Why are cancers acidic? A carrier-mediated diffusion model for H+ transport in the interstitial fluid.
31P MRS studies have shown that the intracellular compartment ot tumours is kept near neutrality, whereas the interstitial fluid is acidic (pH 6.5-6.8). Why is this compartment acidic? Balance studies confirm that tumours produce excessive lactic acid, although less than usually supposed, but this cannot be the whole story, since Tannock and co-workers have shown interstitial acidity in glycolysis-deficient tumours. Another major acid load is caused by hydration of CO2 molecules to carbonic acid, catalysed by carbonic anhydrase. The distance that H+ must diffuse from cancer cells to capillaries is further than in normal tissue and this will increase acidification near the cells. We show that previous quantitative models based on simple H+ diffusion are unsatisfactory. This is because most H+ ions cross the interstitial space bound to buffers such as inorganic phosphate. Although these protonated buffers (i.e. conjugate acids) diffuse much more slowly than H+ ions they carry most of the protons, so the pH predicted by this model is closer to neutrality for a given proton production rate than that predicted by the dissolved H+ model. We have developed a mathematical model of this carrier-mediated system that predicts pHe values as low as those observed in some tumours.
Griffiths
J R
JR
CRC Biomedical Magnetic Resonance Research Group, Department of Biochemistry and Immunology, St George's Hospital Medical School, London, UK.
McIntyre
D J
DJ
Howe
F A
FA
Stubbs
M
M
D004058:Diffusion; D005110:Extracellular Space / Q000502:physiology*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D009369:Neoplasms / Q000503:physiopathology*