Cross-bridge phosphorylation and regulation of latch state in smooth muscle

TLDR

We developed a minimum kinetic model of smooth‑muscle cross‑bridge interactions that posits cycling phosphorylated cross‑bridges and non‑cycling dephosphorylated latch bridges, with Ca²⁺‑dependent myosin phosphorylation as the sole regulatory mechanism and latch bridges formed by dephosphorylation of attached cross‑bridges, and fitted the rate constants to phosphorylation and stress development data. The model predicts that latch‑bridge detachment is rate‑limiting, yields a hyperbolic stress–phosphorylation relationship consistent with experiments, and shows that an initial phosphorylation transient only accelerates stress development without affecting steady‑state stress, supporting that myosin phosphorylation alone is necessary and sufficient for latch state formation.

Abstract

We have developed a minimum kinetic model for cross-bridge interactions with the thin filament in smooth muscle. The model hypothesizes two types of cross-bridge interactions: 1) cycling phosphorylated cross bridges and 2) noncycling dephosphorylated cross bridges ("latch bridges"). The major assumptions are that 1) Ca2+-dependent myosin phosphorylation is the only postulated regulatory mechanism, 2) each myosin head acts independently, and 3) latch bridges are formed by dephosphorylation of an attached cross bridge. Rate constants were resolved by fitting data on the time courses of myosin phosphorylation and stress development. Comparison of the rate constants indicates that latch-bridge detachment is the rate-limiting step. Model simulations predicted a hyperbolic dependence of steady-state stress on myosin phosphorylation, which corresponded with the experimental observation of high values of stress with low levels of phosphorylation in intact tissues. Model simulations also predicted the experimental observation that an initial phosphorylation transient only accelerates stress development, with no effect on the final steady-state levels of stress. Because the only Ca2+-dependent regulatory mechanism in this model was activation of myosin light chain kinase, these results are consistent with the hypothesis that myosin phosphorylation is both necessary and sufficient for the development of the latch state.