Evidence from mathematical modeling that carbonic anhydrase II and IV enhance CO₂ fluxes across Xenopus oocyte plasma membranes

Abstract

Exposing an oocyte to CO2/HCO3 (-) causes intracellular pH (pHi) to decline and extracellular-surface pH (pHS) to rise to a peak and decay. The two companion papers showed that oocytes injected with cytosolic carbonic anhydrase II (CA II) or expressing surface CA IV exhibit increased maximal rate of pHi change (dpHi/dt)max, increased maximal pHS changes (ΔpHS), and decreased time constants for pHi decline and pHS decay. Here we investigate these results using refinements of an earlier mathematical model of CO2 influx into a spherical cell. Refinements include 1) reduced cytosolic water content, 2) reduced cytosolic diffusion constants, 3) refined CA II activity, 4) layer of intracellular vesicles, 5) reduced membrane CO2 permeability, 6) microvilli, 7) refined CA IV activity, 8) a vitelline membrane, and 9) a new simulation protocol for delivering and removing the bulk extracellular CO2/HCO3 (-) solution. We show how these features affect the simulated pHi and pHS transients and use the refined model with the experimental data for 1.5% CO2/10 mM HCO3 (-) (pHo = 7.5) to find parameter values that approximate ΔpHS, the time to peak pHS, the time delay to the start of the pHi change, (dpHi/dt)max, and the change in steady-state pHi. We validate the revised model against data collected as we vary levels of CO2/HCO3 (-) or of extracellular HEPES buffer. The model confirms the hypothesis that CA II and CA IV enhance transmembrane CO2 fluxes by maximizing CO2 gradients across the plasma membrane, and it predicts that the pH effects of simultaneously implementing intracellular and extracellular-surface CA are supra-additive.

Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase II enhances CO2 fluxes across Xenopus oocyte plasma membranes

“…In turn, this increased CO 2 influx enhances the fall in [CO 2 ] S , and thus accentuates the pH S spike. A mathematical model, which is discussed in the third paper in this series (49), supports this hypothesis. …”

“…In the following sections, we present predictions of the extended model that are germane to the analysis and discussion of the CA II data in the present paper. The parameter values for the simulations are those of the "standard oocyte," as defined in the third paper (49).…”

“…Our mathematical model (see third paper in this series; ref. 49) suggests that time to peak reflects in part the time required for the CO 2 /HCO 3 Ϫ buffer components to rise in the BECF, and in part the time required for the buffer components to diffuse throughout the EUF and enter the ICF.…”

“…The purpose of the present study, summarized in this paper and its two companions (46,49), is to elucidate the influence of CAs and non-CO 2 /HCO 3 Ϫ buffers on CO 2 fluxes across the plasma membranes of living cells. 1 In 1984, De Hemptinne and Huguenin (31) used a miniaturized electrode, with a glass sensor, to monitor the pH at the extracellular surface (pH S ) of rat soleus muscle as they applied CO 2 /HCO 3 Ϫ .…”

How carbonic anhydrases and pH buffers facilitate the movement of carbon dioxide through biological membranes. Focus on “Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase II enhances CO2 fluxes across Xenopus oocyte plasma membranes”; “Evidence from simultaneous intracellular- and surface-pH transients that carbonic anhydrase IV enhances CO2 fluxes across Xenopus oocyte plasma membranes”; and “Evidence from mathematical mo

“…Even an oocyte is such a complex system that intuition is of limited value in providing insight into the experimental results. A particular strength of this series of papers is the use in the third paper of a sophisticated mathematical model to interpret the wealth of experimental data and to help provide insights (8) and adapts their previously published, three-dimensional reaction-diffusion model (11).…”