Venous Tone after Cardiovascular Deconditioning in Intact and Autonomic Blocked Conscious Rats

Abstract

Prolonged periods of bed rest and spaceflight result in tachycardia and increased prevalence of orthostatic intolerance, effects that are mimicked by hindlimb unloading (HU) in rats. Regulation of venous function is important for compensation to orthostatic stress; however, the effects of HU on the venous system remain unclear. We hypothesized that 14 days of HU would result in a decrease in venous tone. Male SD (300–400 g) control (n=4) and HU (n=6) rats were instrumented with a femoral venous and arterial catheter and a balloon tipped catheter in the right jugular vein advanced to the right atrium. After recovery (2–5 days) conscious animals were subjected to 4 balloon inflations (0.2–0.3 ml saline for 5–7 seconds once every 10–15 minutes). Mean circulatory filling pressure (MCFP) was calculated from the plateau levels of mean arterial pressure (MAP) and central venous pressure during balloon occlusion. Under baseline conditions, heart rate was increased in HU rats (Con: 346 ± 6 vs HU: 405 ± 7 bpm), while MAP and MCFP were similar in control and HU rats (MAP: 111 ± 6 vs 119 ± 3 mmHg; MCFP: 7.6 ± 0.6 vs 7.5 ± 0.3 mmHg). Some of the rats were ganglionic blocked (hexamethonium, 30 mg/kg and atropine, 0.1 mg/kg) and MCFP was evaluated. MAP and MCFP were reduced in both groups after autonomic blockade. However, MCFP was still similar in control (6.2 ± 0.2 (n=2)) and HU (5.2 ± 0.6 (n=6)) rats. These preliminary data suggest that venous tone under control conditions and after autonomic blockade is unchanged after 14 days of hindlimb unloading. (HL55306)