domain, modulating E1/E2 conformational transition. In El conformation, the NH2 terminus is in association with the first cytoplasmic loop of Na,K-ATPase. Transition from E1 to E2 conformation may release the NH2 terminus from its interaction with the first cytoplasmic loop of Na,KATPase, thus making the NH2 terminus available for interaction either with the InsP3R directly or with a protein bridging between Na,K-ATPase and InsP3R. Such an effect could explain why truncation of the first 32 amino acids of the NH2 terminus will prevent ouabain-induced Ca2 oscillations. An alternative explanation is that NH2-terminal truncation has displaced the E1 3 E2 conformation of Na,K-ATPase in favor of E1 (8, 38). The concept that Na,K-ATPase may, in addition to its function as an ion pump, also act as a signal transducer, is now rapidly evolving (2-5). Most previous studies on the signaling role of Na,K-ATPase have been carried out on cardiac myocytes (for a review, see Ref. 39). In these cells, activation of Src kinase was found to be the primary event in the signaling cascade initiated by the ouabain/Na,K-ATPase complex, and downstream effects included phosphorylation of epidermal growth factor receptor and activation of the mitogen-activated protein kinase pathway. Since cardiac myocytes exhibit spontaneous Ca2 sparks of high frequency, it has not yet been possible to establish whether Na,K-ATPase/InsP3R-triggered Ca2 oscillations also occur in these cells. Another model where Na,K-ATPase-mediated Ca2 signaling is dependent on a close proximity between the plasma membrane and ER Ca2 stores has been presented previously (40). In this model, Na,KATPase 2- and 3-subunits modulate Ca2 release from ER via local changes in intracellular Na concentration. Our study was performed on cells expressing only the 1-subunit of Na,KATPase, and the results from the protocols with the truncated NKA1.M32 imply that increased