The increase in contractility promoted by 4-AP is driven by an increase in the L-type Ca²⁺ current during phase 1. (A and B) Time course of Ca²⁺-dependent ionic currents recorded with LPP before (black trace) and after (dark cyan trace) the hearts were perfused with 100 µM 4-AP. (C) 4-AP increases the amplitude of the early Ca²⁺ current (6.16 ± 0.66 nA/nF, eight measurements, black circles vs. 8.67 ± 0.44 nA/nF, seven measurements, dark cyan circles). (D and E) 4-AP not only increases the amplitude of the early component but also increases its duration and time integral (1.13 ± 0.99 times, 21 measurements, black circles vs. 1.57 ± 0.25 times, 17 measurements, dark cyan circles). Results from five different hearts illustrate that the amount of Ca²⁺ that gets into the cell during 4-AP perfusion is significantly larger (*) respect to their controls at P > 0.001. n = 5 hearts.