Lack of soluble carbonic anhydrase in aortic smooth muscle of the rabbit

Rabbit aorta was investigated for the occurrence of soluble, methazolamide-sensitive carbonic anhydrase (CA) by measuring electrometrically the rate of acidification of a weakly alkaline CO2 solution buffered with 12.5 mM veronal/HCl at 0 degree C. For this purpose, aortae of 10 rabbits with the endothelium carefully preserved, were homogenized, centrifuged, and the supernatants pooled. The proteins were fractionated by FPLC, and tested for their CA activity. In control experiments, the pH change resulting from the spontaneous CO2 hydration was found to be -0.92 +/- 0.01 pH units/min (mean +/- SE, n = 10). Aliquots of the 30,000 dalton protein fraction corresponding to 100 mg of aortic tissue wet weight did not alter the hydration rate significantly (-0.94 +/- 0.01 pH units/min, n = 10). Also, in the presence of 10(-4) M of the CA inhibitor, methazolamide, these rates were not altered significantly (-0.94 +/- 0.01 and -0.93 +/- 0.01 pH units/min, respectively, n = 10). No CA activity was found in the other FPLC fractions, either. These results suggest that soluble CA is absent from the myocytes and the endothelium of the rabbit aorta.