Background: We studied the effect of the calmodulin antagonist trifluoperazine (TFP) on isolated intact rat tracheal and pulmonary artery smooth muscle contractile behaviour.
Methods: Experimental series: 1) TFP-dose-response curves for TFP's effect on force generation were constructed using rat tracheal smooth muscles and rat pulmonary artery preparations (n = 8). A concentration of 1 micromol/l TFP was chosen for the subsequent experimental series. 2) Tracheas and pulmonary arteries (n = 14) were dissected in three segments. One of them was used immediately for experiments ("native"), the other two were treated for 12 h in 4 degrees C Tyrode solution without ("12 h cold storage") or with 1 micromol/l TFP ("12 h cold storage + TFP"). These preparations contracted after supramaximal effective electrical field stimulation. The force-clamping technique was used to analyse kinetic and mechanical parameters of smooth-muscle contraction in both types of preparation (measurement conditions: resting tension 2 mN, 37 degrees C, modified Krebs-Henseleit solution).
Results: 1) TFP decreased developed force dose-dependently in pulmonary artery and tracheal smooth muscle. 2) During sustained tonic activation, the contraction kinetics become slower both with and without TFP treatment (p < 0.0001). 3) TFP caused a dramatic retardation of the velocity of force generation in both types of preparation for any given time interval during the course of a tonic activation (p < 0.0005). 4) The dramatic effects of TFP on the contraction kinetics were not associated with effects on the extent of force generation.
Conclusions: These results support the assumption that tracheal and pulmonary artery smooth muscle cross-bridge rates are controlled by a calcium-calmodulin-dependent myosin light chain kinase. This finding suggests the involvement of a calmodulin-independent regulator process responsible for the changes observed in the cross-bridge cycling rates during sustained tonic activation. A direct intervention on the contractile apparatus level is a measure for reduction of smooth-muscle tone without negative inotropic side effects.