In vivo metabolism of tritiated LHRH by the whole kidney and individual tubules of rats

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Abstract
[3H]LHRH and [14C]inulin were infused into individual nephrons in Inactin-anesthetized rats and the amount of radioactive label and the identity of the radioactively labeled material in urine were determined. The site of infusion was identified by latex injection and microdissection. [3H]LHRH was microinfused at 1.5 .times. 10-5 M (concentration 106-107 higher than in plasma) and analysis of urinary metabolites was performed by high-performance liquid chromatography. The urinary recovery of tritium label was 81% when proximal tubules were infused and 94% when distal tubules were infused. For proximal tubules 90% of the label recovered in urine appeared as pGlu-His (metabolite 2), pGlu-His-Trp (metabolite 3), and pGlu-His-Trp-Ser (metabolite 4), and 10% as LHRH. With distal tubules only LHRH was detected in the urine. [3H]LHRH was presented to the renal artery of the filtering rat kidney in vivo, and urine and renal venous blood were analyzed for breakdown products. The urine contained metabolites 2, 3 and 4 and no LHRH, whereas venous blood contained mainly pGlu, metabolite 4 and LHRH. When [3H]LHRH was perfused in vivo through the nonfiltering rat kidney or rat lower limb, renal or femoral venous blood contained only LHRH. [3H]LHRH apparently undergoes glomerular filtration and contact digestion by brush border enzymes of the proximal tubule to produce metabolites 2, 3 and 4. These metabolites and possibly LHRH are partially reabsorbed and undergo further intracellular degradation to produce pGlu. Endothelial and interstitial cells in the kidney and leg do not appreciably metabolize [3H]LHRH.