Learning Objective

To understand how drug ionization affects renal clearance, including the mechanisms of filtration, reabsorption, and “ion trapping,” and to apply this knowledge clinically to modify urinary pH for enhancing drug elimination.

Ionization Increases Renal Clearance of Drugs

Filtration:

Only free, unbound drugs are filtered at the glomerulus. Both ionized and nonionized forms are filtered.

Reabsorption:

Only the nonionized form can undergo active secretion or active/passive reabsorption from the renal tubule back into circulation.

Ion trapping:

Ionized drugs cannot easily cross tubular membranes, so they remain in the filtrate and are excreted more readily.

Activity

Manipulating Urine pH to Enhance Drug Elimination

Acidify urine

Increases ionization of weak bases → enhances renal elimination.

• Examples: NH₄Cl, vitamin C, cranberry juice

Alkalinize urine

Increases ionization of weak acids → enhances renal elimination.

• Examples: NaHCO₃, acetazolamide

Clinical Example

NOTE on Active Transport

• Primary active transport: Direct energy requirement (e.g., Na⁺/K⁺-ATPase).
• Secondary active transport: Uses energy stored in ionic gradients (usually Na⁺).
  • Antiporters: Exchange one ion for another (e.g., Na⁺/Ca²⁺ exchanger).
  • Symporters: Cotransport ions with another molecule (e.g., Na⁺/glucose).
• Neuronal reuptake of neurotransmitters (dopamine, norepinephrine, serotonin) occurs via symporters.

Activity