Effects of muscle length on intracellular Ca2+ during isometric contraction of tracheal smooth muscle

Critical role of actin-associated proteins in smooth muscle contraction, cell proliferation, airway hyperresponsiveness and airway remodeling

Asthma is characterized by airway hyperresponsiveness and airway remodeling, which are largely attributed to increased airway smooth muscle contractility and cell proliferation. It is known that both chemical and mechanical stimulation regulates smooth muscle contraction. Recent studies suggest that contractile activation and mechanical stretch induce actin cytoskeletal remodeling in smooth muscle. However, the mechanisms that control actin cytoskeletal reorganization are not completely elucidated. This review summarizes our current understanding regarding how actin-associated proteins may regulate remodeling of the actin cytoskeleton in airway smooth muscle. In particular, there is accumulating evidence to suggest that Abelson tyrosine kinase (Abl) plays a critical role in regulating airway smooth muscle contraction and cell proliferation in vitro, and airway hyperresponsiveness and remodeling in vivo. These studies indicate that Abl may be a novel target for the development of new therapy to treat asthma.

Influence of intercellular tissue connections on airway muscle mechanics

Contraction of smooth muscle in visceral organs is modified by structures external to the muscle. Within muscle tissue itself, connective tissue plays an important role in force transference among the contractile cells. Connections arranged radially can affect contractile mechanics by limiting tissue expansion at short lengths. Previous work suggests that increased stiffness at extreme shortening is due to such radial constraints. Two approaches to further study of these effects are reported. To increase radial constraints, very thin Silastic bands were placed loosely about strips of canine trachealis muscle at rest length. The strips were allowed to shorten under light afterloads, expanding until restrained by the bands. Subsequent removal of the bands allowed increased shortening, with less increase in stiffness at short lengths. Related isometric effects were observed. To reduce constraints, muscle strips were partially digested with collagenase. Compared with control conditions, this treatment permitted further shortening, with less increase in stiffness at short lengths. These results emphasize the role of extracellular structures in determining mechanical function of smooth muscle.

Potentiation of smooth muscle contractility after rapid changes in isotonic force

The sudden application of step increases in afterload (0.4-3.0 s in duration) arrested the isotonic shortening of electrically stimulated ovarian ligament smooth muscle strips from rabbits. Force steps were chosen to produce, after initial rapid yielding, a quasi-steady state in which muscle length and force remained constant. Removal of the extra afterload allowed renewed shortening that began with a velocity transiently greater than that measured before the force step (at same muscle length). The rate of force redevelopment was also transiently potentiated under isometric conditions after the removal of the extra load. Both types of potentiation depended on force-step duration, and the transients decayed exponentially with a time constant of approximately 0.25 s. The stiffness of the muscle during the force step was initially depressed but then increased along an exponential time course while force and length remained constant. These observations are consistent with an initial detachment of a portion of the cross-bridge array, which then reattached during the course of the force step, with potentiation being due to either a transient increase in cycling rate or a time-dependent reconfiguration of cytoskeletal elements supporting the contractile system.

Persistent mechanical effects of decreasing length during isometric contraction of ovarian ligament smooth muscle

When isometrically-contracting strips of ovarian ligament smooth muscle were suddenly shortened by 10-20% of their length, force fell rapidly and then redeveloped along an exponential time course. The amount of force recovered fell short of that expected in an isometric contraction at the new length, and this force deficit was proportional to the magnitude of the length step (approximately 80% of force was recovered after a 10% shortening). A sudden imposed decrease in length was more effective in reducing subsequent force than was isotonic shortening. Early in the recovery phase the stiffness of the muscle was decreased to less than its expected value; stiffness recovered to expected levels on an exponential time course approximately three to four times faster than force recovery itself. Force-velocity curves made during the redevelopment phase showed a reduced maximal force (Fmax) and an increased maximal shortening velocity (Vmax) when compared with control contractions matched in force, time and length. The curves crossed at approximately 10% of Fmax. During isometric relaxation the muscles showed an increase in their expected stiffness; prior imposed shortening (as above) reduced the relaxation stiffness increase in proportion to the prior force deficit. The persistent effects of early events on the later phases of the contraction, as well as the increase in shortening velocity with very light loads, are consistent with the hypothesis that the sudden shortening detaches crossbridges and that same fail to reattach during force recovery. During isotonic shortening of unperturbed muscle some slowly-cycling crossbridges may act as an internal load and reduce shortening velocity.