Abstract
Calcineurin, a Ca(2+)-calmodulin-dependent protein phosphatase (PP2B) is one of the links between Ca(2+) signals and regulation of gene transcription in cardiac muscle. We studied the Ca(2+) signal specificity of calcineurin activation experimentally and with modelling. In the rat atrial preparation, an increase in pacing frequency increased nuclear activity of the calcineurin-sensitive transcription factor, nuclear factor of activated T-cells (NFAT), 2-fold in a cyclosporin A (CsA)-sensitive manner. In line with this, modelling results predicted that the frequency of cardiac Ca(2+) transients encodes the stimulus for calcineurin activation. We further observed experimentally that calcineurin inhibition by CsA modulated Ca(2+) release in a Ca(2+)-dependent manner. CsA had no effect on [Ca(2+)](i) at a pacing frequency of 1 Hz but it significantly suppressed the amplitude of Ca(2+) transients, systolic [Ca(2+)](i) and time averaged [Ca(2+)](i) at 6 Hz. Calcineurin had a differential role in the expression of immediate-early genes B-type natriuretic peptide (BNP) and c-fos. CsA inhibited the pacing-induced BNP gene expression, whereas pacing alone had no effect on the expression of c-fos. However, in the presence of CsA, c-fos mRNA levels were significantly augmented by increased pacing frequency. These results show that frequency-dependent calcineurin activation has a specific role in [Ca(2+)](i) regulation and gene expression, constantly recruited by varying cardiac Ca(2+) signals.