Isobaric force-velocity relationship of in situ canine trachealis muscle

We measured the force-velocity relationship of the in situ trachealis muscle during vagal stimulation in six anesthetized dogs. The trachea, about 10 cm in length, was transected above the carina and left in situ with its blood supply intact, and its cut ends artificially closed. The lung was ventilated by a respiratory. First the vagal nerves were electrically stimulated to induce isometric contractions which resulted in an increase in tracheal pressure; then the tracheal volume was reduced to a rate (velocity) which kept the tracheal pressure constant at a given level (force). The force-velocity curve was hyperbolic. Using Hill's equation, we calculated the values of P0 (the maximum force at 10, lo being the trachealis muscle length at which the maximum active tension develops) and Vmax (the maximum velocity at zero afterload). They were P0 = 0.85 +/- 0.20 kg/cm2 and Vmax = 0.08 +/- 0.02 l0/s. We concluded that both force and velocity of the trachealis muscle in situ exhibited lower values than those in vitro.