The latch-bridge hypothesis of smooth muscle contraction

Abstract

In contrast to striated muscle, both normalized force and shortening velocities are regulated functions of cross-bridge phosphorylation in smooth muscle. Physiologically this is manifested as relatively fast rates of contraction associated with transiently high levels of cross-bridge phosphorylation. In sustained contractions, Ca2+, cross-bridge phosphorylation, and ATP consumption rates fall, a phenomenon termed "latch". This review focuses on the Hai and Murphy (1988a) model that predicted the highly non-linear dependence of force on phosphorylation and a directly proportional dependence of shortening velocity on phosphorylation. This model hypothesized that (i) cross-bridge phosphorylation was obligatory for cross-bridge attachment, but also that (ii) dephosphorylation of an attached cross-bridge reduced its detachment rate. The resulting variety of cross-bridge cycles as predicted by the model could explain the observed dependencies of force and velocity on cross-bridge phosphorylation. New evidence supports modifications for more general applicability. First, myosin light chain phosphatase activity is regulated. Activation of myosin phosphatase is best demonstrated with inhibitory regulatory mechanisms acting via nitric oxide. The second modification of the model incorporates cooperativity in cross-bridge attachment to predict improved data on the dependence of force on phosphorylation. The molecular basis for cooperativity is unknown, but may involve thin filament proteins absent in striated muscle.

Figure 1

Cross-bridge models in skeletal muscle. A. Simple two state model depicting force generation in terms of free (myosin, M) and attached (AM) force generating cross-bridges. Each cycle involves the hydrolysis of one ATP. Attachment and cycling depend on 4Ca²⁺ ions binding to troponin in the thin filaments (actin, A) to alter their confirmation (A*) to allow myosin heads to attach, This is a highly cooperative process. B. The generally accepted steps in the hydrolysis of ATP by actin-activated myosin (ATP, T; ADP, D). The rate limiting step for all myosin isoforms involves product release between AM-D-P_i and AM-D. This model also applies to smooth muscle. Adapted from Adelstein and Sellers (1996).

Figure 2

Models for cross-bridge cycling in smooth muscle. A. Two state (free and attached) cross-bridge model for smooth muscle illustrating regulation by a Ca²⁺-dependent phosphorylation switch. Myosin light chain kinase (MLCK) is activated by Ca²⁺-binding in a highly cooperative manner to four sites on calmodulin, thereby enabling it to bind to MLCK. Dephosphorylation is due to the action of myosin light chain phosphatase (MLCP). Note that ATP hydrolysis is required both for the cross-bridge cycle and for phosphorylation/dephosphorylation. B. The four state latch-bridge model of Hai and Murphy (1988a). Only phosphorylated crossbridges (M_p) can attach to actin, but attached phosphorylated cross-bridges are substrates for MLCP and can become dephosphorylated to form a ‘latch-bridge.’ Latch-bridges are postulated to have unaltered force generating capacities, but a slowed detachment rate.

Figure 3

Current working hypothesis for regulating contraction and relaxation in smooth muscle. The latch-bridge hypothesis of Hai and Murphy (1988a, Fig. 2B) is retained. Phosphorylation is a function of the ratio of Ca²⁺-dependent MLCK activity and Ca²⁺-independent MLCP activity. It is now postulated that MLCP activity is regulated physiologically at least during relaxation in response to inhibitory signals such as NO. This model also assumes that the attachment of M_p to thin filaments to form AM_p is cooperative (see Fig. 4).

Figure 4

Steady-state predictions of the current latch-bridge model (Fig. 3) incorporating cooperativity in the attachment of phosphorylated cross-bridges. A. Model predictions for each cross-bridges species where active stress is the sum of AM + AM_p. B. Fit of model to experimental measurements of active stress as a function of myosin phosphorylation determined by a dilution assay procedure (Walker et al., 2000). Adapted from Rembold et al., (2004).