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 (SCs), 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 led to different extents of eccentric contraction-induced injury. Using an established mouse model to deplete SCs, we characterized muscle clock gene expression and ex vivo contractility at two times-of-day (morning: 0700 and afternoon: 1500). Morning-SC + animals demonstrated ∼25%-30% reductions in tetanic/eccentric specific forces and, after eccentric injury, exhibited ∼30% less force-loss and ∼50% less dystrophin negative fibers versus SC - counterparts; no differences were noted between Afternoon groups (Morning-SC + : -5.63 ± 0.61, Morning-SC - : -7.93 ± 0.61; N/cm 2 ; P < 0.05) (Morning-SC + : 32 ± 2.1, Morning-SC - : 64 ± 10.2; dystrophin negative 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 in the afternoon, suggesting that SCs may have a time-of-day influence over contractile function. NEW & NOTEWORTHY Muscle isometric torque fluctuates according to time-of-day with such variation owed to molecular clock regulation. Satellite cells (SCs) have recently demonstrated diurnal characteristics related to muscle physiology. In our work, force production was reduced in the presence versus absence of SCs in the morning but, not in the afternoon. Morning-SC + animals, producing lower force, sustained lesser degrees of injury versus SC - counterparts. One potential mechanism underpinning lower forces produced appears to be lower calcium availability.