Fig. 6

I _CaL in isolated SAN cells from neonatal and adult mice. a Representative traces of I _CaL in SAN cells from neonatal (upper panel) and adult mice (lower panel). Traces shown were obtained by applying 300-ms depolarizing voltage pulses from a holding potential (V _h) of −60 mV to variable voltages (V _t) indicated. The pipette solution was a Cs⁺-rich solution and the external solution was Na⁺ and K⁺-free. Dotted lines indicate the zero current level. b Averaged current–voltage relationships for the initial current (circles) and the current at the end of the 500-ms pulses (squares). Open and filled symbols indicate neonatal and adult mice, respectively. c Voltage-dependent activation and inactivation for I _CaL in SAN cells from neonatal and adult mice. The steady-state inactivation was analysed by a double-pulse protocol; 7-s conditioning prepulses to different potentials were followed by the test pulse to 0 mV. The amplitude of I _CaL was measured, normalized to the maximal value of I _CaL and fitted with Boltzmann equation. Open and filled symbols indicate neonatal and adult mice, respectively. Voltage-dependent activation was obtained by calculating the chord conductance of peak I _CaL at various test potentials in SAN cells from neonatal (open squares) and adult (filled squares) mice. Data are mean ± SEM