INCREASE IN VERTICAL DIMENSION ALTERS MECHANICAL-PROPERTIES AND ISOMETRIC ATPASE ACTIVITY IN GUINEA-PIG MASSETER

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 Ca2+ activations in glycerinated masseter muscles (75 to 150 mu m in diameter and about 3 mm long) from normal and bite opened (6 mm increase in the vertical dimension, period of 1 week) guinea pigs. The isometric tension increased sigmoidally with an increase in Ca2+ concentration in both preparations. However, the bite opening shifted the pCa-relative tension relationship in the direction of increasing Ca2+ required for activation, and the pCa at Km (Ca2+ concentration required to develop half maximum tension) was 6.40 +/- 0.08 in control and 6.27 +/- 0.05 in the bite opened preparations. The maximum isometric tension at the saturating Ca2+ concentration was greater in the bite opened preparation. The ATPase activity changed almost in parallel with the tension (i.e., Ca2+ 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 Ca2+ 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 Ca2+ sensitivity in the isometric tension development and 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.