Ca2+ sensitivities and transient tension responses to step-length stretches in feline mechanically-stripped single-fibre jaw-muscle preparations.

At a muscle length, Lo (just taut), isometric tension at constant levels of various Ca2+ activations and transient tension responses to rapid length stretches (less than 1 per cent of Lo within 2 ms) at maximal Ca2+ activation level were measured in temporal, masseter and digastric (anterior belly) muscles (2-3 mm long and 24-48 micron in diameter). Steady isometric tension increased in a sigmoid fashion with increasing Ca2+ concentration from about pCa 7.28 to 4.49 in temporalis, from about pCa 6.18 to 4.40 in masseter and from about pCa 5.82 to 4.40 in digastric. The maximum tension was 75.5 +/- 10.2 g/mm2 in temporalis, 44.7 +/- 14.1 g/mm2 in masseter, and 46.1 +/- 20.1 g/mm2 in digastric. In the resting state, the sarcomere length at Lo was 2.34 +/- 0.06 micron in temporalis, 2.20 +/- 0.08 micron in masseter, and 2.20 +/- 0.00 micron in digastric. When the sarcomere length was stretched from 2.20 to 2.34 micron (the sarcomere length of temporalis at Lo) in the masseter and digastric, the Ca2+ sensitivity increased without significant change of the maximum tension in either muscle. The transient tension responses in all three muscles showed two distinct phases; an immediate tension increase coincident with the length stretch followed by an exponential tension decrease. The mean value of the time constant in the second phase was 58.5 +/- 19.7 ms in temporalis, 58.5 +/- 12.6 ms in masseter, and 362.6 +/- 16.8 ms in digastric. Thus temporalis showed a higher Ca2+ sensitivity at Lo and a greater maximum tension-producing capability than the other muscles and the cross-bridge turnover rate appears to be slower in digastric than in the others.