The effects of acidosis and bicarbonate on action potential repolarization in canine cardiac purkinje fibers

Studies were performed on canine cardiac Purkinje fibers to evaluate the effects of acidosis and bicarbonate (HCO⁻₃) on action potential repolarization. Extracellular pH (pHe) was reduced from 7.4 to 6.8 by increasing carbon dioxide (CO₂) concentration from 4 to 15% in a HCO3-buffered solution or by NaOH titration in a Hepes-buffered solution. Both types of acidosis produced a slowing of the rate of terminal repolarization (i.e., period of repolarization starting at about -60 mV and ending at the maximum diastolic potential) with an attendant increase in action potential duration of 10-20 ms. This was accompanied by a reduction in the maxium diastolic potential of 2-8 mV. In contrast, if the same pH change was made by keeping C02 concentration constant and lowering extracellular HCO⁻₃ from 23.7 to 6.0 µM, in addition to the slowing of terminal repolarization, the plateau was markedly prolonged resulting in an additional 50- to 80-ms increase in action potential duration. If pHe was held constant at 7.4 and HCO3 reduced from 23.7 mM to 0 (Hepes-buffered solution), the changes in repolarization were nearly identical to those seen in 6.0 µM HCO⁻₃ except that terminal repolarization was unchanged. This response was unaltered by doubling the concentration of Hepes. Reducing HCO⁻₃ to 12.0 µM produced changes in re polarization of about one-half the magnitude of those in 6.0 µM HCO⁻₃. These findings suggest that in Purkinje fibers, HCO⁻₃ either acts as a current that slows repolarization or modulates the ionic currents responsible for repolarization.