Dynamics underlying spontaneous Ca²⁺ oscillations before and after inhibition of SERCA. Time-dependent changes in V_m (top), and [Ca²⁺]_i and [Ca²⁺]_ER (middle), and Ca²⁺ fluxes through J_rel, J_SERCA, I_NaCa, and I_PMCA (bottom) at 11 mM [G] are illustrated with different colors, as indicated in each panel. The scale of [Ca²⁺]_ER is represented on the right y axis (middle), and the zero flux level is indicated by a dotted line (bottom). From 130 s (gray horizontal bar), P_SERCA was reduced to 20% of its control value (from 0.096 to 0.0192 amole ms⁻¹) to simulate the blocking effect of SERCA by thapsigargin. The net Ca²⁺ flux through the ER was calculated by subtracting J_rel from J_SERCA.