthetics, Intravenous / Q000494:pharmacology; D000818:Animals; D000959:Antihypertensive Agents / Q000494:pharmacology / Q000627:therapeutic use; D001794:Blood Pressure / Q000187:drug effects / Q000502:physiology*; D002698:Chloralose / Q000494:pharmacology; D018475:Head-Down Tilt; D006973:Hypertension / Q000209:etiology / Q000503:physiopathology / Q000517:prevention & control*; D007024:Hypotension, Orthostatic / Q000503:physiopathology*; D008297:Male; D011224:Prazosin / Q000494:pharmacology / Q000627:therapeutic use; D051381:Rats; D017208:Rats, Wistar; D013564:Sympathetic Nervous System / Q000502:physiology*; D013686:Telemetry; D018667:Tilt-Table Test
D018686:Anesthetics, Intravenous; D000959:Antihypertensive Agents; D002698:Chloralose; D011224:Prazosin