Stimulus-specific changes in mechanical properties of vascular smooth muscle

To determine whether the mechanical properties of vascular smooth muscle are stimulus specific, force, stiffness, and the unloaded shortening velocity (Vmax) were measured during contractions of aortic smooth muscle strips stimulated with phenylephrine or KCl. After activation, muscle force and stiffness rose to a steady-state plateau where they were maintained. In phenylephrine contractions, Vmax peaked during force development and then fell to a lower steady-state level during force maintenance, whereas in the KCl contractions, Vmax did not decline during sustained contractions. Stimulation with KCl, compared with phenylephrine, produced lower steady-state forces. One possible interpretation is that the muscle formed latch cross-bridges during phenylephrine contractions, but not during KCl depolarizations. The slope of the plot of relative muscle force vs. stiffness for phenylephrine contractions, compared with KCl depolarizations, was reduced. This may imply tht the relative force per attached latch crossbridge could be reduced.