Figure 5. Inward currents in cultured HISM cells developing in divalent cation-free external solution.

A and B, superimposed current traces elicited by 800 ms duration voltage steps from −40 mV to test potentials between +100 and −180 mV in 10 mV increments in control (130 mm Na⁺, 2.5 mm Ca²⁺, 1.2 mm Mg²⁺; A) and after external Ca²⁺ and Mg²⁺ removal (B, denoted as I_HA). Note the instantaneous activation and deactivation of I_HA as well as the noisy appearance of the current at negative potentials. The current seen upon repolarization to −40 mV is denoted as I_DA. C, I-V relationships for currents measured at the end of the pulse (○ and •) and, in the same cell, by applying 1 s duration voltage ramps from +100 to −180 mV (continuous lines). The ramp protocol is illustrated in Fig. 6A.