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Calcium (Ca(2+)) oscillations are ubiquitous signals present in all cells that provide efficient means to transmit intracellular biological information. Either spontaneously or upon receptor ligand binding, the otherwise stable cytosolic Ca(2+) concentration starts to oscillate. The resulting specific oscillatory pattern is interpreted by intracellular downstream effectors that subsequently activate different cellular processes. This signal transduction can occur through frequency modulation (FM) or amplitude modulation (AM), much similar to a radio signal. The decoding of the oscillatory signal is typically performed by enzymes with multiple Ca(2+) binding residues that diversely can regulate its total phosphorylation, thereby activating cellular program. To date, NFAT, NF-κB, CaMKII, MAPK and calpain have been reported to have frequency decoding properties. The basic principles and recent discoveries reporting frequency decoding of FM Ca(2+) oscillations are reviewed here. A limited number of cellular frequency decoding molecules of Ca(2+) oscillations have yet been reported. Interestingly, their responsiveness to Ca(2+) oscillatory frequencies shows little overlap, suggesting their specific roles in cells. Frequency modulation of Ca(2+) oscillations provides an efficient means to differentiate biological responses in the cell, both in health and in disease. Thus, it is crucial to identify and characterize all cellular frequency decoding molecules to understand how cells control important cell programs.

The plant-derived steroid, digoxin, a specific inhibitor of Na,K-ATPase, has been used for centuries in the treatment of heart disease. Recent studies demonstrate the presence of a digoxin analog, ouabain, in mammalian tissue, but its biological role has not been elucidated. Here, we show in renal epithelial cells that ouabain, in doses causing only partial Na,K-ATPase inhibition, acts as a biological inducer of regular, low-frequency intracellular calcium ([Ca ²⁺ ] _i ) oscillations that elicit activation of the transcription factor, NF-κB. Partial inhibition of Na,K-ATPase using low extracellular K ⁺ and depolarization of cells did not have these effects. Incubation of cells in Ca ²⁺ -free media, inhibition of voltage-gated calcium channels, inositol triphosphate receptor antagonism, and redistribution of actin to a thick layer adjacent to the plasma membrane abolished [Ca ²⁺ ] _i oscillations, indicating that they were caused by a concerted action of inositol triphosphate receptors and capacitative calcium entry via plasma membrane channels. Blockade of ouabain-induced [Ca ²⁺ ] _i oscillations prevented activation of NF-κB. The results demonstrate a new mechanism for steroid signaling via plasma membrane receptors and underline a novel role for the steroid hormone, ouabain, as a physiological inducer of [Ca ²⁺ ] _i oscillations involved in transcriptional regulation in mammalian cells.