Figure 5. The voltage dependence of I_p is not shifted by NA when [Ca²⁺]_i is either high or low.

A, the ramp protocol for determining the voltage dependence of I_p. In the upper voltage trace, the cell is held at 0 mV, stepped to +20 mV then ramped to -100 mV in a period of 5 s. The lower trace is the current response to the voltage clamp protocol. The ramp protocol was done in the absence of Str where the response included the background membrane current plus I_p, in the presence of Str where the response was only the background membrane current, and lastly again in the absence of Str to verify the cell was not running down. This protocol was repeated in the presence of NA (10 μm NA in low [Ca²⁺]_i and 1 μm NA in high [Ca²⁺]_i). B, the voltage dependence of I_p is not shifted by NA when [Ca²⁺]_i is low (15 nm). I_p was determined by subtracting the ramp current response in the presence of Str from that in the absence of Str. In each cell, I_p was normalized by its value at 0 mV in the absence of NA (in 5 cells the mean pump current was 170 ± 44 pA). This procedure removes cell-to-cell variation in pump density and membrane area. The normalized pump currents were averaged, and in the left-hand panel normalized I_p in the presence of NA is shown by ▪, whereas the normalized I_p in the absence of NA (Control) is shown by •. The right-hand panel graphs the ratio of the two curves shown in the left-hand panel. The lack of voltage dependence for the ratio indicates a uniform NA stimulation of about 15 % with no voltage shift. C, the voltage dependence of I_p is not shifted by NA when [Ca²⁺]_i is high (1.4 μm). The data were recorded and analysed as described in B. In 4 cells the mean pump current at 0 mV was 160 ± 40 pA. Again, the lack of voltage dependence for the ratio of I_p(NA)/I_p(con) indicates a uniform NA stimulation with no voltage shift.