Electrical and mechanical effects of strontium in sheep cardiac purkinje fibres

The electrical and mechanical effects of strontium were studied in sheep cardiac Purkinje fibres perfused in vitro. In a nominally calcium free solution, strontium (1.35-10.8 mmol·litre^-1): (1) caused a time, rate and concentration dependent shift of the plateau to more positive potentials, prolonged the action potential and decreased the maximum diastolic potential; (2) increased the time to peak and amplitude of the twitch and caused a tonic force which relaxed only on repolarisation; (3) was rapidly overcome in its effects by calcium (1.35-2.7 mmol·litre^-1); (4) was antagonised by manganese (1 mmol·litre^-1) and cadmium (0.1-0.2 mmol·litre^-1); (5) was potentiated by noradrenaline (0.1 μmol·litre^-1); (6) could induce action potentials in 27 mmol·litre^-1 [K]_o; (7) induced a tail following the action potential when the pacemaker potential had been blocked by caesium; (8) could induce a tail in 8 mmol·litre^-1 [K]_o which sustained force development and was reduced by calcium antagonists; (9) if applied to a quiescent fibre, induced a prolongation of the first resumed action potential and tonic force but a small twitch, and these effects were antagonised by calcium and manganese; and (10) induced a strong twitch after a period of quiescence in low [Na]_o. It is concluded that the pronounced and progressive electrical and mechanical effects of strontium in cardiac Purkinje fibres are due to an enhanced strontium influx (due to inability of strontium to substitute for calcium in the inactivation of I_si) and to strontium extrusion through an electrogenic Na-Sr exchange.