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 I-50 for acetazolamide inhibition is 0.07 +/- 0.01 muM. 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 alpha-actin. These data establish the activity, inhibition, and isozyme pattern of carbonic anhydrase expression in mammalian vascular smooth muscle.