A NONISOMETRIC KINETIC-MODEL FOR SMOOTH-MUSCLE

We have modeled the nonisometric contractile dynamics of smooth muscle by modifying a four-state model of actin and myosin bonds originally proposed by Hai and Murphy to simulate the isometric contractions of v ertebrate smooth muscle. The model includes a latch bridge, which cycl es more slowly than regular cross bridges. We generalized this model t o represent the calcium-regulated processes of vertebrate and inverteb rate smooth muscles. We added length dynamics by assuming length-depen dent bonding and unbonding rates for the cross bridges. The calculatio n of the cross-bridge length distribution was simplified by assuming a Gaussian distribution, as first done by Zahalak for skeletal muscle. To test the performance of this model, we simulated isometric and noni sometric responses of different kinds of smooth muscle, including vasc ular smooth muscle, airway smooth muscle, molluscan catch muscle (ante rior byssus retractor muscle), and Aplysia 12 muscle. The model captur es the economical force maintenance property at the later stages of is ometric muscle contraction and responses to imposed lengthening and sh ortening movements.