Fast skeletal muscle troponin activator CK‐2066260 mitigates skeletal muscle weakness independently of the underlying cause

Figures

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  FIGURE 1. CK‐2066260 increases low‐frequency force during the prolonged low‐frequency force depression caused by low‐intensity contractions. (A) Typical recordings of [Ca2+]i and force during repeated low‐intensity stimulation (50 Hz tetani of 350 ms duration given every 5 s for 100 contractions) in an isolated flexor digitorum brevis fibre. (B) Typical example of 30 and 120 Hz tetanic [Ca2+]i and force elicited in the unfatigued state (PRE), at 30 min of recovery (R30), and after an additional 5 min superfusion with 10 μM of CK‐2066260 (CK260).

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  FIGURE 2. CK‐2066260 reverses prolonged low‐frequency force depression after low‐intensity contractions. (A) Mean values of 30 and 120 Hz tetanic [Ca2+]i and force elicited in the unfatigued state (PRE), at 30 min of recovery (R30), and after an additional 5 min superfusion with 10 μM of CK‐2066260 (CK260). (B) Mean force–[Ca2+]i relationship obtained from 15 to 150 Hz stimulations in the unfatigued state (PRE), and 30 and 120 Hz force and tetanic [Ca2+]i elicited at 30 min of recovery (R30), and after 5 min superfusion with 10 μM CK‐2066260. Data shown as mean ± standard error of the mean (n = 5). *Significantly different from PRE at P < 0.05. †Significant main effect with depressed 120 Hz tetanic [Ca2+]i during prolonged low‐frequency force depression at P < 0.05, whereas the post hoc test did not show any significant difference between the individual groups.

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  FIGURE 3. CK‐2066260 increases force during prolonged low‐frequency force depression caused by high‐intensity stimulation. (A) Typical recordings of [Ca2+]i and force during the first and final (sixth) set of high‐intensity stimulation (100 Hz tetani of 250 ms duration given every 0.5 s for 60 contractions) in an isolated flexor digitorum brevis fibre. (B) Representative traces of 30 and 120 Hz tetanic [Ca2+]i and force elicited in the unfatigued state (PRE), at 30 min of recovery (R30), and after an additional 5 min of superfusion with 10 μM CK‐2066260 (CK260). Intriguingly, the almost completely abolished 30 Hz force at R30 is reversed by CK‐2066260 treatment by increasing myofibrillar Ca2+ sensitivity with no alteration of tetanic [Ca2+]i.

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  FIGURE 4. CK‐2066260 reverses the severe prolonged low‐frequency force depression induced by high‐intensity contractions. (A) Mean values of 30 and 120 Hz tetanic [Ca2+]i and force elicited in the unfatigued state (PRE), at 30 min of recovery (R30), and after an additional 5 min superfusion with 10 μM CK‐2066260 (CK260). (B) Mean force–[Ca2+]i relationship obtained from 15 to 150 Hz stimulations in the unfatigued state (PRE), and mean 30 and 120 Hz force–[Ca2+]i values obtained at 30 min of recovery (R30), and after an additional 5 min superfusion with 10 μM CK‐2066260. CK‐2066260 fully recovers force by markedly increasing myofibrillar Ca2+ sensitivity, while tetanic [Ca2+]i remains unaffected. Data shown as mean ± standard error of the mean (n = 5). *Significantly different from value at PRE at P < 0.05.

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  FIGURE 5. Prolonged low‐frequency force depression induced by repeated lengthening contractions is partially ameliorated by CK‐2066260 superfusion. (A) Typical force recordings from the first and last (10th) 100 Hz tetanus involving an initial isometric contraction at optimal length with a 20% lengthening step given midway during the contractions (dashed blue line). Representative traces and mean values (±standard error of the mean, n = 5) of (B) 30 Hz and (C) 120 Hz force elicited in the unfatigued state (PRE), at 30 min of recovery (R30), and after an additional 5 min superfusion with 10 μM CK‐2066260 (CK260). *Significantly different from value at PRE at P < 0.05.