Single-Channel Demonstration of L-Type Ca2+ Window Current

Canine cardiac Purkinje cells contain both Land T-type calcium currents, yet the single Ca2' channels have not been characterized from these cells. Additionally, previous studies have shown an overlap between the steady-state inactivation and activation curves for L-type Ca2' currents, suggesting the presence of L-type Ca2' "window" current. We used the on-cell, patch-clamp technique to study Ca'+ channels from isolated cardiac Purkinje cells. Patches contained one or more Ca2' channels 75% of the time. L-type channels were seen in 69%1 and T-type channels in 73% of these patches. With 110 mM Ba2` as the charge carrier, the conductances of the Land T-type Ca2' channels were 24.2±0.8 pS (n=9) and 9.0±0.5 pS (n=8), respectively (mean±SEM). With 110 mM Ca2' as the charge carrier, the conductance of the L-type Ca2' channel decreased to 9.7± 1.2 pS (n=4), whereas the T-type Ca2' channel conductance