The quantitation of buffering action II . Applications of the formal & general approach Bernhard

Background: The paradigm of "buffering" originated in acid-base physiology, but was subsequently extended to other fields and is now used for a wide and diverse set of phenomena. In the preceding article, we have presented a formal and general approach to the quantitation of buffering action. Here, we use that buffering concept for a systematic treatment of selected classical and other buffering phenomena. Results: H+ buffering by weak acids and "self-buffering" in pure water represent "conservative buffered systems" whose analysis reveals buffering properties that contrast in important aspects from classical textbook descriptions. The buffering of organ perfusion in the face of variable perfusion pressure (also termed "autoregulation") can be treated in terms of "non-conservative buffered systems", the general form of the concept. For the analysis of cytoplasmic Ca++ concentration transients (also termed "muffling"), we develop a related unit that is able to faithfully reflect the time-dependent quantitative aspect of buffering during the pre-steady state period. Steady-state buffering is shown to represent the limiting case of time-dependent muffling, namely for infinitely long time intervals and infinitely small perturbations. Finally, our buffering concept provides a stringent definition of "buffering" on the level of systems and control theory, resulting in four absolute ratio scales for control performance that are suited to measure disturbance rejection and setpoint tracking, and both their static and dynamic aspects. Conclusion: Our concept of buffering provides a powerful mathematical tool for the quantitation of buffering action in all its appearances. Introduction In the preceding article (Buffering I ), we presented a formal and general framework for the quantitation of buffering action. The purpose of the present article is to apply that mathematical tool to the analysis of some scientifically important buffering phenomena. Recall that we formulated buffering phenomena as the partitioning of a quantity into two complementary compartments, and then used the proportions between the respective flows as a simple quantitative criterion of buffering strength. The two measures of buffering action were i) the buffering coefficient b, defined as the differential d(buffered)/d(total), and ii) the buffering ratio B, defined as the differential d(buffered)/d(unbuffered). Moreover, the following analyses will make use of the distinction between various categories of buffered systems (e.g. conservative vs. non-conservative partitioned systems), and Published: 16 March 2005 Theoretical Biology and Medical Modelling 2005, 2:9 doi:10.1186/1742-4682-2-9 Received: 26 August 2004 Accepted: 16 March 2005 This article is available from: http://www.tbiomed.com/content/2/1/9 © 2005 Schmitt; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Page 1 of 15 (page number not for citation purposes) Theoretical Biology and Medical Modelling 2005, 2:9 http://www.tbiomed.com/content/2/1/9 will exploit the equivalencies and interconversions between these categories. To begin, we revisit a classical case of acid-base buffering: H+ ion buffering in a solution of a weak acid. This process can be described easily in terms of a conserved quantity (total H+ ions) that partitions into two complementary compartments or states (bound vs. free). Such a system was termed a "conservative buffered system". Conservative buffered systems constitute the most simple buffered systems according to our buffering concept, and they provide a suitable framework to describe further classical buffering phenomena. An important one among them, the so-called "self-buffering" of H+ ions in pure water, is analyzed in Additional file 2. The concept of a "conservative buffered system" can be applied readily and fruitfully to numerous buffering phenomena that involve quantities other than H+ or Ca++ ions ("non-classical" buffering phenomena). Some examples are presented in the Additional file 3; these include a straightforward approach to the notoriously difficult quantitation of "redox buffering", and examples which demonstrate that the concept of "buffering" is by no means limited to the natural sciences. In the second section, we analyze the buffering of organ perfusion in the face of variable blood pressure. Here, the independent variable is blood pressure, whereas the dependent variables are volume flows. Systems that involve different physical dimensions, however, cannot be formalized in terms of "conservative buffered systems", the basic form of our buffering concept. Here, the general form of our buffering concept (Buffering I ) proves to provide a rigorous and reliable framework for the treatment of such "non-conservative" and "dimensionally heterogeneous" buffered systems. The third section extends the buffering concept to timedependent buffering processes. "Time" as a potentially important aspect of buffering becomes evident, for instance, in the Ca++ concentration transients that are elicited by the brief openings of a calcium channel in the surrounding cytoplasm [1]. It was an important achievement to realize that this blunting of concentration swings represents an independent quantity, and to suggest a term as fitting as "muffling" for it [2]. However, for reasons detailed in Additional file 5, the available units of "muffling strength" are not satisfying. We introduce an extension of our buffering concept that clearly satisfies all criteria required for a muffling strength unit and provides a dimensionless ratio scale for this quantity. Furthermore, this unit is able to connect "muffling" and "buffering" both conceptually and numerically: Steady-state buffering is shown to represent the limiting case of time-dependent muffling for infinitely long time intervals and infinitely small perturbations. Finally, Additional file 6 sketches how our concept of buffering can serve to quantitate "systems level buffering" in the context of control systems. Buffering is an important aspect of homeostasis in physiological systems, and control theory provides a powerful general language to describe homeostatic processes. So far, however, the concept of buffering