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 vertebrate smooth muscle. The model includes a latch bridge, which cycles more slowly than regular cross bridges. We generalized this model to represent the calcium-regulated processes of vertebrate and invertebrate smooth muscles. We added length dynamics by assuming length-dependent bonding and unbonding rates for the cross bridges. The calculation 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 nonisometric responses of different kinds of smooth muscle, including vascular smooth muscle, airway smooth muscle, molluscan catch muscle (anterior byssus retractor muscle), and Aplysia I(2) muscle. The model captures the economical force maintenance property at the later stages of isometric muscle contraction and responses to imposed lengthening and shortening movements.

The Intracellular pH of a Molluscan Smooth Muscle During a Contraction-Catch-Relaxation Cycle Estimated by the Distribution of [14C]DMO and by 31P-NMR Spectroscopy

The intracellular pH (pHi) of the anterior byssus retractor muscle (ABRM) of Mytilus edulis L. was estimated during rest and during a contraction-catch-relaxation cycle by the distribution of [14C]DMO and by 31P-NMR spectroscopy. The pHi of resting muscles was found to be nearly the same whether it was determined with DMO (7.41±0.06, N=5l) or by 31P-NMR spectroscopy (7.44±0.02, N=5). During catch the pHi was not significantly different from its value at rest. Serotonin (10−5moll−1) induced a rapid relaxation and a significant increase in pHi (DMO: 7.56±0.05, N=5; 31P-NMR: 7.57±0.02, N=5). In resting ABRM serotonin (≥10−6moll−1) also induced an alkalosis. It was shown that cyclic AMP, the second messenger of serotonin, elicited an increase of pHi. Dopamine (10−5moll−1) did not cause an increase in either cyclic AMP levels or pHi. The rates by which tonically contracted ABRM loaded at 0.1 N were stretched were significantly increased by an alkalosis elicited by the addition of 10 or 20mmoll−1 methylamine. Acidosis elicited by lO mmoll−1 DMO caused a significant decrease in the rate of relaxation.