Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase IV enhances CO 2 fluxes across Xenopus oocyte plasma membranes

• Published in: American Journal of Physiology: Cell Physiology Vol. 307; no. 9; pp. C814 - C840
• Main Authors: Musa-Aziz, Raif, Occhipinti, Rossana, Boron, Walter F.
• Format: Journal Article
• Language: English
• Published: 01.11.2014
• ISSN: 0363-6143; 1522-1563
• DOI: 10.1152/ajpcell.00050.2014

Human carbonic anhydrase IV (CA IV) is GPI-anchored to the outer membrane surface, catalyzing CO 2 /HCO 3 − hydration-dehydration. We examined effects of heterologously expressed CA IV on intracellular-pH (pH i ) and surface-pH (pH S ) transients caused by exposing oocytes to CO 2 /HCO 3 − /pH 7.50. CO 2 influx causes a sustained pH i fall and a transient pH S rise; CO 2 efflux does the opposite. Both during CO 2 addition and removal, CA IV increases magnitudes of maximal rate of pH i change (dpH i /d t) max , and maximal pH S change (ΔpH S ) and decreases time constants for pH i changes (τ pH i ) and pH S relaxations (τ pH S ). Decreases in time constants indicate that CA IV enhances CO 2 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-CO 2 /HCO 3 − buffer (i.e., HEPES), in the presence of extracellular CA or at high [CO 2 ], accelerates CO 2 influx. Simultaneous measurements with two pH S 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) τ pH S , indicating [CO 2 ] 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 CO 2 or consumes exiting CO 2 at the extracellular surface, enhances the gradient driving CO 2 influx across the cell membrane.