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, l…
During renal excretion, as the glomerular filtrate reaches the DCT, highly permeable lipophilic and nonionized drugs are passively reabsorbed from the tubular fluid into the peritubular capillaries. This tubular reabsorption limits their renal excretion.
Most drugs, however, are weak acids or weak bases, and the extent of their ionization is pH dependent. By manipulating the pH of the urine, reabsorption of such drugs can be prevented.
When the urine is made alkaline, weakly acidic drugs are largely ionized. As the ionized forms are less permeable through the tubular cells, they are retained in the lumen and eventually excreted in the urine. This process of 'ion trapping' prevents reabsorption and increases renal clearance of undesirable drugs.
For instance, an overdose of phenobarbital —a weak acid—is treated with bicarbonate which makes the urine alkaline. This ionizes the drug and prevents its back diffusion.
Similarly, an overdose of weakly basic amphetamines, is treated by acidifying the urine such that the ionized drug does not get reabsorbed.
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Q1: What happens to lipophilic drugs during renal excretion in the distal convoluted tubule?
Highly permeable lipophilic and nonionized drugs undergo passive reabsorption from the tubular fluid into peritubular capillaries as the glomerular filtrate reaches the distal convoluted tubule. This reabsorption process significantly limits their renal excretion and reduces their elimination from the body through the kidneys.
Q2: How does urine pH affect the ionization of weak acid and weak base drugs?
The ionization of weak acids and weak bases is pH-dependent. When urine becomes alkaline, weakly acidic drugs become predominantly ionized. Conversely, acidifying urine ionizes weakly basic drugs. Since ionized forms are less permeable through tubular cells, they remain in the lumen and are excreted rather than reabsorbed.
Q3: What is ion trapping and how does it prevent drug reabsorption?
Ion trapping is a mechanism where manipulating urine pH ionizes drugs, preventing their reabsorption. Ionized drug forms have reduced permeability through tubular cells, so they remain trapped in the tubular lumen and are eventually excreted in urine. This process increases renal clearance of undesirable drugs by blocking their back diffusion into peritubular capillaries.
Q4: Why is bicarbonate used to treat phenobarbital overdose?
Phenobarbital is a weak acid drug that undergoes reabsorption in the renal tubule. Bicarbonate alkalinizes the urine, ionizing the phenobarbital and preventing its back diffusion into the bloodstream. This ion trapping mechanism enhances renal elimination of the drug, reducing its toxicity and accelerating its clearance from the body.
Q5: How is amphetamine overdose managed through urine pH manipulation?
Amphetamines are weakly basic drugs that are reabsorbed when in nonionized form. Acidifying the urine converts amphetamines to their ionized form, which cannot permeate tubular cells. The ionized drug remains in the lumen and is excreted, preventing reabsorption and increasing renal clearance to counteract overdose toxicity.
Q6: Why are most drugs susceptible to pH-dependent reabsorption in the kidney?
Most drugs are weak acids or weak bases whose ionization depends on urine pH. Unlike highly lipophilic nonionized drugs that are readily reabsorbed, ionized forms have low membrane permeability. By controlling urine pH, clinicians can manipulate drug ionization to either promote reabsorption or prevent it through ion trapping, depending on therapeutic goals.
Q7: What is the relationship between drug permeability and tubular reabsorption?
Drug permeability through tubular cells directly determines reabsorption rates. Highly permeable lipophilic and nonionized drugs are readily reabsorbed into peritubular capillaries, limiting renal excretion. Ionized drugs have low permeability and remain in the tubular lumen. This permeability difference is exploited clinically by altering urine pH to control drug elimination.