Solute-free water retention in preascitic cirrhotic rats following intravenous water loading

Increased extracellular fluid volume (ECF) characterizes compensated cirrhosis. To identify the mechanisms of fluid retention in cirrhosis through clearance methods, 10 control and 10 preascitic rats with CCl(4)-induced cirrhosis were studied following i.v. loading with 1 ml 5% glucose solution. Glomerular filtration rate and renal plasma flow were evaluated through inulin and para-aminohippurate clearances; water and electrolyte handling was assessed measuring urine and plasma osmolarity, electrolyte excretions, and tubular solute-free water reabsorption (TFWR = osmolar clearance minus urinary output); ECF was assessed through hormonal status determination. After water loading, cirrhotic rats had increased ECF (lower plasma renin activity and aldosterone and higher atrial natriuretic peptide levels, all P<0.03), solute-free water retention (increased TFWR and decreased plasma osmolarity, all P<0.05), reduced absolute and fractional sodium excretions (P<0.05). Cirrhotic rats showed sodium retention in the medullary thick ascending limb of Henle's loop (i.e. increased values of TFWR for any given value of osmolar clearance). Trans-tubular potassium gradient in medullary collecting duct was similar in the two groups (P=0.55), ruling out aldosterone-dependent sodium retention and potassium hyper-secretion. In experimental preascitic cirrhosis NaCl retention in the ascending limb of Henle's loop increases medullary interstitial tonicity leading to vasopressin-independent water back-diffusion in thin descending limb of Henle's loop and collecting duct.