Introduction / Background
The medication Topamax®, also known as topiramate, was originally developed for treating epileptic seizures, but is commonly used to prevent migraine headaches. While the mechanism of action is not completely understood, there are three observed effects of Topamax on cultured neurons that may be relevant.1 Topamax blocks action potentials elicited by sustained depolarization, which suggests blocking of sodium channels in a state-dependent fashion. It also potentiates the activity of GABAA receptors in a different way than barbituates do. Topamax also antagonizes kainate activation of non-NMDA excitatory amino acid receptors, as well as inhibits some carbonic anhydrase isozymes (CA-II and CA-IV), though this inhibition is weaker than other drugs and is not considered to be a major factor in Topamax’s mechanism.2 Topamax, however, does inhibit renal carbonic anhydrase in some individuals, leading to metabolic acidosis, the accumulation of protons in the blood.
In this case study, the main character, who suffers from migraines, is taken to the emergency room after experiencing shortness of breath and chest pain. As the case unfolds, students review the basics of buffers and equilibrium while learning about the interconnectedness of respiratory and renal regulation of pH.
The case was originally developed for use with a biochemistry course taught in the Chemistry Department at Wabash College, which has a typical enrollment of 24 to 30 students. It was later expanded to include a substantial amount of material relevant to a course in human physiology.
In a biochemistry course, the case is used at the beginning of the semester, after a reading on buffers. Since buffers are typically covered extensively in prerequisite courses, the case focuses on reviewing principles of buffers and applying them to a physiological system. By reducing the complexity of question 2 in the case, which involves more elaborate chemical analysis of the buffer system, and increasing the emphasis on physiological roles of carbonic anhydrase, the case becomes appropriate for a physiology course. In a physiology course, it is used after a consideration of respiratory and renal physiology and during an examination of the principles of acid-base balance.
The story is based upon an actual medical case,3 but is fictional in the details.
Objectives
Biochemistry Edition
- Recall and use buffer system definitions, Henderson Hasselbalch equation, and titration curves, in reference to carbonic acid and bicarbonate.
- Use Hess’s law to adjust the pKa of carbonic acid.
- Use LeChatlier’s principle to predict how physiological responses will alter the blood pH by shifting the equilibrium of the bicarbonate buffer system.
Physiology Edition
- Recall the definition of a buffer and the Henderson Hasselbalch equation.
- Predict the changes in [H+] in the blood when different components of the bicarbonate buffer system are altered.
- Describe the role of the kidney in acid-base balance and explain the cellular mechanisms whereby the kidney regulates the concentration of bicarbonate and nonvolatile acids in the blood.
- Distinguish between the transport processes responsible for bicarbonate reabsorption and for H+ secretion and reaction of H+ with buffers in the urine.
- Describe the respiratory mechanism that compensates for metabolic acidosis.
- Compare and contrast the different causes for metabolic acidosis.
- Explain the effect of exercise on acid-base balance.
Prior Student Knowledge
For the biochemistry edition, familiarity is assumed (either through a prior reading or prerequisite courses) with:
- Buffer definitions and equations
- Equilibrium
- Hess’s Law
- LeChatlier’s principle
For the physiology edition, familiarity is assumed with:
- Buffer definitions
- Kidney and lung anatomy and physiology
- Acid-base balance
- Lactic acid production during exercise
Classroom Management
This case is presented as an interrupted case in which students are given the case in sections, or parts, to work on. Students first read Part I of the case, which includes the results of Mary’s blood work. After the students identify the problem(s) in her blood work, there is a brief in-class discussion as to why the doctor knows Mary did not have a heart attack (if Mary had a heart attack, there would be a disturbance in the creatine kinase isozyme profile). Students then receive Part II, which reviews basic buffer principles, usually discussed in the first or second chapter of biochemistry textbooks. This section is presented in less detail for physiology students. In Part III of the biochemistry edition, students examine the physiological ways that blood pH is regulated, using LeChatlier’s principle to explain alterations in blood pH. Physiology students go into greater depth into the mechanisms of acid-based balance by examining the regulation of bicarbonate and carbon dioxide in the blood by the urinary and respiratory processes.
Once the students have completed the questions, they write a brief summary for the characters in the case study explaining why the main character should stop taking Topamax.
This case can be completed in a single one-hour class period in a biochemistry or physiology class, with the written summary as a recap homework assignment.
Blocks of Analysis
Buffers and pH control are crucial topics in both biochemistry and physiology. Typically, buffers are one of the first concepts covered in a biochemistry course and are used throughout the laboratory section. Consequently, it is crucial that students be able to use buffer concepts throughout the course.
