Steady-state dependence of stress on cross-bridge phosphorylation in the swine carotid media

Myoplasmic [ca], crossbridge phosphorylation and latch in rabbit bladder smooth muscle

TONIC SMOOTH MUSCLE EXHIBIT THE LATCH PHENOMENON: high force at low myosin regulatory light chains (MRLC) phosphorylation, shortening velocity (Vo), and energy consumption. However, the kinetics of MRLC phosphorylation and cellular activation in phasic smooth muscle are unknown. The present study was to determine whether Ca(2+)-stimulated MRLC phosphorylation could suffice to explain the agonist- or high K(+)-induced contraction in a fast, phasic smooth muscle. We measured myoplasmic [Ca(2+)], MRLC phosphorylation, half-time after step-shortening (a measure of Vo) and contractile stress in rabbit urinary bladder strips. High K(+)-induced contractions were phasic at both 22℃ and 37℃: myoplasmic [Ca(2+)], MRLC phosphorylation, 1/half-time, and contractile stress increased transiently and then all decreased to intermediate values. Carbachol (CCh)-induced contractions exhibited latch at 37℃: stress was maintained at high levels despite decreasing myoplasmic [Ca(2+)], MRLC phosphorylation, and 1/half-time. At 22℃ CCh induced sustained elevations in all parameters. 1/half-time depended on both myoplasmic [Ca(2+)] and MRLC phosphorylation. The steady-state dependence of stress on MRLC phosphorylation was very steep at 37℃ in the CCh- or K(+)-depolarized tissue and reduced temperature flattend the dependence of stress on MRLC phosphorylation compared to 37℃. These data suggest that phasic smooth muscle also exhibits latch behavior and latch is less prominent at lower temperature.

The latch-bridge hypothesis of smooth muscle contraction

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.

A physiological model of cerebral blood flow control

The construction of a computational model of the human brain circulation is described. We combine an existing model of the biophysics of the circulatory system, a basic model of brain metabolic biochemistry, and a model of the functioning of vascular smooth muscle (VSM) into a single model. This represents a first attempt to understand how the numerous different feedback pathways by which cerebral blood flow is controlled interact with each other. The present work comprises the following: Descriptions of the physiology underlying the model; general comments on the processes by which this physiology is translated into mathematics; comments on parameter setting; and some simulation results. The simulations presented are preliminary, but show qualitative agreement between model behaviour and experimental results.

Regulation of smooth muscle calcium sensitivity: KCl as a calcium-sensitizing stimulus

KCl has long been used as a convenient stimulus to bypass G protein-coupled receptors (GPCR) and activate smooth muscle by a highly reproducible and relatively “simple” mechanism involving activation of voltage-operated Ca2+channels that leads to increases in cytosolic free Ca2+([Ca2+]i), Ca2+-calmodulin-dependent myosin light chain (MLC) kinase activation, MLC phosphorylation and contraction. This KCl-induced stimulus-response coupling mechanism is a standard tool-set used in comparative studies to explore more complex mechanisms generated by activation of GPCRs. One area where this approach has been especially productive is in studies designed to understand Ca2+sensitization, the relationship between [Ca2+]iand force produced by GPCR agonists. Studies done in the late 1980s demonstrated that a unique relationship between stimulus-induced [Ca2+]iand force does not exist: for a given increase in [Ca2+]i, GPCR activation can produce greater force than KCl, and relaxant agents can produce the opposite effect to cause Ca2+desensitization. Such changes in Ca2+sensitivity are now known to involve multiple cell signaling strategies, including translocation of proteins from cytosol to plasma membrane, and activation of enzymes, including RhoA kinase and protein kinase C. However, recent studies show that KCl can also cause Ca2+sensitization involving translocation and activation of RhoA kinase. Rather than complicating the Ca2+sensitivity story, this surprising finding is already providing novel insights into mechanisms regulating Ca2+sensitivity of smooth muscle contraction. KCl as a “simple” stimulus promises to remain a standard tool for smooth muscle cell physiologists, whose focus is to understand mechanisms regulating Ca2+sensitivity.

Hypoxic vasorelaxation: Ca2+-dependent and Ca2+-independent mechanisms

The mechanisms of oxygen sensing in vascular smooth muscle have been studied extensively in a variety of tissue types and the results of these studies indicate that the mechanism of hypoxia-induced vasodilation probably involves several mechanisms that combined to assure the appropriate response. After a short discussion of the regulatory mechanisms for smooth muscle contractility, we present the evidence indicating that hypoxic vasorelaxation involves both Ca2+-dependent and Ca2+-independent mechanisms. More recent experiments using proteomic approaches in organ cultures of porcine coronary artery reveal important changes evoked by hypoxia in both Ca2+-dependent and Ca2+-independent pathways.

