Inhibition of muscle carbonic anhydrase slows the Ca2+ transient in rat skeletal muscle fibers

A countertransport of H+ is coupled to Ca2+ transport across the sarcoplasmic reticulum (SR) membrane. We propose that SR carbonic anhydrase (CA) accelerates the CO2-HCO3- reaction so that H+ ions, which are exchanged for Ca2+ ions, are produced or buffered in the SR at sufficient rates. Inhibition of this SR-CA is expected to reduce the rate of H+ fluxes, which then will retard the kinetics of Ca2+ transport. Fura 2 signals and isometric force were simultaneously recorded in fiber bundles of the soleus (SOL) and extensor digitorum longus (EDL) from rats in the absence and presence of the lipophilic CA inhibitors L-645151, chlorzolamide (CLZ), and ethoxzolamide (ETZ), as well as the hydrophilic inhibitor acetazolamide (ACTZ). Fura 2 and force signals were analyzed for time to peak (TTP), 50% decay time (t(50)), and their amplitudes. L-645151, CLZ, and ETZ significantly increased TTP of fura 2 by 10-25 ms in SOL and by 5-7 ms in EDL and TTP of force by 6-30 ms in both muscles. L-645151 and ETZ significantly prolonged t50 of fura 2 and force by 20-55 and 40-160 ms, respectively, in SOL and EDL. L-645151, CLZ, and ETZ also increased peak force of single twitches and amplitudes of fura fluorescence ratio (R340/380) at an excitation wavelength of 340 to 380 nm. All effects of CA inhibitors on fura 2 and force signals could be reversed. ACTZ did not affect TTP, t50, and amplitudes of fura 2 signals or force. L-645151, CLZ, and ETZ had no effects on myosin-, Ca2+-, and Na+-K+-ATPase activities, nor did they affect the amplitude and half-width of action potentials. We conclude that inhibition of SR-CA by impairing H+ countertransport is responsible for deceleration of intracellular Ca2+ transients and contraction times.