Buffers are solutions that contain a weak acid and its conjugate base or a weak base and its conjugate acid. Buffers resist pH because the weak acid can neutralize any hydroxide ions, while the weak base neutralizes any H+ ions, yet the conjugate pair does not react with each other because they are in equilibrium. This equilibrium is described by the equation HA 
A- + H+ and the equilibrium expression Ka=([A-][H+])/[HA]. By taking the log of both sides and rearranging, the Henderson-Hasselbalch can be obtained: pH = pKa + log [A-]/[HA]. The Ka (and pKa) are a measure of the degree of dissociation of the acid, and are dependent upon the identity of the weak acid.
Carbonic acid is a diprotic acid; it has two titratable protons. Consequently, the ionization of carbonic acid can be described with two sequential equilibrium expressions:
H2CO3 H+ + HCO3- pKa1 = 3.83
HCO3- H+ + CO3-2 pKa2 = 10.33
For buffering to occur, there must be a significant fraction of both the acid and conjugate base (or base and conjugate acid) present. This occurs at the midpoint of the titration (where the mole equivalents of base added to an acid is 0.5 on the titration curve), but extends to a ratio of 10:1 and 1:10 (0.1 to 0.9 mol equivalents of base added on the titration curve). Outside of this range, there is insufficient base or acid to react with either OH- or H+ and the solution does not resist pH change well. Consequently, buffers work best within one pH unit of the pKa value.
At first glance, the carbonic acid/bicarbonate buffer system seems to disobey this rule. While it is the main buffer component of blood, both pKas of carbonic acid, 3.83 and 10.33, are far from physiological pH, 7.40. However, carbonic acid is in equilibrium with dissolved carbon dioxide, a reaction that is catalyzed by carbonic anydrase, H2O + CO2 H2CO3 (pKeq=2.52). At physiological pH, the resultant carbonic acid rapidly ionizes to form bicarbonate:
H2O + CO2 H2CO3 H+ + HCO3-
The equilibrium constant for the overall equation, H2O + CO2 H+ + HCO3-, is:
using logarithmic rules, pK = pKeq + pKa. Using this approach, the equilibrium constant is 3.83 + 2.52 = 6.35, the value most textbooks report for carbonic acid (the detailed ones include a footnote about the dissolution of carbon dioxide being included in the calculation). This equilibrium is also the reason that bicarbonate is considered a “volatile” acid, as carbon dioxide can be exhaled. While this pKa value is slightly outside of the “buffers-work-best-within-one- pH-unit-of-their-pKa-value” rule, it is the predominant buffer in the blood, with proteins and phosphate compounds also contributing to the overall buffer capacity.
Part III of the case examines the regulation of blood pH through bicarbonate equilibria. In the biochemistry edition, students simply use LeChatlier’s principle (disturbing an equilibrium system will shift the relative concentrations of reactants and products to partially undo the effects of the disturbance) to evaluate the effect of various stressors, including inhibition of renal carbonic anhydrase by Topamax.
Part III of the physiology edition of the case is more detailed, and examines the role of carbonic anhydrase in various tissues in much greater detail. To begin, students must also analyze various perturbations to the bicarbonate buffer system using LeChatlier’s principle. This work challenges them to make sense of some of the common pathophysiological challenges to acid-base balance by predicting the resulting changes in blood pH. It may be necessary to help students understand the impact of diarrhea on acid-base balance since they may not recognize the loss of bicarbonate from the blood in the diarrheal fluid. Students are then asked to survey the function of carbonic anhydrase in different cells and tissues and to predict the alterations in physiological functions at each site, including osteoclasts,4 parietal cells, red blood cells, and epithelial cells in the proximal tubules. Not only does this task enable students to identify similarites and differences in the functions of carbonic anhydrase at different locations in the body, it helps them focus on where and how the blockage of carbonic anhydrase might lead to metabolic acidosis. Students will need to thoroughly research the carbonic anhydrase-related aspects of cell function in each circumstance. In some instances, this question requires students to make a couple of inferences beyond what they might ordinarily learn about the functions of these cells from a physiology text book. The key to successfully answering this question is the students’ ability to think about the effect of carbonic anhydrase inhibition on the functions of these cells irrespective of the specific impact of Topamax on acid-base balance.
Students are then directed to explain how Topamax acts on the transport mechanisms found in the epithelial cells of the renal tubule. This question forces students to review the physiology of renal tubular bicarbonate transport.5 The understanding that the drop in bicarbonate concentration in the blood caused by Topamax results from the inhibition of luminal carbonic anhydrase should produce a greater appreciation for the key role played by tubular bicarbonate reabsorption in preventing the bicarbonate loss caused by filtration from the blood. In addition, by examining the effect of Topamax on the intracellular carbonic anhydrase, students are challenged to consider how Topamax reduces bicarbonate transport from epithelial cell to the blood.6
By predicting the effect of Topamax on the pH of the urine, students have the opportunity to integrate their understanding of the contribution of the proximal tubule to acid-base balance with the actions of other segments of the nephron. In considering the urine pH, they are required to reexamine what the kidney is doing to the blood by not reclaiming bicarbonate from the filtrate. This is another teachable moment for students where they can reexamine the effect of bicarbonate depletion on the bicarbonate buffer system of the blood. Moreover, by correctly predicting the pH of the urine, students demonstrate that they have properly understood the impact of an elevation of bicarbonate concentration on the pH of a biological fluid, and have accurately accounted for a gain or loss of an acid or base from the blood (see diagram for question 5).
