Contractile and endurance properties of feline triangularis sterni muscle

The triangularis sterni muscle has recently been found to play an active role in the modulation of airflow during expiration in several species; furthermore its electrical activity is influenced by many chemical and mechanical stimuli which influence breathing. To determine feline triangularis sterni contractile and endurance properties, strips of triangularis sterni and costal diaphragm muscle were removed from anesthetized ventilated cats, and studied in vitro. The isometric contractile kinetics of the two muscles were similar; contraction times were 37 +/- 2 and 36 +/- 1 ms for the triangularis sterni and diaphragm, respectively. However, the twitch to tetanic tension ratio of the triangularis sterni was lower than that of the diaphragm (0.19 +/- 0.01 versus 0.37 +/- 0.03; P less than 0.001), and the force frequency relationship of the triangularis sterni was located to the right of that of the diaphragm. Repetitive stimulation (40 Hz trains, duty cycle 0.33) produced a greater decline in force for the diaphragm than the triangularis sterni. The fatigue index (ratio of force at 2 min to initial force) was significantly higher for the triangularis sterni (0.31 +/- 0.04) than for the diaphragm (0.18 +/- 0.02; P less than 0.01). These data indicate that the contractile and endurance properties of the feline triangularis sterni are different in some but not all respects from those of the diaphragm, which may reflect adaptations to patterns of use during breathing.