Maintenance Of Tension Research Articles

Does a tensioning device pinned to the tibia improve knee anterior–posterior load-displacement compared to manual tensioning of the graft following anterior cruciate ligament reconstruction? A cadaveric study of two tibial fixation devices

Devices that are pinned to the tibia to tension an anterior cruciate ligament (ACL) graft produce joint reaction loads that in turn can affect the maintenance of graft initial tension after tibial fixation and hence knee anterior-posterior (AP) load-displacement. However, the effect of these devices on AP load-displacement is unknown. Our objectives were to determine whether tensioning by device versus tensioning by hand causes differences in AP load-displacement and intraarticular graft tension for two commonly used tibial fixation devices: a bioresorbable interference screw and a WasherLoc. AP load-displacement and intraarticular graft tension were measured in 20 cadaveric knees using a custom arthrometer. An initial tension of 110 N was applied to a double-looped tendon graft with the knee at extension using a tensioning device pinned to the tibia and a simulated method of tensioning by hand. After inserting the tibial fixation device, the 134 N anterior limit (i.e., anterior position of the tibia with respect to the femur with a 134 N anterior force applied to the tibia) and 0 N posterior limit (i.e., AP position of the tibia relative to the femur with a 0 N force applied to the tibia) were measured with the knee in 25 degrees flexion. Intraarticular graft tension was measured at extension. These limits and intraarticular graft tension were also measured after cyclically loading the knee 300 times. Compared to a simulated method of tensioning by hand, tensioning with a device pinned to the tibia did not decrease the 134 N anterior limit and did not cause posterior tibial translation. However, intraarticular graft tension was maintained better with a tensioning device pinned to the tibia for the Washerloc, but not the interference screw. For two commonly used tibial fixation devices, a tensioning device pinned to the tibia does not improve AP load-displacement at 25 degrees flexion over tensioning by hand when the graft is tensioned at full extension, but does improve the maintenance of intraarticular graft tension for the Washerloc.

Localization of Myosin and Actin in the Pelage and Whisker Hair Follicles of Rat

The combined effects of myosin II and actin enable muscle and nonmuscle cells to generate forces required for muscle contraction, cell division, cell migration, cellular morphological changes, the maintenance of cellular tension and polarity, and so on. However, except for the case of muscle contraction, the details are poorly understood. We focus on nonmuscle myosin and actin in the formation and maintenance of hair and skin, which include highly active processes in mammalian life with respect to the cellular proliferation, differentiation, and movement. The localization of nonmuscle myosin II and actin in neonatal rat dorsal skin, mystacial pad, hair follicles, and vibrissal follicles was studied by immunohistochemical technique to provide the basis for the elucidation of the roles of these proteins. Specificities of the antibodies were verified by using samples from the relevant tissues and subjecting them to immunoblotting test prior to morphological analyses. The myosin and actin were abundant and colocalized in the spinous and granular layers but scarce in the basal layer of the dorsal and mystacial epidermis. In hair and vibrissal follicles, nonmuscle myosin and actin were colocalized in the outer root sheath and some hair matrix cells adjoining dermal papillae. In contrast, most areas of the inner root sheath and hair matrix appeared to comprise very small amounts of myosin and actin. Hair shaft may comprise significant myosin during the course of its keratinization. These results suggest that the actin-myosin system plays a part in cell movement, differentiation, protection and other key functions of skin and hair cells.

Ten-Year Experience Using an Articulating Antibiotic Cement Hip Spacer for the Treatment of Chronically Infected Total Hip

Infection is a devastating complication after total-hip arthroplasty. Between June 1991 and December 2001, 42 patients were treated at our center for chronically infected total-hip arthroplasty using a 2-stage articulating antibiotic hip spacer technique. Of the 27 patients available for review, 26 (94%) remain clinically free of infection at an average 76 months (range, 28-148 months) postoperatively. Twenty-two (81%) patients had a positive identification of the infecting organism. All patients received a minimum of 6 weeks of intravenous antibiotics and the Harris Hip Scores improved from 53 (range, 36-68) to 92 (range, 81-99) postoperatively. Advantages of this technique include improved patient function, maintenance of bone stock and soft tissue tension, thus simplifying reimplantation.

