Mechanical changes after histamine activation of intact single vascular smooth muscle cells.

Although several different models have been proposed to explain force maintenance in vascular smooth muscle, the molecular mechanism responsible for this phase of contraction has yet to be elucidated. To investigate the molecular mechanism for force maintenance, force and stiffness were measured during (1 microM histamine) activation of single intact arterial smooth muscle cells. After histamine stimulation, the rise in quadrature stiffness preceded force (P < 0.05) and reached a steady state plateau before force (P < 0.05). These data suggest that the number of cycling cross-bridges increases during force activation and then remains constant during force maintenance. In-phase stiffness, on the other hand, continued to increase in amplitude after force had reached a steady state. The increase in the in-phase stiffness during force maintenance suggests that during force maintenance either a population of cross-bridges in an attached nonforce producing state develop or noncross-bridge force bearing structures are formed.