Albumin absorption and catabolism by isolated perfused proximal convoluted tubules of the rabbit

Abstract

Overall characteristics and kinetics of tubular absorption of albumin (Alb) were studied in isolated perfused proximal convoluted tubules of the rabbit. The fate of absorbed Alb was determined in tubules perfused with low [Alb]. Alb was labeled with tritium by reductive methylation ([3H3C]Alb). At [Alb] = 0.03 mg/ml, ~80% of the absorbed [3H3C]Alb was released to the peritubular bathing solution as catabolic products. Transcellular transport of intact [3H3C]Alb was negligible. Iodoacetate (IAA, 4 mM) inhibited albumin absorption (J(Alb)) by >95% and fluid reabsorption (J(v)) by 55%. At [Alb] = 0.1 mg/ml the absorption rate of a derivatized cationic Alb (pI = 8.4) was 5-fold greater (P < 0.01) than that of anionic Alb. Higher cationic [Alb] had deleterious effects on tubular functions. Overall Alb absorption was of high capacity and low affinity (J(Alb)(max) = 3.7 ng/min per mm tubule length, apparent Michaelis constant (K(m)) = 1.2 mg/ml). A low capacity system that saturates at near physiological loads was also detected (J(Alb)(max) = 0.064 ng/min per mm, apparent K(m) = 0.031 mg/ml). High [Alb] did not alter the rate of endocytic vesicle formation as determined by the tubular uptake of [14C]inulin. Results show that Alb absorption is a saturable process that is inhibited by high IAA concentrations and is affected by the charge of the protein. Absorbed Alb is hydrolyzed by tubular cells and catabolic products are readily released to the peritubular side. The dual kinetics of Alb absorption may be due to a combination of adsorptive endocytosis (low capacity system) and fluid endocytosis of albumin aggregates (high capacity system). Results indicate that albuminuria occurs much before albumin absorption is saturated. The kinetic characteristics of the process of tubular absorption of albumin helps to explain the concomitance of albuminuria, increased renal catabolic rates of albumin, and renal cell deposition of protein absorption droplets in severe glomerular proteinurias.