This is really cool, congratulations! Was there any indication of a change in resting I1,1/I1,0 in the T2DM group?

I've just had a paper published which uses cartoons of the thick and thin filaments that may also be helpful to explain how muscle X-ray people use each part of the X-ray patterns to ascribe to certain sarcomere structures and how they change with contraction:
www.pnas.org/doi/10.1073/...

To summarise, removing load is more effective at relaxing the muscle than Ca2+. When Ca2+ is removed, a conformational change in troponin may initiate the initial detachment of myosin from actin. Yielding of weak sarcomeres and tendon compliances determine muscle relaxation.

When examining sarcomere length, SL remains constant during the first isometric phase of relaxation (blue). When force starts to decline, weak sarcomeres being to yield and stretch as identified by two populations of sarcomeres, eventually resulting in exponential force decay.

In this period called "isometric relaxation" motors begin to detach as signalled by a decrease in the equatorial intensity ratio as myosin moves away from actin (blue), meaning the strain per attached motor increases.

If both the thick and thin filaments are remaining ON given the high load and low Ca2+ after electrical stimulation, how does myosin escape their actin-attached, force-generating state?

IAL2 increases as motors attach to actin and tropomyosin moves into the actin grooves. When load is removed and myosin detaches, IAL2 ↓ despite the high Ca2+ as tropomyosin is allowed to move out of the grooves between actin. When Ca2+ ↓, the change in IAL2 is similar to force.

When load is removed (grey shade) the thick filament begins to switch off as motors detach, and occurs much faster than force due to the small number of motors required to drive shortening. When Ca2+ is removed IML1 remains low (blue shade) and slowly returns to its OFF state.

X-ray diffraction patterns were collected and reflections ascribed to certain sarcomere structures. (See ALT text for description of each reflection examined). I want to draw attention to two reflections in particular - the ML1 and AL2 reflections.

To test this, we visited ID02 at
@esrf.fr to perform time-resolved X-ray diffraction on mouse EDL muscles. To separate out the contribution of load and [Ca2+]i we imposed ramp shortening to remove load when [Ca2+]i was high, and by allowing reuptake of Ca2+ by the SR.

This paradox poses a fundamental question: how does a muscle relax following contraction given the above paradox of high load and actin-bound motors preventing the thin filament switching off

Muscle relaxation has been long viewed as a reversal of this activation paradigm, but actin-bound myosin motors prevent the return of tropomyosin back to the "blocked" position, suggesting that motor detachment rather than Ca2+ dissociation may be a key determinant of relaxation.

When [Ca2+]i increases on electrical stimulation, troponin becomes saturated with Ca2+ triggering a movement of troponin and tropomyosin exposing binding sites. Motors then leave their helical tracks in a load-dependent manner. High load + mechanosensing = rapid activation.

In resting muscle myosin motors in the thick filaments are arranged in an OFF state unavailable for actin binding, with a tiny fraction of sentinel motors. Troponin and tropomyosin in the actin-containing thin filaments are also OFF at low [Ca2+]i, preventing myosin binding.

The meeting can be accessed via the below link:

duke.zoom.us/j/91574693682 (Meeting ID: 915 7469 3682)

#MyoBlue