Length-dependent filament formation assessed from birefringence increases during activation of porcine tracheal muscle

Abstract

Birefringence and force produced by pig tracheal smooth muscles were recorded every 100 ms during electrically stimulated tetani at muscle lengths that varied 1.5-fold and at the peak of acetylcholine contractures at the same lengths. Isometric force was nearly the same at all lengths. Resting birefringence at the longest length was 30% greater than that at the shortest length. During tetani, birefringence increased with approximately the same time course as force, rising by 20% at the shortest length and 9% at the longest length, and continued to increase by an additional 0.5-1.5% of the resting value for 2-8 s after stimulation ended and force began to fall. This late increase was greatest and more sustained at longer lengths. During contractures, birefringence increased by 25 and 18% at the shortest and longest lengths, respectively. Comparison of these results with our published thick-filament densities suggests that thick-filament density increased by about 80, 72 and 50% during contractures at the short, intermediate and long lengths, and that approximately 35% of birefringence in the resting muscle at the longest length was not due to thick filaments. These findings support the hypotheses that tracheal smooth muscle adapts to longer lengths by increasing thick-filament mass and that myosin thick filaments are evanescent, dissociating partially during relaxation and reforming upon activation. The results further suggest that thick-filament formation is sufficiently rapid to account for the velocity slowing and some of the force increase observed during the rise of activation of tracheal smooth muscle.

Figure 7. Patterns traced by the electric vectors of initially plane-polarized light beams before and after passing through a muscle and then a ¹/₄-waveplate compensator

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Figure 1. Birefringence microscope

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Figure 2. Birefringence signals on an absolute scale

Muscles were adapted at the three lengths indicated at the ends of the records, and recording began 0.1 s before a 12.5 s electrical stimulation. The inset below the upper record shows the first 0.8 s of recording at this length, with the birefringence scale amplified 50-fold and the time scale amplified 15-fold.

Figure 3

Increase in force and birefringence with 12.5 s electrical stimulation, normalized to their increases during the stimulation.

Figure 4

Birefringence records at three lengths superimposed to show the similarity of time course during the rise of activation and the divergence when stimulation ended.

Figure 5. Birefringence (upper) and force (lower) at three muscle lengths

Each panel plots separately the resting values (□), values at the end of electrical stimulation (○), and peak values achieved during acetylcholine contractures (▿). For the electrically stimulated muscles, two birefringence values are plotted: the value at the end of stimulation (○) and the peak value achieved after stimulation ended (▵). Active force plotted is total force, i.e. rest force plus developed force. Error bars indicate s.e.m. for the absolute values.

Figure 6. Birefringence versus previously published filament densities in muscles during contracture (filled symbols and fitted line)

For relaxed muscles (open symbols), birefringence values have been reduced by 10% to account for crossbridge movement during activation and plotted on the line fitted to the data for muscles in contracture. The values for filament density in the relaxed muscle were read from the abscissa scale.