In the present study we have explored the role of calmodulin (CaM) and inositol 1,4,5-trisphosphate receptor (IP3R) in the communication process activated after the release of calcium from the endoplasmic reticulum. (ER) and the activation of calcium influx via endogenous TRP1 channels from Chinese hamster ovary cells. Experiments using combined rapid confocal calcium and electrophysiology measurements uncovered a consistent delay of around 900 ms between the first detectable calcium released from the ER and the activation of the calcium current. This delay was evident with two different methods used to release calcium from the ER: either the blockade of the microsomal calcium ATPase with thapsigargin or activation of bradykinin receptors linked to the IP3 cascade. Direct application of IP3 or a peptide from the NH2-terminal region of the IP3R activated store operated calcium, reducing the delay period. Introduction of CaM into the cell via the patch pipette increased the delay period from 900 +/- 100 ms to 10 +/- 2.1 s (n = 18). Furthermore, the use of selective CaM antagonists W7 and trifluoperazine maleate resulted in a substantial reduction of the delay period to 200 +/- 100 ms with 5 muM trifluoperazine maleate (n = 16) and 150 +/- 50 ms with 500 nM W7 (n = 22). CaM reduced also the current density activated by thapsigargin or brandykinin to about 60% from control. The CaM antagonists did not affect significantly the current density. The results presented here are consistent with an antagonistic effect of lP(3)R and CaM for the activation of store operated calcium after depletion of the ER. The functional competition between the activating effect of IP3R and the inhibiting effect of CaM may modulate the delay period between the release of calcium from the ER and the activation of calcium influx observed in different cells, as well as the amount of current activated a er depletion of the ER.