Effects of protein meals on the urinary excretion of various plasma proteins in healthy subjects

To examine whether hemodynamic changes in response to acute protein loading s with different protein sources cause increases in urinary excretion of pl asma proteins in healthy subjects, urinary excretions of various plasma pro teins with various molecular radii and isoelectric points, namely albumin ( Alb), IgG, lgG4, ceruloplasmin (CRL), and alpha(2)-macroglobulin (A2), were measured in healthy subjects after ingestion of a beef meal or of a tuna f ish meal. Significant increases in urinary excretions of the negatively cha rged lgG4 and CRL and of the neutrally charged IgG were found in parallel w ith enhanced creatinine clearances after each protein ingestion. These rena l responses returned to basal levels 9 h after the test. This finding sugge sts that in healthy subjects, the increase in glomerular filtration rate af ter acute protein loading caused selective enhancement of urinary excretion s of plasma proteins with a molecular radius of approximately 55 Angstrom ( the radius of IgG, lgG4, and CRL), irrespective of the charge barrier of th e glomerulus. The increases in these three plasma proteins may be induced b y leakage via the shunt pathway in the glomerulus, as proposed earlier (see text). In contrast, increases in urinary excretions of A2 and Alb were not found. The former finding may be explained by the possibility that A2 woul d not pass through this pathway, since the molecular radius of A2 (88 Angst rom) is larger than that of IgG, although the latter finding may be partial ly explained by preferential renal tubular reabsorption of Alb.