[The changes in transmembrane potential and ionic currents induced by "ischemia" in sheep cardiac Purkinje fibers]

Ischemia-mimic solution (hypoxia, hyperkalemia, acidosis, no substrate) induced changes of transmembrane potential and ionic current were observed in 24 sheep cardiac Purkinje fibers. The sequence of changes of transmembrane potential was as follows: Two to three min after perfusion with ischemia-mimic solution, the maximum diastole potential (MDP) was depolarized slightly, and attended by a decrease of depolarization rate of phase 4 while the action potential duration (APD) was shortened or underwent a shortening-prolongation-re-shortening process. Then, the plateau of action potential disappeared gradually. As MDP was depolarized further, the action potential amplitude (APA) and the excitability of the Purkinje fibers decreased continuously until action potential could no longer be elicited. The whole process took 30 min to 160 min in different preparations. When the APD was shortened, the instantaneous outward current was always increased at all membrane potential levels. The steady-state current-voltage relationship (CVR) curve was changed from S shape to a straight line, and the inward rectification phenomenon disappeared. Under ischemic condition, the amplitude of Isi was decreased from 6.74 +/- 4.48 nA to 0.86 +/- 1.39 nA, and the CVR curve shifted more to the negative. The above results indicate that under the ischemic condition, the pacemaker function of cardiac Purkinje cell is inhibited, a large amount of K+ ions outflows and inward current Isi is decreased. These changes may be the reasons for the genesis of ischemic arrhythmia.