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carbonic_anhydrase_inhib

Carbonic anhydrase inhibitors

see also diuretics, glaucoma

Introduction

  • Carbonic anhydrase 1st discovered in RBC's (1930's), & sulfanilimide-induced metab.acidosis shown to be inhib. of this enzyme.
  • Little used as diuretic but still for glaucoma & prevention of ac. mountain sickness;

Acetazolamide:

Renal effects:

  • More than 99% enzyme must be inhib. before physiol.effects apparent but acetazolamide potent reversible inhib. → 50% inhib. in renal cortex @ 10nm [];
  • inhib. of prox. tub. cytoplasmic carb.anhydrase:
    • → decr. availability of protons for Na-H exchange → decr. H excretion;
  • inhib. of prox. tub. brush border carb.anhydrase:
    • → decr. destruction of lumen bicarb. → decr. CO2 diffusion into cell
      • → decr. bicarb. prox. tub. reabs. by 80% → incr. bicarb. excretion;
  • THUS in urine:
    • decr. titratable acidity; decr. ammonia; decr. Cl; incr.pH; incr. bicarb.; incr.Na; incr. K; incr. volume;
    • may cause urinary calculi as decr. decr. urinary citrate but N or incr. urinary Ca;

Plasma Effects:

  • decr. ECF bicarb. (as excreted incr.) → metab. acidosis → decr. renal acetazolamide effect!
  • decr. ECF K (as excreted incr. via incr. distal tub. secretion) → EC alk. & IC acidosis incr.

Eye effects:

  • decr. rate aq. humor formation as normally ciliary processes → high [bicarb] in aq.humor;

CNS effects:

  • decr. CSF rate of formation; decr. epileptic seizures - ? via acidosis or incr. local CO2;
  • may transiently incr. CSF pressure via incr. cerebral blood flow;
  • may cause drowsiness, paraesthesiae; disorientation if hep.cirrhosis ( incr. ammonia?)

Resp. effects:

  • inhib. CO2 transport system → incr. CO2 in tissues & decr. CO2 in expired air;
  • P/K:
  • readily absorbed from GIT → peak 2hrs with complete excretion within 24hrs;
  • tightly bound to carbonic anhydrase → incr. [ ] in tissues with incr. [carb.anhyd.] (RBC's/renal);
  • not metabolised, solely excreted by kidneys with active secretion & passive reabs.;
  • dose: 250-500mg once every 1-2days if as diuretic or tds to induce metab.acidosis;

Interactions:

  • phenytoin → osteomalacia;
  • methenamine (urinary antiseptic) actions decr. as alkaline urine;
carbonic_anhydrase_inhib.txt · Last modified: 2008/10/01 17:24 (external edit)