Fig. 5. I_Na under action potential clamp. Representative atrial and ventricular action potentials (top) were used to elicit tetrodotoxin-sensitive I_Na (middle) at physiological external Na⁺ concentration (154 mmol l^-1). The bottom panel shows V_max mean (± s.e.m.; N=12) calculated from I_Na, V_max measured in the intact tissue and their difference. Asterisks show a significant difference (P<0.05) between atrial and ventricular myocytes.