Many molecules foreign to the body—known generally as xeno-biotics and including toxins and drugs—are eliminated in the urine more rapidly than would be possible by just glomerular filtration. This implies that they are secreted by membrane carriers that somehow recognize them as foreign to the body. Considering that membrane carriers are specific and that there are so many possible xenobiotic molecules, how is this accomplished?
Scientists have discovered that there are a large number of different carriers, usually in the basolateral membrane of the proximal tubule, that are organic anion transporters. These carriers are each specific for a broad range of molecules; they are described as being polyspecific. The specificity of one type
Many antibiotics are secreted by the renal tubules and thus are rapidly cleared from the body. Penicillin, for ^ example, is secreted into the tubular filtrate and, because of this, large amounts of the drug have to be administered. When penicillin was first used during World War II, however, it was in short supply. Scientists then discovered that a different organic anion (benzoic acid) would compete with penicillin for the carrier proteins and thus prevent penicillin from being too rapidly eliminated from the body. A newer competitor for these carriers, probenecid, is sometimes used to inhibit the tubular secretion of certain antibiotics. This improves the effectiveness of the antibiotic and reduces its potential toxicity to the kidneys (nephrotoxicity).
of carrier overlaps with the specificity of other carriers, so that they can transport a wide variety of exogenous ("originating outside") and endogenous ("originating inside") molecules across the nephron tubules. This allows the kidneys to rapidly eliminate potentially toxic molecules from the blood. However, tubular secretion of therapeutic drugs can interfere with the ability of those drugs to work.
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