Strontium induces oscillatory potentials in sheep cardiac Purkinje fibers

The induction of strontium overload and its electromechanical manifestations, the factors influencing and the mechanism underlying Sr overload were studied in Purkinje fibers perfused in vitro. Strontium: (1) can induce an oscillatory potential (V_os) and repetitive spontaneous activity at low concentrations (1.35-2.7 mM); (2) at high concentrations (5.4-10.8 mM) less frequently causes a V_os but during recovery in Tyrode solution V_os appears as Sr overload recedes; (3) decreases the maximum diastolic potential by inducing a prolonged depolarization (V_ex) which subsides slowly during an interruption of drive; (4) induces a larger V_ex after procedures that increase Sr loading (fast driving rates, higher [Sr]_o or longer action potentials); (5) does not induce V_os and V_ex when the slow channel is blocked; (6) exaggerates V_ex (but not V_os) in calcium overloaded fibers; (7) exchanges with Na since in low [Na]_o the twitch amplitude increases; (8) is removed from the cell at the resting potential since after a period of quiescence the first resumed twitch decreases as a function of the preceding pause duration; (9) needs Na as charge carrier since the slope of diastolic depolarization decreases in low [Na]_o. Thus, Sr causes overload even at low concentrations and induces an oscillatory potential and the prolonged depolarization V_ex, whose mechanism appears to be an electrogenic Sr extrusion through NaSr exchange.