Latch state

As noted in Table 49-3, slow cycling of the crossbridges permits slow prolonged contraction of smooth muscle (eg, in viscera and blood vessels) with less utilization of ATP compared with striated muscle. The ability of smooth muscle to maintain force at reduced velocities of contraction is referred to as the latch state this is an important feamre of smooth muscle, and its precise molecular bases are under smdy. 

Hai, C. and Murphy, RA. (1988). Cross-bridge phosphorylation and regulation of the latch state in smooth muscle. Am. 

Constraints imposed by caldesmon that disrupt potentiation may not block myosin docking on actin. This process of modulating ATPase could in turn lead to or permit the development of the latch-state of tension maintenance displayed by tonic smooth muscles and observed at low Ca + concentration. This phenomenon probably involves stable actin-myosin binding and is associated with low actomyosin ATPase activity (Hai and Murphy, 1988 McDaniel et al, 1990). As envisioned, such tension maintenance would be incompatible with a troponin-tropomyosin form of regulation since tropomyosin would in that case block myosin docking at low Ca + concentrations. Hence, the caldesmon-tropomyosin system may be adapted for muscles that enter a latch-state. 

Accordingly, it has been suggested that CP contributes to the development of the latch state of smooth muscle that is characterized by tension maintenance with slowly cycling cross-bridges (Haeberle,... 

In intact smooth muscle, phosphorylation often declines while force is maintained, the so-called "latch" state (Dillon etal., 1981). Regulation of the latch state is still poorly understood. It has been very difficult to induce a latch state in skinned smooth muscle. The closest may be triton skinned chicken gizzard, which contracts independent of MLC phosphorylation (Wagner and Ruegg, 1986). In skinned chicken gizzard, but also in other types of smooth muscle (Bialojan et al., 1987 Siegman et al., 1989 Kenney et al., 1990 Schmidt et al, 1995), a steep and nonlinear relation between force and MLC phosphorylation was observed, which was postulated by the "latch model" of Hai and Mur-... 

Fig. 4. Schematic diagram (modified from Rembold and Murphy 1993) for the crossbridge cycling in smooth muscle according to the latch bridge model (Hai and Murphy 1988a, 1988b). Actin is indicated with A M and Mp denotes the unphosphorylated and phosphorylated myosin, respectively. Note that the latch state (AM) refers to a myosin bound state which is not identical to the rigor A.M state shown in Figure 1, chapter 2.

An important problem with any latch theory is that the latch-bridge in many ways remains descriptive and has not been coupled to specific biochemical actomyosin states or mode of actomyosin turnover. Skinned fibres could provide a way to explore this problem but at present there is a striking lack of a latch state in skinned smooth muscle preparations with a few exceptions. The hyperbolic phosphorylation force relationship has been demonstrated in e.g. skinned smooth muscle strips from chicken... 

FIGURE 1.61 State diagram with explicit transition commands (a) state diagram element, (b) SR latch state diagram. 

Timing through a latch is possible provided a timing arc exists between the latch data input and the latch state output. The basic approach is to disable the setup/hold timing relationship between the data input and the enable input of a latch. In order to time through all latches of a certain reference type in the technology library, the timing arc... 

---