Depolarization of skeletal muscle cells in culture induced by K+ or by electrical stimulation, generates a complex Ca2+ release sequence of events both cytoplasmic and nuclear. A fast Ca2+ transient mediated by ryanodine receptors and associated to contraction, is followed by a slower Ca2+ transient induced by InsP3 receptors activation. K+-induced depolarization activates the transcription factors fos, jun and CREB, effect mediated by the slow but not by the fast Ca2+ component. These results suggest that Ca2+ signals specific properties are required for the activation of these particular factors in skeletal muscle cells. Myotubes depolarized by K+ or by tetanic electrical stimulation (45 Hz, 400 pulses) present a transient increase in calcium/calmodulin-dependent protein phosphatase calcineurin (CaN) activity (BIOMOL kit) of 2- and 3-4 fold, respectively. The K+-depolarization of myotubes, transfected with a NF-κB luciferase-reporter vector, enhances NF-κB transcription which is partially blocked both by cyclosporin A (CsA), a CaN inhibitor, and by inhibitors of the InsP3-induced slow Ca2+ component. Further evidence for CaN activation is that the increase in ERKs and CREB phosphorylation induced by this electrical stimulation protocol, is significantly increased by CsA. To further investigate the Ca2+ dependence over NF-κB, we have studied its activation by electrical stimulation in skeletal muscle cells kept in primary culture. Sprague-Dawley newborn rats were killed humanely, following the protocols approved by the Ethics Committee from Facultad de Medicina, Universidad de Chile. Different protocols of electrical stimulation permit the expression of fast and slow or only fast Ca2+ transients. Fast and slow Ca2+ transients are induced by 45 and 10 Hz protocols, while 1 Hz induces only the fast component. The fold induction of NF-κB-dependent transcription by 400 pulses of 45 and 10 Hz, was 2.02 ± 0.24 and 1.76 ± 0.28, respectively (n =4). Meanwhile, 1 Hz (400 pulses) activated the reporter gene by 1.30 ± 0.24 (n =4). Increasing to 1000 pulses, the reporter gene activation was 4.26 ± 0.29, 4.08 ± 0.55 and 3.50 ± 0.34 for 45, 10 and 1 Hz, respectively (n =4). These results, indicating a differential response of NF-κB to Ca2+ signals, are supported by IkBα degradation and p65 translocation experiments. Currently, we are assessing the contribution of both fast and slow Ca2+ transients on NF-κB activation by using pharmacological inhibitors. These experiments will allow to determine what kind of membrane electrical activity modulates CaN and NF-κB activation.