Effects of PMCA and SERCA pump overexpression on the kinetics of cell Ca2+ signalling

The dynamic interactions of the main pathways for active Ca2+ transport have been analysed in living cells by altering the expression of their components. The plasma membrane (PMCA) and the endoplasmic reticulum (ER) (SERCA) Ca2+ pumps were transiently overexpressed in CHO cells, and the Ca2+ homeostasis in the subcellular compartments was investigated using specifically targeted chimaeras of the Ca2+-sensitive photoprotein aequorin, In resting cells, overexpression of the PMCA and SERCA pumps caused a reduction and an increase in ER [Ca2+] levels, respectively; while no significant differences were detected in cytosolic and mitochondrial [Ca2+]. Upon stimulation with an inositol 1,4,5-trisphosphate (IP3)-generating agonist, the amplitude of the mitochondrial and cytosolic Ca2+ rises correlated,vith the ER [Ca2+] only up to a threshold value, above which the feedback inhibition of the IP3 channel by Ca2+ appeared to be limiting.