General Pharmacology MCQ Answer 001

• The glomerular filtrate contains similar concentrations of low molecular weight solutes as does plasma.
  
• In contrast, molecules with a molecular weight of 66 000 or above (which includes plasma proteins and drug-protein complexes) do not pass the glomerulus.
• The organic acid transport mechanism excretes hippuric acid, endogenous phenols, sulfates and glucuronides as well as acidic drugs such as probenecid, penicillin and some sulfonamides.
• The organic base transport mechanism contributes to the elimination of basic drugs (e.g. quinidine, cimetidine) from the body.
• Because secretion of free drug occurs up a concentration gradient from peritubular fluid into the lumen, the equilibrium between unbound and bound drug in plasma can be disturbed, with bound drug dissociating from protein binding sites.
• Unlike glomerular filtration, tubular secretion can therefore eliminate drugs efficiently even if they are highly protein bound.
  

• Reabsorption of drugs that are weak acids (AH) or bases (B) depends upon the pH of the tubular fluid, because this determines the fraction of acid or base in the charged, polar form and the fraction in the uncharged non-polar lipid-soluble form.
• For acidic drugs, the more alkaline the urine, the greater the renal clearance and vice versa for basic drugs, since:

AH = A^- + H⁺

and

B + H⁺ = BH⁺

Thus high pH (alkaline conditions) favours A^-, the charged form of the weak acid which remains in the tubular fluid and is excreted in the urine, while low pH (acid conditions) favors BH⁺, the charged form of the base. This is made use of in treating overdose with aspirin (a weak acid) by causing an alkaline urine, thereby accelerating elimination of salicylate.

Category: Pharmacology MCQs