Albumin homeostasis in the nephrotic rat: nutritional considerations

Albumin catabolism and the relationship between plasma albumin concentration and albuminuria were studied in male Sprague-Dawley rats with Heymann nephritis. The rats were placed on isocaloric diets of 8.5, 21, or 40% protein. Serum albumin concentration correlated negatively with urinary albumin excretion for each of these dietary groups, but the correlation was dependent on dietary protein intake. The magnitude of albuminuria reflected the increase in albumin synthesis rate plus the decrease in albumin catabolic rate. Maximal urinary albumin loss was dependent on dietary protein intake. Albumin catabolism was studied in the different groups of nephrotic animals. Albumin catabolism correlated inversely with the rate of albuminuria in the 21 and 40% protein-fed rats and contributed nearly half of the albumin that was lost in these groups of animals. Albumin catabolism was independent of albuminuria in the rats fed 8.5% protein. The rats were fed 18% of their normal caloric intake, and albumin catabolism was studied in nephrotic and control animals. Albumin catabolism increased with increased albuminuria, in contrast to the well-nourished group, and there was no relationship between serum albumin concentration and urinary albumin excretion. Increased catabolism of albumin plays little or no role in albumin homeostasis in the well-nourished nephrotic rat but may be significant in protein- and calorie-malnourished animals.