Effects of vanadate, phosphate and 2,3-butanedione monoxime (BDM) on skinned molluscan catch muscle

The effects of orthovanadate (V(i)), inorganic phosphate (P(i)) and 2,3-butanedione monoxime (BDM) on tension, force transients and the catch state (passive tension maintenance) were investigated in saponin-skinned fibre bundles of the anterior byssus retractor muscle (ABRM) of the bivalve mollusc Mytilus edulis at pH 6.7. During maximal Ca(2+) activation isometric force was depressed by V(i) (0.03-10 mM), P(i) (10 mM) and BDM (50 mM). Force transients following quick stretches (0.1-0.3% of fibre length) were accelerated substantially by 1 mM V(i), 10 mM P(i) or 50 mM BDM. These compounds also accelerated force responses in experiments in which ATP was released rapidly from caged ATP by flash photolysis at both pCa 4.7 (force rise) and at pCa>8 (force decline). The effects on the catch state were investigated in two types of experiments: (1) Ca(2+) removal after maximal Ca(2+) activation and (2) rapid ATP release during high-force rigor at pCa>8. In both cases rapid relaxation was followed by slow relaxation (slower than 2% of initial force per min). This later slow relaxation (catch) was insensitive to V(i) (1-10 mM), P(i) (10 mM) and BDM (50 mM) but was accelerated by 0.12 mM cAMP. Complete relaxation to almost zero force was attained by changing pH from 6.7 to 7.7 (pCa>8). We conclude that catch depends on cAMP- and pH-sensitive structures linking the myofilaments and not on the force-generating actomyosin cross-bridges that are sensitive to V(i), P(i) and BDM.

Functional Divergence of Human Cytoplasmic Myosin II: KINETIC CHARACTERIZATION OF THE NON-MUSCLE IIA ISOFORM

Cytoplasmic (or non-muscle) myosin II isoforms are widely expressed molecular motors playing essential cellular roles in cytokinesis and cortical tension maintenance. Two of the three human non-muscle myosin II isoforms (IIA and IIB) have been investigated at the protein level. Transient kinetics of non-muscle myosin IIB showed that this motor has a very high actomyosin ADP affinity and slow ADP release. Here we report the kinetic characterization of the non-muscle myosin IIA isoform. Similar to non-muscle myosin IIB, non-muscle myosin IIA shows high ADP affinity and little enhancement of the ADP release rate by actin. The ADP release rate constant, however, is more than an order of magnitude higher than the steady-state ATPase rate. This implies that non-muscle myosin IIA spends only a small fraction of its ATPase cycle time in strongly actin-bound states, which is in contrast to non-muscle myosin IIB. Non-muscle myosin II isoforms thus appear to have distinct enzymatic properties that may be of importance in carrying out their cellular functions.

Regulation of smooth muscle contraction by calcium, monomeric GTPases of the Rho subfamily and their effector kinases.

A key event in the activation of smooth muscle contraction ist the phosphorylation of the regulatory light chains of myosin (r-MLC) at Ser19 which is predominantly catalyzed by the Ca2+ and calmodulin dependent myosin light chain kinase, MLCK (Gallager et al., 1997). Recently, it has been suggested that, in addition, r-MLC phosphorylation and contraction may be induced in a Ca2+-independent manner by several protein kinases, such as Rho associated kinase, ZIP kinase, and integrin linked kinase, ILK (reviewed in Ganitkevich et al, 2002). These kinases may, hence, be involved in Ca2+-independent contractions (Kureishi et al., 1999) leading to an increased Ca2+-sensitivity of r-MLC phosphorylation and contraction. Although the physiological role of these kinases is far from clear, they may be of importance during the maintained phase of a contraction when intracellular [Ca2+] has returned to near resting values (Himpens and Somlyo, 1988; Lucius et al., 1998). One goal of this study, therefore, was to test whether Ca2+ is required for tension maintenance of an agonist induced contraction using the membrane permeant form of the caged Ca-chelator, diazo2.