Ultimately these questions challenge students to make specific connections between their understanding of the mechanisms of membrane transport in the kidney and the regulation of bicarbonate concentration in the blood. By drawing these conclusions, the student reinforces his/her understanding of the physiological processes responsible for the kidney’s role in maintaining normal acid-base balance.
Primary disturbances in acid-base balance caused by the dysfunction of either the respiratory or urinary system are compensated by the actions of the unaffected system. In question 7 in the physiology edition of the case, students learn this valuable lesson about compensation by outlining the pathway whereby metabolic acidosis stimulates increased rate and depth of breathing. By identifying the acid-base disturbance (decreased blood pH) and by noting the compensatory response (hyperventilation), students have the opportunity to understand how a change in blood pH leads to an appropriate, albeit alarming, physiological response. Students should be encouraged to use the rubrics for identifying acid-base disturbances that are found in many physiology textbooks. This work prepares them to perform the same sort of analysis on other types of acidosis described in question 8 in the physiology edition of the case. By comparing and contrasting the acidosis caused by inhibition of carbonic anhydrase with the metabolic acidosis caused by other means, the student is able to evaluate the relevant features of each type of disturbance and determine which one is most analogous to Topamax inhibition. This analysis by identification of best analogy forces students to clarify their understanding of the perturbation caused by Topamax, and encourages them to specifically apply their understanding of the chemistry of the bicarbonate buffer system to other common types of acid-base disturbances. This is also a point where the instructor may need to help students recognize that a loss of bicarbonate, such as occurs in diarrhea, can lead to an acidosis.
To determine the most appropriate treatment for the metabolic acidosis caused by Topamax, the students must predict the effect of each treatment on the pH and bicarbonate concentration of the blood. Picking the correct answer for question 9 in the physiology edition of the case requires that students have a correct understanding of the Topamax-induced acid-base disturbance and can reason through the process whereby each treatment alters blood pH. It may be necessary for the instructor to advise the students on the outcomes and side effects of some of the proposed remedies in order for students to successfully arrive at the correct answer. Question 10 in the physiology edition of the case challenges students to consider how the additive effects of Topamax inhibition and the increased muscular activity due to exercise alter the pH of the blood. By recognizing that challenges to blood pH come from two different locations in the body, students have the opportunity to gain the understanding that acid-base disturbances can be caused by the combined subclinical outcomes of one or more simultaneous pathophysiological processes. On the other hand, instructors need to realize that, in the context of the case, students may consider alternate reasons for the development of Mary’s hyperventilation, including an elevated blood pCO2 and a decreased blood pO2.
Answer Key
Answers to the questions posed in the case study are provided in two separate answer keys that are password-protected: the Biochemistry Answer Key and the Physiology Answer Key. You will be prompted for a username and password when you access either of the keys. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.
References and Notes
1 Topamax Sprinkle Capsules Approved Labeling Text. http://www.fda.gov/downloads/Drugs/DrugSafety/UCM152837.pdf Last accessed: 12/28/09.
2 S.J. Dodgson, R.P. Shank, and B.E. Maryanoff. 2000. Topiramate as an inhibitor of carbonic anhydrase isoenzymes. Epilepsia 41:S35–S39. http://www.blackwell-synergy.com/doi/abs/10.1111/j.1528-1157.2000.tb06047.x Last accessed: 12/28/09.
3 J.E. Burmeister, R.R. Pereira, E.M. Hartke, and M. Kreuz. 2005. Topiramate and severe metabolic acidosis. Neuro-Psiquiatr 63(2-B): 532–534. http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0004-282X2005000300032&lng=en&nrm=iso&tlng=en, accessed 12/28/09.
4 S.L. Teitelbaum. 2000. Bone resorption by osteoclasts. Science 289: 1504–1508.
5 S. Drage and D. Wilkinson. 2001. Acid base balance. Anaesthesia 13; Article 12. http://www.nda.ox.ac.uk/wfsa/html/u13/u1312_01.htm Last accessed 12/28/09.
6 R.P. Shank, D.R. Doose, A.J. Streeter, and M. Bialer. 2006. Plasma and whole blood pharmacokinetics of topiramate: the role of carbonic anhydrase. Epilepsy Research 63:103–112.
Acknowledgements: This case was published with support from the National Science Foundation under CCLI Award #0341279. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Date Posted: December 28, 2009.
Originally published at http://www.sciencecases.org/topiramate/notes.asp
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