To verify whether a long-term weightlessness simulation was associated with development of cardiovascular deconditioning, male Wistar rats were tail suspended for 13 days and then removed for a 24-h recovery. Blood pressure (BP) and heart rate (HR) responses, their spectral properties, and the pharmacologically tested baroreceptor reflex sensitivity were studied throughout the suspension period and after removal from the tail suspension device. BP, HR, and their variability were not altered over the experimental period, and there were no indications of orthostatic intolerance on release from head-down suspension. Spectral properties of BP and HR were unchanged during the experiment, and tail suspension did not induce modifications in the baroreceptor reflex sensitivity. These results taken together suggest that cardiovascular deconditioning may not be developed even after long-term hindlimb suspension in rats, in contrast to humans exposed to actual or simulated weightlessness. Our results raise issue with the use of tail-suspended rats as a valid model for the study of alterations in cardiovascular function induced by spaceflight in humans.

Exposure to microgravity in humans causes cardiovascular deconditioning affecting blood pressure, heart rate and vascular responsiveness. This study investigated cardiac output, arterial blood pressure and regional blood flows [radioactive microspheres: 57Co, 15.5 (SEM 0.1) microm in diameter] in conscious and freely moving rats subjected to 14 days of simulated microgravity (head-down suspension, HDS) in male Wistar rats: control (horizontally attached, n = 7), suspended for 14 days (n = 8) and suspended/allowed to recover for 10 min (R10min, n = 5) or 24 h (n = 9). Compared to the control group, 14 days of HDS resulted in reduced total peripheral resistance (37%); an increased cardiac index (65%) was associated with no significant change in the mean arterial pressure BPa. There were elevated brain (63%), visceral (> 20%), hindlimb (> 80%) and forelimb (> 215%) muscle blood flows. In the R10min group, the BPa decreased (18%) and the regional blood flows returned to control values. Within 24 h the BPa as well as cardiac index and total peripheral resistance were restored. In conclusion, 14 days of HDS engendered local circulatory changes resulting in transient blood pressure instability during recovery.

To determine the effect of hindlimb suspension on body fluid volume, salt and water balance, and relevant hormones, two series of experiments were performed in an experimental protocol including periods of isolation (7 days), horizontal attachment (7 days), and suspension (14 days). 1) During the first experiment, water and electrolyte balance, arginine vasopressin (AVP), and guanosine 3',5'- cyclic monophosphate (cGMP) were determined in urine, atrial natriuretic peptide in plasma and atria, and renin concentration and AVP in plasma in 30 rats. 2) During the second experiment, blood volume and extracellular fluid volume were measured by a dilution technique (Evans blue and sodium thiocyanate) in another 30 rats. We observed a pronounced and early effect of horizontal attachment on the renal variables. After 48 h, diuresis (49%), natriuresis (44%), kaliuresis (36%), osmotic load (39%), creatinine (28%), and AVP excretion (155%) were significantly increased in attached rats (P < 0.05). There was no short-term (24-h) effect of suspension on urine flow and Na+, K+, creatinine, and AVP excretion, but the urine cGMP decreased significantly (45%; P < 0.05). Significant decreases in natriuresis, kaliuresis, urine creatinine, and osmotic load occurred in the suspension group 7 days after suspension. After the 14-day tail suspension, plasma volume and extracellular fluid volume measured in suspended rats were not different from isolated rat values, whereas plasma volume increased by 15% (P < 0.05) in the attached rats. Plasma immunoreactive plasma atrial natriuretic levels of suspended rats were significantly reduced by 35% vs. isolated rats (P < 0.001) and by 18% vs. attached rats (P < 0.05). By using this experimental protocol, the physiological alterations revealed that suspension produced some acute and long-term effects, but the fixation to the suspension device, restraint, and confinement have their own influence on fluid distribution and renal function.

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