The Kinetic Mechanism of Myo1e (Human Myosin-IC)

Myo1e is the widely expressed subclass-1 member of the myosin-I family. We performed a kinetic analysis of a truncated myo1e that consists of the motor and the single IQ motif with a bound calmodulin. We determined the rates and equilibrium constants for the key steps in the ATPase cycle. The maximum actin activated ATPase rate (V(max)) and the actin concentration at half-maximum of V(max) (K(ATPase)) of myo1e are similar to those of the native protein. The K(ATPase) is low (approximately 1 microm), however the affinity of myo1e for actin in the presence of ATP is very weak. A weak actin affinity and a rapid rate of phosphate release result in a pathway under in vitro assay conditions in which phosphate is released while myo1e is dissociated from actin. Actin activation of the ATPase activity and the low K(ATPase) are the result of actin activation of ADP release. We propose that myo1e is tuned to function in regions of high concentrations of cross-linked actin filaments. Additionally, we found that ADP release from actomyo1e is > 10-fold faster than other vertebrate myosin-I isoforms. We propose that subclass-1 myosin-Is are tuned for rapid sliding, whereas subclass-2 isoforms are tuned for tension maintenance or stress sensing.

Alterations of myocardial dynamic stiffness implicating abnormal crossbridge function in human mitral regurgitation heart failure.

Mitral regurgitation (MR) causes ventricular dilation, a blunted myocardial force-frequency relation, and increased crossbridge force-time integral (FTI). The mechanism of FTI increase was investigated using sinusoidal length perturbation analysis to compare crossbridge function in skinned left ventricular (LV) epicardial muscle strips from 5 MR and 5 nonfailing (NF) control hearts. Myocardial dynamic stiffness was modeled as 3 parallel viscoelastic processes. Two processes characterize intermediate crossbridge cycle transitions, B (work producing) and C (work absorbing) with Q(10)s of 4 to 5. No significant differences in moduli or kinetic constants of these processes were observed between MR and NF. The third process, A, characterizes a nonenzymatic (Q(10)=0.9) work-absorbing viscoelasticity, whose modulus increases sigmoidally with [Ca(2+)]. Effects of temperature, crossbridge inhibition, or variation in [MgATP] support associating the calcium-dependent portion of A with the structural "backbone" of the myosin crossbridge. Extension of the conventional sinusoidal length perturbation analysis allowed using the A modulus to index the lifetime of the prerigor, AMADP crossbridge. This index was 75% greater in MR than in NF (P=0.02), suggesting a mechanism for the previously observed increase in crossbridge FTI. Notably, the A-process modulus was inversely correlated (r(2)=0.84, P=0.03) with in vivo LV ejection fraction in MR patients. The longer prerigor dwell time in MR may be clinically relevant not only for its potential role as a compensatory mechanism (increased economy of tension maintenance and increased resistance to ventricular dilation) but also for a potentially deleterious effect (reduced elastance and ejection fraction).

Vascular Smooth Muscle Calcium Channels

Voltage-gated Ca2+ channels of the plasma membrane open in response to changes in cell membrane potential, providing an important pathway for regulated entry of extracellular Ca2+ ions. Ca2+ channels comprise a family of structurally related proteins displaying tissue- and stage-specific expression. They are classified as either high voltage-activated (HVA) or low voltage-activated (LVA) and can be subdivided further based on their pharmacological properties. HVA channels include L-, N-, P-/Q-, and R-types, and LVA channels are designated as T-type. Members of each subtype have been identified by molecular cloning, facilitating studies of their biophysical properties, relative distribution, and contribution to Ca2+-dependent cellular functions. In recent years, it has become increasingly clear that in most, if not all, cases, more than one class of voltage-dependent Ca2+ channel are expressed in a given cell type. The functional relevance of different Ca2+ channels within a cell, however, remains unclear. In contractile cells, voltage-gated Ca2+ channels are particularly important because influx of extracellular Ca2+ is essential for muscle contraction and maintenance of tension. Ca2+ channels have received a great deal of attention in vascular smooth muscle (VSM), where calcium influx affects arterial vasoconstriction and vasorelaxation, and ultimately influences systemic blood pressure. Resistance arteries are an important site of blood pressure regulation. These arteries normally exist in a relatively contracted state and can undergo either dilation or constriction, depending on the contractile state of the arterial VSM. Ca2+ channel activity is a key determinant of VSM contractile state, and L-type Ca2+ channels are the classic target of calcium channel blockers (CCBs) widely prescribed as antihypertensive drugs. Therefore, a wealth of information has been published comparing different calcium antagonists, or CCBs, and their inhibitory affects on L-type calcium currents and vasoconstriction. It is important to …

