Figure 6. Kinetics, [Ca²⁺]_o dependence and ion permeation properties of I_HA and I_DA.

A, typical currents during a slow voltage ramp from +100 to −180 mV and a voltage step to −40 mV slowly developing after Ca²⁺ and Mg²⁺ removal from the external solution. Superimposed current traces were recorded at 25 s intervals. B, top, maximal amplitudes of inward currents at −180 mV (•, I_HA) and at −40 mV after the ramp (8, I_DA) plotted against time where time zero represents the moment of Ca²⁺ and Mg²⁺ removal from the external solution. Bottom, dependence of I_HA (•) and I_DA (8) in the same cell on the external free Ca²⁺ concentration. Relative currents were fitted by logistic functions with IC₅₀ values of 20 μm for I_HA and 311 nm for I_DA. C and D, external cation substitution experiments were performed on two different cells after I_HA(C) and I_DA(D) had been stabilized in a divalent cation-free external solution. The control I-V relationship for I_HA (voltage protocol as in A) and control current response for I_DA (voltage step from −100 to −40 mV) were obtained in high-Na⁺, 2.5 mm CaCl₂-containing external solution. Traces labelled ‘Na⁺’ were obtained after Ca²⁺ removal whereas all other traces were obtained in solutions with Na⁺ replacement as described in the Methods. High-Cs⁺ pipette solution was used.