To measure sarcomere length myocytes were placed in a perfusion chamber on the stage of an inverted microscope (Nikon) and continu- ously superfused with a Krebs-HEPES solution containing (mmol/L) NaCl 146.2; HEPES 10; glucosa 11; KCl 4.7; NaHPO 4 0.35; MgSO4 1; CaCl2 1.35 (pH 7.4) after continuous bubbling with 99.5% O2. The myocytes were stimulated via 2-platinum electrodes on either side of the bath at 0.5 Hz. The sarcomere length was recorded with specific software (Ion Wizard).