Oxygenation and Perfusion

The measurement of intracellular pH by NMR is an important application of this technique. For both cell suspensions and perfused organs the techniques that are used most often today involve the measurement of the equilibrium distribution of acid or base indicators across cell membranes. Questions arise concerning possible binding of the indicator to intracellular compounds and membranes, thereby introducing contributions other than pH to the distribution. In the case of ischemic organs, difficulties arise because these methods require flow (Poole-Wilson, 1978). Thus, 

The basis for the use of NMR to measure pH is easily understood, using orthophosphate (Pj) as an example. H3PO4 is a tribasic acid which dissociates according to the three equilibria  

at physiological pH, near neutrality, only the second equilibrium need be considered. Since the exchange of a proton between H2PO4 and HPOl is extremely rapid the observed phosphate chemical shift is given by  

Where A is the observed chemical shift, A and A are the chemical shifts of H2PO4 and HPO , respectively, and X and X are their mole fractions. Since 

The constants were found experimentally, titrating the pH and measuring chemical shift at each pH of an aqueous phosphate solution, to be pK = 6.90, A4 = 3.29, and A = 5.81, where A and A are the limiting chemical shifts at high and low pH, and Aq is the observed chemical shift. The chemical shifts were measured relative to internal PCr, the chemical shift of which is practically independent of pH in the physiological pH range (Hoult et al, 1974 Dawson et a/., 1977). 

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