Effects of reduced PO₂ on rapid-drive-induced hyperpolarization of diastolic transmembrane potential in feline cardiac Purkinje strands

Using standard microelectrode techniques, we evaluated effects of diminished oxygen tension on the magnitude and time course of frequency dependent changes in maximum diastolic transmembrane potential (MDP) and on alteration of action potential duration (APD) in feline Purkinje fibers. MDP was recorded continuously during a control period (cycle length (CL) = 1,000 ms), during a 5-min period of rapid drive (CL = 400 ms) and following return to pacing CL = 1,000 ms. Rapid drive resulted in hyperpolarization of MDP from control value; and after return of pacing CL = 1,000 ms, MDP gradually depolarized, eventually attaining a steady state value within ± 0.5 mV of the control value. The difference between hyperpolarized MDP value and final steady-state value was designated V(H), and the decline of MDP towards steady-state value approximated an exponential function (time constant = τ(VH)). Exposure to reduced PO₂ (75 ± 2.1 mmHg vs. control 473 ± 39.1 mmHg) (1 mmHg = 133.322 Pa) resulted in reduction in the magnitude of V(H) (6.2 ± 3.43 mV vs. 7.8 ± 2.73 mV, mean ± SD, p < 0.005) and shortening of APD within 0-24 min, while measurable prolongation of τ(VH) (75 ± 18.5 vs. 54 ± 9.0 s, p < 0.005) began at 25-49 min following onset of reduced PO₂. These observations suggest that rate-related changes of MDP in cardiac tissues are oxygen dependent, and they support previously reported analagous observations in nerve which suggested that frequency dependent potential changes may in part reflect alterations of electrogenic Na-K pump activity.