Ouabain, a steroid hormone that signals with slow calcium oscillations

The plant-derived steroid, digoxin, a specific inhibitor of Na,K-ATPase, ha s 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 e pithelial cells that ouabain, in doses causing only partial Na,K-ATPase inh ibition, acts as a biological inducer of regular, low-frequency intracellul ar calcium ([Ca2+](i)) oscillations that elicit activation of the transcrip tion factor, NF-KB. Partial inhibition of Na,K-ATPase using low extracellul ar K+ and depolarization of cells did not have these effects. Incubation of cells in Ca2+-free media, inhibition of voltage-gated calcium channels, in ositol triphosphate receptor antagonism, and redistribution of actin to a t hick layer adjacent to the plasma membrane abolished [Ca2+](i) oscillations , indicating that they were caused by a concerted action of inositol tripho sphate receptors and capacitative calcium entry via plasma membrane channel s. Blockade of ouabain-induced [C-a2+](i) oscillations prevented activation of NF-kappaB. The results demonstrate a new mechanism for steroid signalin g via plasma membrane receptors and underline a novel role for the steroid hormone, ouabain, as a physiological inducer of [Ca2+](i) oscillations invo lved in transcriptional regulation in mammalian cells.