Figure 3. Activation of I_p by [Na⁺]_pip at various membrane potentials.

I_p activation as a function of [Na⁺]_pip in cardiomyocytes dialysed with NMDG⁺-containing (A) or TEA⁺-containing (B) pipette solutions. Activation was measured at +40 mV (circles), 0 mV (triangles), −50 mV (diamonds), and −100 mV (squares) (n = 4–8). The curves fitted to the data points obey the Hill equation (eqn (1)) with the following parameters (for A and B): + 40 mV: K_0.5= 2.3 and 2.1 mM Na⁺_pip, I_p(max)= 0.75 and 0.81 pA pF⁻¹, n_H= 1.5 and 0.8; 0 mV: K_0.5= 3.1 and 3.3 mM Na⁺_pip, I_p(max)= 0.68 and 0.72 pA pF⁻¹, n_H= 1.6 and 1.8; −50 mV: K_0.5= 4.3 and 4.2 mM Na⁺_pip, I_p(max)= 0.70 and 0.68 pA pF⁻¹, n_H= 1.4 and 2.3; −100 mV: K_0.5= 5.8 and 5.4 mM Na⁺_pip, I_p(max)= 0.70 and 0.70 pA pF⁻¹, n_H= 2.3 and 5.2. r²= 0.984–1.000. Note that K_0.5 values increase with hyperpolarization, whereas I_p(max) remains constant.