Ca2+ transients activate calcineurin/NFATc1 and initiate fast-to-slow transformation in a primary skeletal muscle culture.

The calcineurin-mediated signal transduction via nuclear factor of activated T cells (NFATc1) is involved in upregulating slow myosin heavy chain (MHC) gene expression during fast-to-slow transformation of skeletal muscle cells. This study aims to investigate the Ca2+ signal necessary to activate the calcineurin-NFATc1 cascade in skeletal muscle. Electrostimulation of primary myocytes from rabbit for 24 h induced a distinct fast-to-slow transformation at the MHC mRNA level and a full activation of the calcineurin-NFATc1 pathway, although resting Ca2+ concentration ([Ca2+]i) remained unaltered at 70 nM. During activation, the calcium transients of these myocytes reach a peak concentration of approximately 500 nM. Although 70 nM [Ca2+]i does not activate calcineurin-NFAT, we show by the use of Ca2+ ionophore that the system is fully activated when [Ca2+]i is >or=150 nM in a sustained manner. We conclude that the calcineurin signal transduction pathway and the slow MHC gene in cultured skeletal muscle cells are activated by repetition of the rapid high-amplitude calcium transients that are associated with excitation-contraction coupling rather than by a sustained elevation of resting Ca2+ concentration.