Acetazolamide and smooth muscle: possible mechanisms conditioning the contractile responses induced by various experimental procedures in several isolated preparations

Carbonic Anhydrase in Mammalian Vascular Smooth Muscle

Carbonic anhydrase (CA) is ubiquitously expressed and plays a pivotal role in acid-base balance, ion transport, and gas exchange. Limited observations by others, derived from functional, pharmacological, and histochemical studies, suggest that CA is present in vascular smooth muscle and is involved in vasoregulation. The present study, using measurements of bioactivity, inhibition characteristics, and immunohistochemical analysis, was undertaken to more fully evaluate CA in vascular smooth muscle. In isolated bovine aortic smooth muscle, which is devoid of erythrocytes, CA is present in low concentrations with a CO2 hydration activity (at 0C) of 3.5 ± 2.7 U/g. The I50 for acetazolamide inhibition is 0.07 ± 0.01 μM. Results with dorzolamide and bromopyruvate, selective inhibitors of the CA II and I isozymes, respectively, show that roughly 75% of the CA activity is accounted for by CA I, with 20% due to CA II. These results accord qualitatively with immunocytochemical staining with specific CA I and II antibodies, showing that both isozymes are present and that their staining co-localizes with cells positive for smooth muscle α-actin. These data establish the activity, inhibition, and isozyme pattern of carbonic anhydrase expression in mammalian vascular smooth muscle.

New insights into carbonic anhydrase inhibition, vasodilation, and treatment of hypertensive-related diseases

Carbonic anhydrase (CA) and its inhibitors are relevant to many physiological processes and diseases. The enzyme is differentially expressed throughout the body, in concentration and subcellular location, and as 13 catalytically active isoforms. Blood vessels contain small amounts of CA, but the enzyme's role in vascular physiology and blood pressure regulation is uncertain. However, considerable recent evidence points to vasodilation by CA inhibitors. CA inhibition in vascular smooth muscle, endothelium, heart, blood cells, and nervous system could all contribute. It is equally plausible that other targets besides CA for all known CA inhibitors may account for their vascular effects. I will review this knowledge and important remaining gaps relating to treatment of hypertensive-related diseases with potent sulfonamide inhibitors, such as acetazolamide; but also the possibility that CA inhibition by thiazides and loop diuretics, although generally weaker, may have antihypertensive effects beyond their inhibition of renal sodium transporters.