Vaughan-Jones Research, main group website

Wellcome Trust Physiome Project

Intracellular pH  (normally ~7.10) is a powerful modulator of cell function.  In the heart, it influences processes as varied as intracellular Ca²⁺-signalling, electrical excitation, and contraction.  Changes of pH_i occur physiologically, for example, during changes of heart-rate and cardiac work-load.  A large fall of pH_i is also prominent in clinical conditions such as myocardial ischaemia where it causes acute contractile failure, abnormal Ca²⁺_i-signalling and electrical arrhythmia.  The efficient regulation of pH_i is therefore essential for the maintenance of normal cardiac function.

We have shown that pH_i in heart is controlled at the level of each constituent myocyte.  Specialised ion transport proteins, expressed at the sarcolemma, move H⁺-ions or their ionic equivalent (OH^-, HCO₃^-) into or out of the cell, thereby compensating for displacements of pH_i.  We have recently discovered that pH_i is also regulated spatially within cells by means of intracellular carrier-molecules (e.g. histidyl dipeptides) that shuttle H⁺-ions throughout the cytoplasmic compartment, and couple it functionally to the sarcolemmal transport proteins.  A further level of pH_i control is afforded by the flow of H⁺-loaded carrier-molecules through gap junctional channels (connexin-channels comprised of Cx proteins) that bridge between adjacent cells, thereby regulating the local spread of H⁺-ions within the myocardium.  We find that this cell-to-cell H⁺-traffic is gated by pH_i and by Ca²⁺_i.

Our overall aim is to elucidate the integrated control of pH_i in larger structures of the heart, such as the vascularised myocardium, the conduction system, and eventually the whole organ, in an attempt to discover how pH_i-control contributes to normal cardiac physiology and how its dysfunction contributes to the pathology of ischaemic heart disease and arrhythmogenesis. 

The following specific lines of inquiry are being pursued:

• The chemical structure of H⁺-equivalent ion-transporters, such as the AE2 Cl-HCO₃ exchanger, in order to define the pH-sensing sites responsible for regulating the molecule’s transport activity in the face of changes of intracellular and extracellular pH.
• The diffusive flow of H⁺-ions within myocytes isolated from ventricular, atrial and Purkinje tissue, in order to clarify the intracellular spatial control of pH.
• The regulation of H⁺-permeation through selected Cx channel isoforms in wildtype coupled myocytes and in heterologous cell expression-systems, in order to elucidate the spatial control of pH in cardiac tissue
• Interactions between the pH_i-regulatory and Ca²⁺_i-regulatory systems in the cardiac myocyte in order to elucidate the well-documented (but poorly understood) link between acid/base disorders in the heart and contractile and electrical dysfunction.
• Computational models of pH_i-regulation in cardiac cells, tissues, and the whole organ, in order to learn how pH_i-regulation is integrated into normal and abnormal cardiac physiology. 

The techniques being used are:

• Molecular biology: site-directed mutagenesis, and gene-product expression.
• Whole cell pH, Ca²⁺ & Na⁺ epifluorescence and confocal cellular/tissue ionic fluorescence imaging.
• Optical imaging of contraction.
• Cellular voltage clamp.
• Intracellular flash photolysis (ionic uncaging of H⁺ and Ca²⁺)
• Computational analysis and modelling. 

Further information and a list of recent publications can be found at: http://www.physiol.ox.ac.uk/Research_Groups/Proton_Transport/

Richard Vaughan-Jones, main group website

Wellcome Trust Physiome Project