could not be accomodated explicitly in this framework. We show that "buffering" and "control theory" can be connected conceptually and numerically in a straightforward and meaningful way. To quantitate systems level buffering, we need to exploit simultaneously all possibilities and features of our buffering concept, because control systems may be conservative or non-conservative, dimensionally homogeneous or heterogeneous, and time-invariant or time-dependent. The buffering of H+ ions by weak acids or bases – Buffering as partitioning of a conserved quantity and the concept of "Langmuir buffering" Weak acids in conjunction with their conjugate base, and weak bases in conjunction with their conjugate acids, are the prototypical "buffers". They were the first buffers put to action by biochemists in order to stabilize the pH of solutions, and they were also the first buffers to receive thorough theoretical analysis. Numerous textbook definitions explicitly equate "buffers" with "mixtures of weak acids plus conjugate base" (or vice versa), and this notion became so inextricably woven into our thinking about buffering that the distinction between the chemical substrate of this process and the abstract quantitative pattern manifest in it fell into oblivion. In a wider sense, however, the manifold varieties of ligand binding are indeed responsible for a large number of buffering phenomena encountered in biochemistry or physiology. For instance, ions such as Ca++ are buffered by physico-chemical processes analogous to H+ buffering, albeit without the involvement of literal weak acids or bases. Compared to buffering involving other mechanisms, such as blood pressure buffering or systems level buffering (see below), buffering via ligand binding exhibits some distinct quantitative patterns. The terminology of the original acid-base concept of buffering, however, is too specific as to serve as a general framework for the treatment of these phenomena. The following section demonstrates how the quantitative patterns of buffering via ligand binding can be caught with the aid of the four parameters t, b, T, and B. The analysis explores the classic case of a weak monoprotic acid dissolved in water. Page 2 of 15 (page number not for citation purposes) Theoretical Biology and Medical Modelling 2005, 2:9 http://www.tbiomed.com/content/2/1/9 Mathematical model of free and bound H+ ion concentrations in a solution of a weak acid To obtain an explicit quantitative description of buffering by weak acids, we first recapitulate the mathematical model that describes the concentrations of H+ ions in an aqueous solution of a weak acid as a function of free H+ ion concentration. Subsequently, we reformulate the concentrations of free and bound H+ ions as functions of total H+ ion concentration, and combine these functions into a buffered system. Finally, we derive the four parameters t, b, T, and B from this system. A weak monoprotic acid HA can dissociate into a free H+ ion and a conjugated base A-, to an extent that is dictated by KA, the acid constant in water, according to: KA × [HA] = [H]free × [A]free. The total amount of weak acid [A]total equals the sum [A]free+ [HA] of dissociated and undissociated weak acid; the system is "conservative". We can therefore substitute [A]free by [A]total[HA] and obtain: and after several intermediate steps: For the sake of readability, we express the same relationship in a more general notation: where c and d stand for the constants [A]total and KA, respectively, and the variables y and z correspond to [H]free and [HA], respectively. This latter equation describes a hyperbola that approaches c as y increases to infinity. It was first used empirically by Hill [3], but became a much more meaningful mathematical model after Langmuir had supplied a mechanistical interpretation, namely in terms of non-cooperative binding of a ligand to a finite number of binding sites [4]. That model is widely applicable to numerous phenomena, e.g. receptorligand interactions, adsorption processes at surfaces, or enzyme kinetics, to name but a few. The same rules of non-cooperative binding apply to the binding of H+ ions to the conjugate base of a monoprotic weak acid. In order to move from specific acid-base terminology to a general ligand binding terminology, we re-interpret the symbols in the equation as follows: Variable z represents the concentration of bound ligand, y the concentration of free ligand, c the total number of binding sites, and d the equilibrium constant Kd of the complex with respect its dissociation products. "Langmuir buffers" So far, we have described bound ligand z as a function of free ligand y. Next, we express the concentration of free ligand y as a function of total ligand x = y+z: and the concentration of bound ligand z as a function of total ligand x: The relation between total, free, and bound ligand for non-cooperative binding to a fixed number of binding sites with similar affinity is shown in Figure 1A. It is easy to double-check that the sum y(x)+z(x) equals x, i.e., the conservation condition is satisfied. To turn these two functions into a "buffered system", we assign the role of "transfer function" τ(x) to the free H+ concentration y(x), and the role of "buffering function" β(x) to the bound H+ concentration z(x). Because many common and important systems follow this quantitative pattern, it might be useful to have a specific term to refer to them. We suggest the term "Langmuir-type buffers" or "Langmuir-type buffered systems". Briefly, a Langmuir-type system can be defined as an ordered pair of two functions {y = τ(x), z = β(x)} that satisfies the three conditions (x = y+z) and z = c × y/(d+y) and c, d ∈ +. In systems that involve ion concentrations, both τ(x) and β(x) naturally assume a value of zero at x = 0, i.e., they pass through the origin. However, we can relax this fourth constraint by allowing for offsets τ0 and β0, respectively, without altering the buffering properties (Buffering I ). Thus, we obtain the general form of a Langmuir buffer BLangmuir as: The buffered system constituted by the solution of a weak acid in water is "dimensionally homogeneous": the variables x, y, and z are either all dimensionless (e.g. when [ ] [ ] [ ] HA H [A] -[HA] K [H ] A [H free total