Figure 1.

Effect of RDV on delayed-rectifier K⁺ current (I _K(DR)) in pituitary GH₃ cells. Cells were bathed in Ca²⁺-free, Tyrode’s solution and the recording electrode was backfilled up with K⁺-containing solution. (A) Superimposed I _K(DR) traces obtained in the control (1, i.e., RDV was not present), and during the exposure to 0.3 μM RDV (2), 1 μM RDV (3) or 3 μM RDV (4). The upper part is the voltage-clamp protocol applied to the cell. (B) Concentration-dependent inhibition of RDV on I _K(DR) amplitude measured at the beginning (□) and end (○) of depolarizing command potential (mean ± SEM; n=8 for each point). I _K(DR) amplitudes (i.e., transient or late component) in different RDV concentrations were taken at the beginning or end of depolarizing pulse for 1 sec from −50 to +50 mV. Continuous lines were well fitted with Hill equation as detailed in Materials and Methods. The IC₅₀ value (as indicated by the vertical dashed line) measured in initial peak or late component of I _K(DR) was yielded to be 10.1 or 2.8 μM, respectively. (C) Relative block (i.e., (I _control-I _RDV)/I _control) of I _K(DR) in the presence of 1 or 3 μM RDV. Smooth line in the presence of 1 or 3 μM RDV denotes the exponential fit with the time constant of 113.5 or 98.9 ms, respectively. (D) Relationship of the RDV concentration as a function of the rate constant (1/τ) (mean ± SEM; n=8 for each point). Based on minimal kinetic scheme described in Materials and Methods, the value of k ₊₁ ^* or k _-1 was estimated to be 2.01 s⁻¹μM⁻¹ and 6.12 s⁻¹, respectively; and the K _D value (k _-1/k ₊₁ ^*, i.e., dissociation constant) was resultantly yielded to be 3.04 μM.