Hypermuscular mice with mutation in the myostatin gene display altered calcium signalling

Key points Hypermuscularity associated with naturally occurring mutations in the myostatin gene as found in Compact mice results in increased muscle mass but reduced specific force. The calcium sensitivity of the contractile apparatus as assessed on chemically skinned skeletal muscle fibres under isometric conditions is not altered in these animals. While the resting calcium concentration remains unaffected, depolarization-evoked increases in intracellular calcium concentration are suppressed. Spontaneous calcium release events from sarcoplasmic reticulum are also decreased in frequency, amplitude and spatial spread. Our results suggest that mutations in the myostatin gene are accompanied by alterations in excitation contraction coupling, which manifest as a reduction in sarcoplasmic calcium release. Myostatin, a member of the transforming growth factor beta family, is a potent negative regulator of skeletal muscle growth, as myostatin-deficient mice show a great increase in muscle mass. Yet the physical performance of these animals is reduced. As an explanation for this, alterations in the steps in excitation-contraction coupling were hypothesized and tested for in mice with the 12 bp deletion in the propeptide region of the myostatin precursor (Mstn(Cmpt-dl1Abc) or Cmpt). In voluntary wheel running, control C57BL/6 mice performed better than the mutant animals in both maximal speed and total distance covered. Despite the previously described lower specific force of Cmpt animals, the p(Ca)-force relationship, determined on chemically permeabilized fibre segments, did not show any significant difference between the two mouse strains. While resting intracellular Ca2+ concentration ([Ca2+](i)) measured on single intact flexor digitorum brevis (FDB) muscle fibres using Fura-2 AM was similar to control (72.0 +/- 1.7 vs. 78.1 +/- 2.9 nm, n = 38 and 45), the amplitude of KCl-evoked calcium transients was smaller (360 +/- 49 vs. 222 +/- 45 nm, n = 22) in the mutant strain. Similar results were obtained using tetanic stimulation and Rhod-2 AM, which gave calcium transients that were smaller (2.42 +/- 0.11 vs. 2.06 +/- 0.10 Delta F/F-0, n = 14 and 13, respectively) on Cmpt mice. Sarcoplasmic reticulum (SR) calcium release flux calculated from these transients showed a reduced peak (23.7 +/- 3.0 vs. 15.8 +/- 2.1 mMs(-1)) and steady level (5.7 +/- 0.7 vs. 3.7 +/- 0.5 mm s(-1)) with no change in the peak-to-steady ratio. The amplitude and spatial spread of calcium release events detected on permeabilized FDB fibres were also significantly smaller in mutant mice. These results suggest that reduced SR calcium release underlies the reduced muscle force in Cmpt animals.