A macrophage cell model for pH and volume regulation

Abstract

A whole-cell model of a macrophage ( m φ ) is developed to simulate pH and volume regulation during a NH 4Cl prepulse challenge. The cell is assumed spherical, with a plasma membrane that separates the cytosolic and extracellular bathing media. The membrane contains background currents for Na + , K + and Cl - , a Na + - K + pump, a V-type H + -extruder (V-ATPase), and a leak pathway for NH 4 + . Cell volume is controlled by instantaneous osmotic balance between cytosolic and extracellular osmolytes. Simulations reveal that the m φ model can mimic alterations in measured pH i and cell volume ( Vol i ) data during and after delivery of an ammonia prepulse, which induces an acid load within the cell. Our analysis indicates that there are substantial problems in quantifying transporter-mediated H + efflux solely from experimental observations of pH i recovery, as is commonly done in practice. Problems stemming from the separation of effects arise, since there is residual NH 4 + dissociation to H + inside the m φ during pH i recovery, as well as, proton extrusion via the V-ATPase. The core assumption of conventional measurement techniques used to estimate the H + extrusion current ( I H ) is that the recovery phase is solely dependent on transporter-mediated H + extrusion. However, our model predictions suggest that there are major problems in using this approach, due to the complex interactions between I H , NH 3 / NH 4 + buffering and NH 3 / NH 4 + efflux during the active acid extrusion phase. That is, the conventional buffer capacity-based I H estimation must also take into account the perturbation that a prepulse challenge brings to the cytoplasmic acid buffer itself. The importance of this whole-cell model of m φ pH i and volume regulation lies in its potential for extension to the characterization of several other types of non-excitable cells, such as the microglia (brain macrophage) and the T-lymphocyte.