Fig. 4. Characterisation of the cloned ccKir2 channels in COS-1 cells and native I_K1 of the crucian carp ventricular myocytes by Ba²⁺ sensitivity and inward rectification. (A) Representative whole-cell recordings of the inward-rectifying K⁺ current through the cloned ccKir2.5 channel and I_K1 of the warm-acclimated (WA) crucian carp myocyte, demonstrating a reversible block by 0.3 mmol l^–1 external Ba²⁺. On the basis of current density, expression levels of the three ccKir2 proteins were similar (P>0.8). (B) Dose–response relationship of Ba²⁺ block of the cloned ccKir2 proteins and endogenous I_K1 of the ventricular myocytes. The upper panel shows dose–response curves and the lower panels give mean values of K_d (left) and Hill coefficient (H, right), respectively. CA, cold-acclimated carp. Different letters indicate statistically significant differences between the cloned ccKir2 channels. Number of tested cells is 7–11. (C) Current–voltage relationship of cloned ccKir2 channels (left) and I_K1 of warm- and cold-acclimated ventricular myocytes (right). The lower panels show the relative outward current for the same recordings. Note the higher inward and outward current density of cold-acclimated myocytes in comparison to warm-acclimated myocytes. (D) Boltzmann fits (top) and half-voltage (V_1/2) and effective valency (z) of the inward rectification (bottom). Different letters indicate a statistically significant difference between cloned ccKir2 channels and an asterisk indicates a statistically significant difference between acclimation groups. Number of tested cells is 7–13.