Myofilament length-dependent activation develops within 5 ms in guinea-pig myocardium

Abstract

Myofilament length-dependent activation is a universal property of striated muscle, yet the molecular mechanisms that underlie this phenomenon are incompletely understood. Additionally, the rate by which sarcomere length (SL) is sensed and then transduced to form length-dependent activation is unknown. Here, using isolated guinea-pig myocardium, we employed a rapid solution-switch single myofibril technique that allows for the study of contractile action/relaxation dynamics in the virtual absence of diffusion delays. We compared contraction kinetics obtained at submaximal activation at steady-state SL with contractions observed after rapid SL ramps to that same SL just before activation. Neither the activation and relaxation kinetics nor the final submaximal force development differed significantly between the two contraction modes for SL ramps as fast as 5 ms. We conclude that the transduction of the length signal by the cardiac sarcomere to modulate thin filament activation levels occurs virtually instantaneously, possibly resulting from structural rearrangements of the contractile proteins.

Figure 1

Myofibrils were attached to glass micro tools (A) that were placed in a laminar solution flow emanating from a double-barreled pipette (B). Rapid translation of the pipette allows for rapid solution switching. (C) Force (top), myofibril length (middle), and PMT output (bottom). (D) The five consecutive myofibril activation-relaxation cycles (open bar, pCa = 9; closed bar, pCa = 4.0; hatched bar, pCa = 5.7). Calibration bars: 40 nN for force, 0.15 L_o for length, 2 s (D), and 200 ms (C).

Figure 2

(A) Submaximal force at steady-state SL = 2.3 μm (blue trace) or after a quick stretch from steady-state SL = 2.0 μm (red trace; black trace = passive force). (B) The developed force (total force minus passive force). Calibration bars: 40 nN and 2 s.

Figure 3

(A) Submaximal force at steady-state SL = 2.3 μm (blue trace) or after a quick stretch from steady-state SL = 2.0 μm (red trace). (B) Submaximal activation at steady SL = 2.3 μm (blue trace) or after a quick release from steady-state SL = 2.5 μm (green trace). Inorganic phosphate was removed by an enzymatic phosphate mop. Calibration bars: 40 nN for force, 0.15 L_o for muscle length, and 2 s for time.