Recent studies have indicated that Na,K-ATPase may, in addition to being the key regulator of intracellular Na(+) and K(+) concentration, act as a signal transducer. Despite extensive research, the biological role for ouabain, a natural ligand of Na,K-ATPase, is not well understood. We have reported that exposure of rat proximal tubular cells (RPTC) to doses of ouabain that inhibit the Na,K-ATPase activity by less than 50% (10 nM - 500 micro M), will induce intracellular [Ca(2+)](i) oscillations and that this calcium signal leads to activation of the transcription factor NF-kappaB. The ouabain-induced calcium oscillations were blocked by an inhibitor of the IP(3) receptors but not by phospholipase C inhibitors nor by cellular depletion of IP(3), suggesting that the calcium signal is not due to phospholipase C-mediated IP(3) release. Fluorescence resonance energy transfer (FRET) studies suggested a close proximity between the Na,K-ATPase and IP(3) receptor. Our findings demonstrate a novel principle for calcium signaling via Na,K-ATPase.