Fig. 2. The rapidly inactivating I_Ca is carried by the L-type Ca²⁺ channels and not by T-type.

Initially the cells with rapidly inactivating I_Ca were perfused with 3 μM nifedipine (blue line-Nife, 2 Ca²⁺), which reduced almost all of the I_Ca signal (Panels A, B); then the perfusion solution was changed with 2 mM Ba²⁺ (instead of Ca²⁺) maintaining the drug nifedipine (purple line-Nife, 2 Ba²⁺) which in turn modified the τ_i of the I_Ba (Panels A, C); the subsequent return to the previous nifedipine-Ca²⁺ solution (black line-Nife, 2 Ca²⁺) again reverses the characteristics of the I_Ca (Panels A–C). The same set of experiments was conducted in cells with slowly inactivating I_Ca (Panels F–H). 50 μM Ni²⁺ was used to block T-type calcium channels in rN-CMs with rapidly (Panel D) or slowly inactivating I_Ca (Panel I). Panel E, shift of the voltage depolarization −40 to −60 mV to recruit possible T-type calcium channels. The duration of each different experimental conditions was 100 s with nifedipine, where the wash/out was extended for 2–3 minutes, nickel where the steady-state was reached in 30 seconds or the experiments with different holding potential (−40/−60 mVs) where the protocol extended only by 15 s.