The effects of the current (iex) generated by the Na-Ca exchange mechanism have been investigated on the electrical activity of a cardiac cell using the model of MC ALLISTER et al. (1975) for the Purkinje fibre action potential (AP). 2. The reversal potential and the steady-state value of iex were described by the same equations as for the squid axon (MULLINS, 1976). The maximal intensity and the time constant of iex were extrapolated from experiments using frog hearts. 3. A compact program written with the Adams method in Fortran IV (PLANT, 1979) allowed a minicomputer to be used. 4. The addition of iex to the previous model induced a prolongation of the AP and a reduction of the duration of the diastolic phase. 5. Increasing the maximal steady-state amplitude of iex may lead to early after depolarizations and oscillatory behaviour. These effects can be prevented by adequate changes in the amplitude of some potassium outward currents. 6. It is concluded that : (i) alterations of the Na-Ca exchange, e.g., by cellular Na loading, should induce variations of both AP repolarization and diastolic phase durations and, consequently, alter the beating rate ; (ii) iex could interfere with the outward currents whose characteristics should be reconsidered.
