Barium ions reduce contraction kinetics in rat tracheal smooth muscle

In isolated rat tracheal smooth muscle, contraction was induced by electrical field stimulation (= EL; frequency = 30 Hz; pulse duration = 0.17 ms), 10 μmol/l acetylcholine (= ACh), potassium depolarization (137 mmol/l K), or by 10 mmol/l barium (= BA). Contraction kinetics were studied by analyzing tension recovery after cessation of a 1.8s length vibration (100 Hz sinusoida; amplitude = 6% of the muscle length). The contraction speed was high during EL as can be seen from the short time constant of post-vibration tension recovery (τ=5.90±0.14 s) found 30 s after onset of stimulation. The time constants of tension recovery during long-term (50 min) activation averaged 12.88±0.32 s (K) and 13.24±0.17 s (ACh) when the vibration was stopped 8–45 min after onset of activation. As both of these stimuli act mainly via cholinergic receptors, similar down-regulated contraction kinetics occur under steady-state conditions of tonic contraction. However, during barium activation steady-state conditions need a 45 min agonist incubation, and the time constant of post-vibration tension recovery was extended to 34.05±2.21 s. Thus, calcium may be replaced by barium in the force generation process but it produces slower cycling of cross-bridges.