Time-of-day effects on ex vivo muscle contractility following short-term satellite cell ablation.

Muscle isometric torque fluctuates according to time-of-day with such variation owed to the influence of circadian molecular clock genes. Satellite cells (SC), the muscle stem cell population, also express molecular clock genes with several contractile related genes oscillating in a diurnal pattern. Currently, limited evidence exists regarding the relationship between SCs and contractility, although long-term SC ablation alters muscle contractile function. Whether there are acute alterations in contractility following SC ablation and with respect to the time-of-day is unknown. We investigated whether short-term SC ablation affected contractile function at two times of day, and whether any such alterations lead to different extents of eccentric contraction-induced injury. Utilizing an established mouse model to deplete SCs, we characterized muscle clock gene expression and ex vivo contractility at two times-of-day (morning 0700 h and afternoon, 1500 h). Morning-SC+ animals demonstrated ~25-30% reductions in tetanic/eccentric specific forces and, after eccentric injury, exhibited ~30% less force-loss and ~50% less dystrophinnegative fibers versus SC- counterparts; no differences were noted between Afternoon groups (Morning-SC+ : -5.63 ± 0.61, Morning-SC- : -7.93 ± 0.61; N/cm2 ; p < 0.05) (Morning-SC+ : 32 ± 2.1, Morning-SC- : 64 ± 10.2; dystrophinnegative fibers; p < 0.05). As Ca++ kinetics underpin force-generation, we also evaluated caffeine-induced contracture-force as an indirect marker of Ca++ availability, and found similar force reductions in Morning-SC+ vs SC- mice. We conclude that force-production is reduced in the presence of SCs in the morning but not the afternoon, suggesting that SCs may have a time-of-day influence over contractile-function.