Figure 2. Relative amounts of myosin heavy chain (MHC) isoform IIx (a) and IIb (b) mRNA (mean ± SEM, n=4) in cultured rat myocytes (muscle cells) measured with Northern blotting (Higginson et al., ). ▲ Control (no pharmacological treatment). ● Cyclosporin A (calcineurin pathway inhibitor) treated. ■ U0126 (ERK1/2 pathway inhibitor) treated. Blockade of the calcineurin pathway resulted in significantly higher levels of MHC IIx while blockade of the ERK1/2 pathway resulted in significantly higher levels of MHC IIx and MHC IIb. These results suggest that the calcineurin pathway suppresses the MHC IIx isoform when activated by exercise. The ERK1/2 pathway suppresses the MHC IIx and MHC IIb isoforms when activated by exercise.