Selective availability of protein bound estrogen and estrogen conjugates to the rat kidney

Estrogen glucuronides are selectively cleared by the kidney as compared to estrogen sulfates. The selective trafficking of circulating serum estrogen conjugates to kidney and urine may arise from differential transport properties of the various estrogen conjugates in the renal microcirculation. In the present study, the effects of glomerular and peritubular permeability barriers, and plasma protein binding on the influx of unconjugated and conjugated estrogens into rat kidney were studied. Experiments were carried out utilizing an in vivo double isotope, single injection, timed tissue sampling technique. The extractions of these steroids by the renal cortex were examined utilizing inulin as the reference substance, as it is freely permeable through the glomerular and tubular capillary permeability barriers, but not extracted by tubular epithelial cells. The method was validated by studying the extraction of para-amino hippuric acid (PAH) before and after probenecid treatment. In the absence of plasma proteins, all the estrogens and estrogen conjugates readily diffused through both the glomerular and peritubular capillary permeability barriers and were extracted by tubular epithelia. The addition of 4% albumin to the injection solution led to significant inhibitions of extraction of estradiol (E2) and estrone sulphate (E1-S) only. The extraction of E1-S was reduced to a value less than that of inulin; the extraction of E2 was less than that of control value but significantly more than that of inulin. The addition to the injection solution of human pregnancy sera containing sex hormone binding globulin and albumin was associated with a reduction in the extraction of all estrogens and estrogen conjugates except estriol and E1-S to values approximating that of inulin.(ABSTRACT TRUNCATED AT 250 WORDS)