The interplay between plasma membrane and endoplasmic reticulum Ca 2+ ATPases in agonist-induced temporal Ca 2+ dynamics

A change in the intracellular free Ca2+ concentration ([Ca2+]i) functions as a transmitter for signal transduction and shows a broad temporal pattern. Even genetically homogeneous cell types show different Ca2+ response patterns under permanent agonist stimulation. In Ca2+ signaling, the dynamics of the Ca2+ release from the Ca2+ channels during continuous agonist stimulation and the simultaneous effect of the pumps are unclear. In this study, the dynamic interaction of the Ca2+ ATPases in the plasma membrane (PMCA) and the endoplasmic reticulum membrane (SERCA) during continuous ACh stimulation is monitored using Fluo-3 and Fura-2 loaded HEK 293 cells. We characterize Ca2+ release patterns at the sub-maximal and maximal stimulation doses in the absence of extracellular Ca2+. We analyze the responses regarding their types, oscillation frequency and response times. La3+ (PMCA blocker) do not change the frequency and time courses in sub-maximal ACh treatment, while with the maximal stimulation oscillation frequency increase as oscillations superimpose on robust release, and response time of [Ca2+]i is elongated. A similar effect of La3+ is observed in quantal Ca2+ release phenomenon. In the presence of CPA, a SERCA blocker, oscillations are completely abolished, but response time does not change. We also observe that during continuous receptor stimulation, Ca2+ release do not cease. These data may suggest that Ca2+ release continues during agonist stimulation, but SERCA and PMCA form a new steady state and return [Ca2+]i to its physiological concentration.