Cooperative attachment of cross bridges predicts regulation of smooth muscle force by myosin phosphorylation

Serine 19 phosphorylation of the myosin regulatory light chain (MRLC) appears to be the primary determinant of smooth muscle force development. The relationship between MRLC phosphorylation and force is nonlinear, showing that phosphorylation is not a simple switch regulating the number of cycling cross bridges. We reexamined the MRLC phosphorylation-force relationship in slow, tonic swine carotid media; fast, phasic rabbit urinary bladder detrusor; and very fast, tonic rat anococcygeus. We found a sigmoidal dependence of force on MRLC phosphorylation in all three tissues with a threshold for force development of approximately 0.15 mol P(i)/mol MRLC. This behavior suggests that force is regulated in a highly cooperative manner. We then determined whether a model that employs both the latch-bridge hypothesis and cooperative activation could reproduce the relationship between Ser(19)-MRLC phosphorylation and force without the need for a second regulatory system. We based this model on skeletal muscle in which attached cross bridges cooperatively activate thin filaments to facilitate cross-bridge attachment. We found that such a model describes both the steady-state and time-course relationship between Ser(19)-MRLC phosphorylation and force. The model required both cooperative activation and latch-bridge formation to predict force. The best fit of the model occurred when binding of a cross bridge cooperatively activated seven myosin binding sites on the thin filament. This result suggests cooperative mechanisms analogous to skeletal muscle that will require testing.

A dilution immunoassay to measure myosin regulatory light chain phosphorylation

We examined the quantitation of myosin regulatory light chain phosphorylation (MRLCP) by Western blot and found both offset and saturation errors. The desirable characteristics of an MRLCP assay are that the dynamic range be 60- to 100-fold and that the detection threshold be known and preferably very small relative to total MRLC concentration. No technique examined provided all these characteristics. However, accurate measurements can be obtained by including serial dilutions of the sample to provide a fractional calibration scale in terms of the dephosphorylated light chain and by using interpolation of the phosphorylated band signal intensity to provide values for the relative phosphorylation ratio. We found that this method offers several advantages over methods that rely on signal ratios from single samples: The dilution ratio method is less subject to errors from differences in protein load, it offers estimates of the error in the individual measurement, and has some redundancy that increases the likelihood of obtaining a valid measurement despite gel or membrane artifacts.

Inhibition of Ca2+-dependent contraction in swine carotid artery by myosin kinase inhibitors

Experiments were designed to examine the efficacy of the MLCK inhibitors wortmannin and ML-9 in intact smooth muscle to determine whether contractile agonists can induce a Ca(2+) and myosin light chain phosphorylation-independent contraction. Both wortmannin and ML-9 reduced active stress in a dose-dependent manner. Both inhibitors interfered with Ca2+ mobilization in either the K(+)-depolarized or agonist activated swine carotid media at concentrations greater than 10 microM. Wortmannin reduced MRLC phosphorylation and stress to resting levels in stimulated tissues while Ca2+ remained above resting levels. There was no evidence for Ca2+ and MRLC phosphorylation-independent stress generation in swine arterial smooth muscle.

Dependence of force on length at constant cross-bridge phosphorylation in the swine carotid media

1. The dependence of force (F) on length (L) in smooth muscle remains uncertain since (i) it is influenced by changes in activation (myosin light chain phosphorylation), (ii) no anatomical reference length for the contractile unit is available, (iii) the length at which optimum force is generated (L(o)) exhibits a broad, flat optimum, and (iv) the presence of an extensive connective tissue network makes it difficult to stretch tissues without damage. 2. A swine carotid medial ring preparation prepared by removal of the adventitia and endothelium could be stretched to 1.8 L(o) without decreasing active force generation on return to shorter lengths. 3. A highly reproducible mechanically defined reference length, L(o), was obtained by fitting force-length data between 0.3 and 1.6 L(o) with a third-order polynomial where L = L(o) when dF/dL = 0. 4. Activation as assessed by myosin regulatory light chain (MRLC) phosphorylation increased with length in 100 microM histamine-stimulated tissues from 0.6 to 1.8 L(o). 5. Activation was constant in K(+)-depolarized and field-stimulated tissues from 1.0 to 1.8 L(o) allowing determination of the descending limb of the force-length relation to be assessed independently of activation. 6. The slope of the descending limb of the force-length relation was linear except at very long lengths, which often produced tissue damage. The slope was not statistically different from that estimated for sarcomeres in vertebrate skeletal muscle. 7. The medial ring preparation and the procedures used to define the reference length provide advantages for the measurement of length-dependent variables.

