Mathematical modeling of the role of carbonic anhydrase II and IV on the influx of CO 2 in a Xenopus oocyte

• Published in: The FASEB Journal
• Main Authors: Occhipinti, Rossana, Musa-Aziz, Raif, Boron, Walter F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2012
• Subjects: Musa; Carbonic acid
• Online Access: http://dx.doi.org/10.1096/fasebj.26.1_supplement.882.9

Exposing oocytes to a solution containing CO 2 /HCO 3 causes the familiar fall in intracellular pH (pH i ) and a rise in surface pH (pH S ) followed by a decay. Musa‐Aziz et al (ASN, 2005) examined the effects of carbonic anhydrases (CAs) on pH transients caused by CO 2 influx. They found that injecting CAII into oocytes or expressing CAIV on the oocyte surface accelerates the initial rate of pH i acidification (dpH i /dt) and amplifies the height of the pH S spike (ΔpH S ). They proposed that both enzymes enhance CO 2 influx by maximizing CO 2 gradients across the plasma membrane. We use a mathematical model of a Xenopus oocyte—which accounts for CO 2 , carbonic acid (H 2 CO 3 ), HCO 3 , protons, and a multitude of non‐HCO 3 buffer pairs—to investigate the above findings. We simulate the experimental protocol in which the CO 2 /HCO 3 solution is delivered from the bulk to the oocyte surface by raising exponentially the concentrations of CO 2 , H 2 CO 3 and HCO 3 in the bulk and assuming that initially no CO 2 , H 2 CO 3 and HCO 3 are present in the extracellular unconvected fluid. We use the experimental data for 1.5% CO 2 /10mM HCO 3 (pH o =7.5) to find parameter values that simultaneously match ΔpH S , the time that it takes for pH S to reach its peak (t P ), dpH i /dt, and the time delay in the pH i decay. We validate the model against data collected when exposing oocytes to 5% CO 2 /33mM HCO 3 and to 10% CO 2 /66mM HCO 3 . The model confirms that CAII and CAIV enhance CO 2 influx.