|
To study the changes in mechanical and metabolic properties associated with an increase in the vertical dimension of the face, isometric tension, isometric ATPase activity, unloaded shortening velocity (V_max), and the tension transients in response to step stretches in length were measured at constant levels of various Ca^2+ activations in glycerinated masseter muscles (75 to 150㎛ in diameter and about 3㎜ long) from normal and bite opened (6㎜ increase in the vertical dimension, period of 1 week) guinea pigs. The ATPase activity changed almost in parallel with the tension (i.e., Ca^2+ concentration) in both preparations. However, the tension cost (ATPase activity/tension) was significantly(p<0.05) lower in the bite opened preparation, compared with the control. The V_max at the saturating Ca^2+ concentration was lower in the bite opened preparations. The tension transients in response to step stretch in length were characterized by four distinct phases; the first phase of an immediate tension increase coincident with the stretch, the second phase of a rapid quasi-exponential tension reduction, the third phase of a minute tension increase and the fourth phase of a slow tension reduction. The time constant of the second phase (denoted by t_63%), which is believed to reflect kinetics of turnover rate of crossbridges, was 4.7 msec in control and 11.0 msec in the bite opened preparations. These results suggest that the increase in the vertical dimension for 1-week decreases the Ca^2+ sensitivity in the isometric tension development ans alters the dynamic mechanical properties of masseter muscle so as to decrease the tension cost by decreasing the cycling rate of crossbridges. The economical ATP consumption in the bite opened preparation may partly contribute to the relief of muscle spasm with an increase in the vertical dimension. (This paper is the Korean translation of AM J ORTHOD DENTOFAC ORTHOP 1993;104:484-91.)
|