Active tubular secretion is a robust, energy-demanding process that utilizes carrier systems to transport drugs into renal tubules. The active renal secretion systems include the organic anion transporter (OAT) for weak acids and the organic cation transporter (OCT) for weak bases. Structurally similar drugs can compete for the same transporter, potentially leading to drug accumulation and toxicity. However, this principle can be exploited therapeutically. One example is probenecid (Probalan), which inhibits active tubular secretion of organic acids, increasing their plasma concentration, and is beneficial in treating gout by suppressing uric acid reabsorption.
The rate of active tubular secretion is directly dependent on renal plasma flow (RPF). Drugs like p-aminohippuric acid (PAH) and iodopyracet (Diodrast) are frequently used to measure active tubular secretion. Both are filtered by glomeruli, secreted by tubular cells, and eliminated in a single pass. Their clearance reflects the effective renal plasma flow (ERPF), which ranges from 425 to 650 mL/min. ERPF is determined by RPF, and the fraction of the drug effectively extracted by the kidney is relative to the concentration in the renal artery.
Interestingly, the elimination half-life of drugs excreted solely by active secretion is minimally affected by drug-protein binding. For instance, although extensively protein-bound, penicillins have short half-lives due to rapid elimination by active secretion.
Blood exits glomeruli via efferent arterioles, reaching the capillary plexus surrounding the proximal tubule.
Here, unfiltered drugs are secreted into the tubular lumen through secondary active transporters such as the organic anion transporter, OAT, the organic cation transporter, OCT, and ATP-powered carriers such as multidrug resistance protein 4, MRP4, and P-glycoprotein. This constitutes active tubular secretion.
OAT transports deprotonated weak acids, while OCT carries protonated weak bases to the tubular lumen, ensuring efficient drug elimination.
The rate of active tubular secretion depends on renal plasma flow, RPF.
Drugs like p-amino-hippuric acid, and iodopyracet undergo rapid tubular secretion and are eliminated in a single pass. As a result, their clearance indicates effective renal plasma flow, ERPF.
ERPF depends on renal plasma flow, RPF, and the drug fraction extracted effectively by kidneys.
The elimination half-life of drugs undergoing tubular secretion is least affected by reversible drug-protein binding. For example, penicillin has a short half-life despite being highly protein-bound.
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