Figure 1.

Both voltage-dependent and Ca²⁺-dependent inactivation of I_Ca play a significant role during a sustained train of APW. A, The top left shows the APW used to stimulate the cells, and the bottom left shows a representative I_Ca. The right panel shows a series of I_Ca elicited by a train of APW applied at 5 Hz for 15 s. B, Cells were stimulated with a 5 Hz train of APW first in extracellular solution containing 5 mm Ca²⁺ and then in solution containing 5 mm Ba²⁺. Peak current amplitude elicited by each APW was normalized to the first pulse within the train and plotted against time. The left panel shows data from cells recorded with 0.3 mm EGTA in the patch-pipette solution. The decline in current amplitude during the train was significantly greater in Ca²⁺ than in Ba²⁺ (p < 0.0001) indicating that Ca²⁺-dependent inactivation plays a role. The right panel is from cells recorded with 10 mm EGTA in the patch-pipette solution. The decline in current amplitude during the train was slightly but significantly less in Ca²⁺ than in Ba²⁺ (p < 0.002). Error bars indicate SEM.