RESTRAINT VS HINDLIMB SUSPENSION ON FLUID AND ELECTROLYTE BALANCE IN RATS

To determine the effect of hindlimb suspension on body fluid volume, s alt and water balance, and relevant hormones, two series of experiment s were performed in an experimental protocol including periods of isol ation (7 days), horizontal attachment (7 days), and suspension (14 day s). 1) During the first experiment, water and electrolyte balance, arg inine vasopressin (AVP), and guanosine 3',5'-cyclic monophosphate (cGM P) 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 we re measured by a dilution technique (Evans blue and sodium thiocyanate ) in another 30 rats. We observed a pronounced and early effect of hor izontal 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 attache d 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 uri ne cGMP decreased significantly (45%; P < 0.05). Significant decreases in natriuresis, kaliuresis, urine creatinine, and osmotic load occurr ed in the suspension group 7 days after suspension. After the 14-day t ail 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. Plasm a immunoreactive plasma atrial natriuretic levels of suspended rats we re significantly reduced by 35% vs. isolated rats (P < 0.001) and by 1 8% vs. attached rats (P < 0.05). By using this experimental protocol, the physiological alterations revealed that suspension produced some a cute and long-term effects, but the fixation to the suspension device, restraint, and confinement have their own influence on fluid distribu tion and renal function.