Figure 3

Slow inactivation of the Ca²⁺ current in γ ₁ knockout myotubes. A) Diagram to scale of the two-pulse protocol used to inactivate the L-type Ca²⁺ current at each of 15 potentials and then the test potential (+20 mV) to measure the remaining non-inactivated current. Ca²⁺ currents during the pre-pulse and test-pulse phases of the protocol are shown for a pre-pulse depolarization to -70 mV (trace 1), +30 mV (trace 2) and +60 mV (trace 3) in a control, a γ₁ het, and a γ₁ null myotube. The cell capacitance was 317 pF for the control, 175 pF for the γ₁ het and 348 pF for the γ₁ null cell. B) The maximum Ca²⁺ current during the test pulse is plotted as a function of the pre-pulse potential for 6 control, 8 γ₁ het and 8 γ₁ null cells. C) The non-activating component was subtracted and the curves were normalized to show the voltage-dependence of the inactivating component. I is the maximum test current, Imin is the test current at +50 mV and Imax is the test current at -70 mV. The curves correspond to a Boltzmann fit of the population mean with the following parameters. [(I-Imin)/Imax]_max = 1, V_1/2 = -3.8 mV and k = 8.4 mV for control cells. [(I-Imin)/Imax]_max = 1, V_1/2 = +15.6 mV and k = 8.1 mV for γ₁ het cells. [(I-Imin)/Imax]_max = 1, V_1/2 = +9.7 mV and k = 7.9 mV for γ₁ null cells.