Sex-related differences in the handling of fluorescent ovalbumin by the proximal tubule of the rat kidney.

Sex-dependent protein handling in the rat renal tubular system was studied both qualitatively and quantitatively using the method of direct fluorescent protein tracing. The protein tracer, fluorescent ovalbumin, was synthesized by conjugating hen ovalbumin with fluorescein isothiocyanate (FITC), and the fluorescence characteristics of fluoresceinthiocarbamyl (FTC)-ovalbumin conjugates with different degrees of labelling were studied. Heavily labelled tracer was intravenously injected into male and female rats, and both kidneys were perfused; the right kidney was then homogenized and used for quantitative fluorometric measurements, while the left kidney was perfusion fixed and prepared for fluorescence microscopy. The tubular reabsorption of fluorescent ovalbumin was studied 4 min and 10 min after the injection of different doses (1.4, 7.0 and 14.0 mg/kg body weight) of the tracer, and the tubular catabolism was investigated in animals killed 60 and 120 min after the injection. Fluorescence microscopy demonstrated that, in both sexes and regardless of the dose administered and the time after injection, specifically fluorescent protein or its degradation products was only present in the epithelial cells of the proximal tubule. With regard to sex-dependent differences in protein handling, fluorometry indicated that at 4 min (7.0 mg) and at 10 min (all doses) after injection, female animals had reabsorbed more fluorescent protein than males. With regard to the catabolic phase, both the fluorescence microscopy and the fluorometric results showed that the female rats had degraded the fluorescent tracer at a significantly higher rate than males. The results are discussed in connection with sex-dependent proteinuria in rats.