The equilibrium potential for Ca ++ is +134mV.

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In cardiac pacemaker cells the action potential (phase 3) is caused by an influx of Ca++ ions. The equilibrium potential for Ca++ is +134mV. Despite the high equilibrium potential for Ca++ , the peak of the action potential in a cardiac pacemaker cells is approximately 20mV. Why does the peak magnitude of the cardiac pacemaker action potential not increase to levels closer to the Ca++ equilibrium potential?