i_K i_Ca i_K_Ca i_Na_Ca Cm lamda n_infinity n tau_n 1 1 V_n V S_n c V V_ a V_ V b g_K n V V_K 1 1 V_m V S_m g_Ca m_infinity V V_Ca g_K_Ca Ca_i K_d Ca_i V V_K g_Na_Ca Ca_i nH K_1_2 nH Ca_i nH V V_Na_Ca RT_F 3 Na_o Na_i Ca_o Ca_i f alpha i_Ca 2 i_Na_Ca k_Ca Ca_i k_rel Ca_ret Ca_i k_pump Ca_i k_rel Ca_ret Ca_i k_pump Ca_i