The variable coupling between force and myosin light chain phosphorylation in Triton-skinned chicken gizzard fibre bundles: role of myosin light chain phosphatase

The mechanism responsible for the regulation of smooth muscle tone at low levels of myosin light chain (MLC) phosphorylation is still poorly understood. According to one model, so-called latchbridges, which contribute to force maintenance at low levels of MLC phosphorylation, are generated by dephosphorylation of attached and phosphorylated crossbridges. The model predicts that the force generated for a given level of MLC phosphorylation depends on the activity of the MLC phosphatase. We tested this hypothesis by reducing the activity of the phosphatase by at least 80% in two ways: inhibition with okadaic acid and extraction. Under both conditions, higher levels of MLC phosphorylation were required to support a given level of force, suggesting a decreased flux of attached phosphorylated to attached dephosphorylated crossbridges, as predicted by this model. Although, under both conditions, the relationship between force and MLC phosphorylation was shifted to the right, the curves did not superimpose as would have been expected if the phosphatase activity were the only determinant of the coupling between force and phosphorylation. In the extracted fibres, two more proteins, calponin and SM22, were significantly reduced in addition. Therefore, these proteins might be involved in modulating the relationship between force and MLC phosphorylation.

Dependence of ATP consumption on cross-bridge phosphorylation in swine carotid smooth muscle

1. Ca(2+)-dependent phosphorylation of the myosin regulatory light chain (MRLC) initiates cross-bridge cycling and contraction in smooth muscle. A four-state cross-bridge model, in which Ca(2+)-dependent phosphorylation is the only proposed regulatory mechanism, can predict the mechanical output of the swine carotid media. Our aims were to determine whether ATP consumption rates and the economy of force maintenance are regulated functions of MRLC phosphorylation as predicted by the model. 2. Steady-state force and oxygen consumption were measured in medial rings of swine carotid arteries activated with depolarizing solutions and agents capable of maintaining a wide range of steady-state myoplasmic Ca2+ and MRLC phosphorylation levels. 3. Suprabasal ATP consumption increased almost linearly with MRLC phosphorylation and exhibited a hyperbolic increase with active stress, as predicted. 4. The economy of stress maintenance fell with increases in suprabasal phosphorylation. 5. In absolute terms the energetic cost of covalent regulation by cross-bridge phosphorylation was small, although it may be a significant fraction of the ATP consumption associated with contraction.

Models of the mechanism for crossbridge attachment in smooth muscle

The mechanism responsible for formation of attached, dephosphorylated crossbridges (latchbridges) in smooth muscle is controversial. Myosin light chain phosphorylation may be obligatory for crossbridge attachment; if this were the case, latchbridges would arise solely by dephosphorylation of attached, phosphorylated crossbridges. Alternatively, the presence of attached crossbridges could induce cooperative activation by allowing dephosphorylated crossbridges to attach to the thin filament. We evaluated whether four-state models based on dephosphorylation and/or cooperativity-regulated attachment could quantitatively predict smooth muscle contractile behaviour. Five quantitative models for transitions between crossbridge states were developed. Mechanisms for latchbridge formation included: (1) dephosphorylation, (2) cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, (3) cooperativity-regulated attachment dependent on all attached crossbridges, (4) dephosphorylation and cooperativity-regulated attachment dependent only on attached, phosphorylated crossbridges, and (5) dephosphorylation and cooperativity-regulated attachment dependent on all attached crossbridges. All five models approximated the time course of contraction and the dependence of steady-state stress on myosin phosphorylation in the swine carotid artery. In the two models that had cooperative attachment regulated by all attached crossbridges, small increases in the rate constant for cooperativity-regulated attachment resulted in positive feedback and irreversible contraction. We suggest that a number of four-state crossbridge models can predict contractile behaviour in arterial smooth muscle. Potentially, latchbridges could be formed by both dephosphorylation and cooperativity-regulated attachment. If cooperativity-regulated latchbridge attachment does exist in smooth muscle, we suggest that it should be dependent only on the number of phosphorylated crossbridges rather than all attached crossbridges.