3.20: Drug Elimination by Renal Route: Tubular Reabsorption
During the process of renal excretion, as the glomerular filtrate progresses to the distal convoluted tubule (DCT), drugs that are highly permeable, lipophilic, and nonionized undergo passive reabsorption from the tubular fluid into the surrounding peritubular capillaries. This reabsorption process restricts their elimination through the kidneys. However, the majority of drugs are either weak acids or weak bases, and their ionization level is dependent on pH. By altering the pH of urine, the reabsorption of these drugs can be hindered. When urine becomes alkaline, weak acidic drugs become predominantly ionized. Due to the decreased permeability of ionized forms through tubular cells, these drugs remain in the lumen and are ultimately expelled in the urine. This mechanism, known as 'ion trapping,' prevents reabsorption and enhances renal clearance of unwanted drugs. For example, in cases of phenobarbital overdose—a weak acid—treatment with bicarbonate is used to alkalinize the urine. This action ionizes the drug, impeding its back diffusion. Similarly, when dealing with an overdose of weakly basic drugs such as amphetamines, urine acidification is employed to ensure the ionized drug is not reabsorbed.
