No abstract

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.

No abstract

Abstract-The SA gene is expressed in the proximal tubule of the kidney and may be involved in blood pressure (BP) regulation. However, direct evidence for this is lacking. We constructed and analyzed an SA-null mouse in which exons 2 and 3 of the SA gene (including the start codon) had been deleted by homologous recombination. Basal BP and BP changes in response to increased salt and to treatment with losartan were compared between mice homozygous for the targeted SA allele (SA -/-mice) and littermates carrying the wild-type allele (SA ϩ/ϩ mice). Molecular and biochemical analysis confirmed the lack of SA gene product in SA -/-mice. SA -/-mice grew normally, were fertile, and had no overt phenotype. With both indirect and direct techniques, basal BP was similar in SA -/-and SA ϩ/ϩ mice. A high salt diet for 4 weeks caused a significant increase in BP in SA -/-and SA ϩ/ϩ , mice but there was no difference between the 2 strains. Losartan caused a significant decrease in BP, but again the response was similar between SA -/-and SA ϩ/ϩ mice, as were their kidney renin mRNA levels. SA is not involved in the regulation of either basal or salt related BP, and the lack of differential effect in SA -/-mice is not a consequence of compensatory activation of the renin-angiotensin system.

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.