Protein kinase A treatment increased stretch activation by up to 50% during low calcium activation in rat slow-twitch skeletal muscle fibers compared to baseline.
Slow-skeletal myosin binding protein-C phosphorylation and cardiac troponin I phosphorylation regulate stretch activation through cross-bridge recruitment and faster cycling kinetics, offering potential targets for enhanced contractility.
Stretch activation is defined as a delayed increase in force after rapid stretches. Although there is considerable evidence for stretch activation in isolated cardiac myofibrillar preparations, few studies have measured mechanisms of stretch activation in mammalian skeletal muscle fibers. We measured stretch activation following rapid step stretches ∼1%–4% sarcomere length (SL) during submaximal Ca 2+ activations of rat permeabilized slow-twitch skeletal muscle fibers before and after protein kinase A (PKA), which phosphorylates slow myosin binding protein-C. PKA significantly increased stretch activation during low (∼25%) Ca 2+ activation and accelerated rates of delayed force development ( k ef ) during both low and half-maximal Ca 2+ activation. Following the step stretches and subsequent force development, fibers were rapidly shortened to original sarcomere length, which often elicited a shortening-induced transient force overshoot. After PKA, step shortening-induced transient force overshoot increased ∼10-fold following an ∼4% SL shortening during low Ca 2+ activation levels. k df following step shortening also increased after PKA during low and half-maximal Ca 2+ activations. We next investigated thin filament regulation of stretch activation. We tested the interplay between cardiac troponin I (cTnI) phosphorylation at the canonical PKA and novel tyrosine kinase sites on stretch activation. Native slow-skeletal Tn complexes were exchanged with recombinant human cTn complex with different human cTnI N-terminal pseudo-phosphorylation molecules: 1) nonphosphorylated wild type (WT), 2) the canonical S22/23D PKA sites, 3) the tyrosine kinase Y26E site, and 4) the combinatorial S22/23D + Y26E cTnI. All three pseudo-phosphorylated cTnIs elicited greater stretch activation than WT. Following stretch activation, a new, elevated stretch-induced steady-state force was reached with pseudo-phosphorylated cTnI. Combinatorial S22/23D + Y26E pseudo-phosphorylated cTnI increased k df . These results suggest that slow-skeletal myosin binding protein-C (sMyBP-C) phosphorylation modulates stretch activation by a combination of cross-bridge recruitment and faster cycling kinetics, whereas cTnI phosphorylation regulates stretch activation by both redundant and synergistic mechanisms; and, taken together, these sarcomere phosphoproteins offer precision targets for enhanced contractility.
Robinett et al. (Tue,) conducted a other in None (Healthy animal model). Protein kinase A (PKA) treatment and cTnI pseudo-phosphorylation vs. Baseline (before PKA) or Wild Type (WT) cTnI was evaluated on Stretch activation (PTO/PO) and rate of delayed force development (kdf). Protein kinase A treatment increased stretch activation by up to 50% during low calcium activation in rat slow-twitch skeletal muscle fibers compared to baseline.