Inhibition properties and inhibition kinetics of an extracellular carbonic anhydrase in perfused skeletal muscle

Abstract

We have investigated the properties of the carbonic anhydrase which is functionally available to CO 2 and HCO 3 − in the capillary bed of skeletal muscle. We used essentially the indicator-dilution technique of Effros and Weissman ( J. Appl. Physiol. 47, 1090–1098, 1979). Into hindlimbs of rabbits perfused with dextran solution we injected boli containing H 14CO 3 − or 36Cl −, and 3H-dextran (MW 80 000) as an intravasal indicator, and observed the washout of these indicators by fractioned collection and analysis of the venouse effluent. In agreement with previous studies we found that addition of 10 −4 M of the carbonic anhydrase inhibitor acetazolamide to the perfusate considerably speeds up the washout of 14C, reducing the extraction of 14C from 0.7 to 0.45. A half-maximal effect on 14C extraction was achieved with 1 · 10 −6 M acetazolamide (IC 50). The carbonic anhydrase inhibitors methazolamide and benzolamide both yielded IC 50 values of 10 −5 M. This patterns of inhibitory potency of the three sulfonamides is incompatible with their inhibitory effects on the two known cytosolic isoenzymes of skeletal muscle. CAII and CAIII. While cells take up acetazolamide and benzolamide extremely slowly, with half-times of several minutes to hours, the effect of both sulfonamides on 14C washout occurred very rapidly: less than 1 min, probably not more than a few seconds, were necessary to achieve inhibitory effects. We conclude that (1) a tissue carbonic anhydrase converts the injected H 14CO 3 − quickly into 14CO 2 which then diffuses into the intracellular space thus causing a washout of 14C that is much slower than that of the intravasal indicator or that of 36Cl −, (2) this carbonic anhydrase is not intra- but extracellular and presumably membrane-bound, and (3) its properties suggest that it is distinct from the well-known cytosolic carbonic anhydrases and represents a different isoenzyme.