The Buffering Power in the Human Atrial Myocardium

The quantification of intracellular buffering power is essential for calculating sarcolemmal acid-equivalent fluxes from intracellular pH (pH(subscript i))-recordings. However, it is not well established in the human atrial myocardium. The buffering power (• •(subscript tot)) in general has two components: the intrinsic buffering power (•(subscript i)•) and the buffering capacity (• •(subscript CO2)). Thus, we calculate the buffering power by microspectrofluorimetry with a fluorescence probe 2',7'-bis (carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Experiments are performed under conditions free of Na(superscript +), Cl(superscript -) and high K(superscript +) to prevent the operation of pH regulators. Small stepwise reductions of external NH4Cl (from 30 to 0 mM) result in stepwise reductions of pH(subscript i). Similar procedures are performed in two kinds of solutions which are buffered with the CO2/HCO3(superscript -)-condition or the HEPES-condition. Results show that, in the CO2/HCO3(superscript -)-condition, the values of • •(subscript tot) can be described as • •(subscript tot)=-1633.3 pH(subscript i)+12544.9 (R^2=0.86) in the pH(subscript i) ranges of 7.1~7.5. The values of • •(superscript tot) increase while pH(subscript i) decreasing. In HEPES-condition, the values of •(subscript i)• can be described as •(subscript i)•=-212.1 pH(subscript i)+1931.9 (R^2=0.026) in the pH(subscript i) ranges of 7.1~7.5. Note that there is not a significant correlation between •(subscript i)• and pH(subscript i). Moreover, between •(subscript tot) and •(subscript i)•, there is a significant difference in the pH(subscript i) ranges of 7.1~7.3 (p＜0.01; n=4, n=15, respectively). This means that the factor of • •(subscript CO2) becomes more important while in the acidic conditions. Also, the magnitude of intracellular • •(subscript CO2), derived from • •(subscript tot)-(subscript i)•, has been described as • •(subscript CO2)=3670.7 pH(subscript i)^2-54082.7 pH(subscript i)+199255.4 (R^2=0.95) in the pH(subscript i) ranges of 7.1~7.5 (p＜0.01; n=12). The result supports that CO2-permeation and CO2-hydration/dehydration reaction are not rapid enough to behave as an open system for CO2 in human atrial myocardium. In conclusion, in our present study, for the first time, we quantify the buffering power in human atrial myocardium.