Evidence from simultaneous intracellular- and surf ace-pH transients that carbonic anhydrase IV enhances CO2 fluxes across Xenopus oocyte plasma membranes

摘要

Musa-Aziz R, Occhipinti R, Boron WF. Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase IV enhances CO2 fluxes across Xenopus oocyte plasma membranes. Am J Physiol Cell Physiol 307: C814-C840, 2014. First published June 25, 2014; doi: 10.1152/ajpcell.00050.2014.—Human carbonic anhydrase IV (CA IV) is GPI-anchored to the outer membrane surface, catalyzing CO2/HCO^~ hydration-dehydration. We examined effects of heterologously expressed CA IV on intracellular-pH (pHi) and surface-pH (pHs) transients caused by exposing oocytes to CCVHCO^/pH 7.50. CO2 influx causes a sustained pHi fall and a transient pHs rise; CO2 efflux does the opposite. Both during CO2 addition and removal, CA IV increases magnitudes of maximal rate of pHi change (dpHi/d?)max, and maximal pHs change (ApHs) and decreases time constants for pHi changes (TpHi) and pHs relaxations (tphs). Decreases in time constants indicate that CA IV enhances C02 fluxes. Extracellular acetazolamide blocks all CA IV effects, but not those of injected CA II. Injected acetazolamide partially reduces CA IV effects. Thus, extracellular CA is required for, and the equivalent of cytosol-accessible CA augments, the effects of CA IV. Increasing the concentration of the extracellular non-CC?2/HCOJ buffer (i.e., HEPES), in the presence of extracellular CA or at high [CO2], accelerates CO2 influx. Simultaneous measurements with two pHs electrodes, one on the oocyte meridian perpendicular to the axis of flow and one downstream from the direction of extracellular-solution flow, reveal that the downstream electrode has a larger (i.e., slower) tphs, indicating [CO2] asymmetry over the oocyte surface. A reaction-diffusion mathematical model (third paper in series) accounts for the above general features, and supports the conclusion that extracellular CA, which replenishes entering CO2 or consumes exiting CO2 at the extracellular surface, enhances the gradient driving CO2 influx across the cell membrane.