In 2 mM of external Ca²⁺, L-type currents are activated by weak depolarizations and inactivate very slowly. (A) Comparison of I-V relationships for normal myotubes in 10 mM of external Ca²⁺ (•; n = 10) or 2 mM of external Ca²⁺ (○; n = 4) elicited at 0.1 Hz by test potentials ranging from −20 through +80 mV in 10-mV increments after a prepulse protocol. The smooth curves are plotted according to Eq. 1. The best-fit parameters for each plot are presented in Table I. (B) Recordings of L-type currents in 2 mM of external Ca²⁺ elicited by 9,800-ms depolarizations from −80 mV to the indicated test potentials.