Light-dependent osmoregulation in pea stem protoplasts. photoreceptors, tissue specificity, ion relationships, and physiological implications.

Light-induced changes in the volume of protoplasts bathed in a medium of constant osmolarity are useful indications of light-dependent cellular osmoregulation. With this in mind, we investigated the effect of light on the volume of protoplasts isolated from the elongating stems of pea (Pisum sativum) seedlings raised under red light. The protoplasts were isolated separately from epidermal peels and the remaining peeled stems. Under continuous red light, the protoplasts of peeled stems swelled steadily, but those of epidermal peels maintained a constant volume. Experiments employing far-red light and phytochrome-deficient mutants revealed that the observed swelling is a light-induced response mediated mainly by phytochromes A and B with a little greater contribution by phytochrome A. Protoplasts of epidermal peels and peeled stems shrank transiently in response to a pulse of blue light. The blue light responsiveness in this shrinking response, which itself is probably mediated by cryptochrome, is under the strict control of phytochromes A and B with equal contributions by these phytochromes. We suggest that the swelling response participates in the maintenance of high tissue tension of elongating stems and that the shrinking response is involved in stem growth inhibition. Other findings include the following: The swelling is caused by uptake of K+ and Cl-. The presence of Ca2+ in the bathing medium is required for phytochrome signaling in the swelling response, but not in the response establishing blue light responsiveness. Phytochrome A mediates the two responses in a totally red/far-red light reversible manner, as does phytochrome B.

Muscle phenotype remains unaltered after limb autotomy and unloading.

Loss of chelipeds in crustaceans results in severe atrophy of the major muscle responsible for lifting the limb, the anterior levator. We decided to test if this loss of mechanical load altered muscle phenotype as measured by SDS-PAGE analysis of levator total protein and actomyosin fractions. Levator muscles of adult crayfish, Procambarus clarkii, with either functional regenerate limbs or lack of limb buds (papilla stage) were compared with those from normal contralateral limbs and those from pristine animals. We find that there is no difference in protein profiles among the three conditions. However, the total protein profile for the dually excited levator muscle is unique compared to those of fast or slow muscles of the abdomen (L and SEL, respectively), which receive only phasic or tonic excitatory innervation. The levator myosin heavy chain profile is similar to that of slow phenotype muscles such as the SEL and opener. We conclude that load does not influence levator phenotype. This is likely due either to the intact innervation and continued activation of the levator during atrophy or to the maintenance of passive tension on the muscle. J. Exp. Zool. 289:10-22, 2001.

The Real Separation in Separation of Powers Law

This Article argues that contemporary separation of powers commentary is misconceived. Despite the disagreement that dominates the commentary, a closer look at that debate reveals a surprise: commentators subscribe to a consensus about separation of powers. Once exposed, however, that consensus turns out to be underdeveloped, confused, and possibly incoherent. This Article, first, identifies the latent consensus about separation of powers, and, second, critically examines the consensus. The Article argues that the present consensus must be abandoned or refashioned in some as-yet-undeveloped way. Separation of powers commentary is conventionally thought to be dominated by a contest between adherents of or methodologies. A closer look at that debate, however, reveals that the description is mistaken; it disguises a robust consensus about separation of powers that one finds at the base of every approach to separation of powers - formalist and functionalist alike. That consensus is civics-class familiar: it calls for dispersal of three government functions among three separate government institutions and equipping each institution with select powers to protect itself and police the other departments. Having exposed the consensus, this Article critically examines it. The consensus simultaneously embraces two different substantive conceptions that are assumed to, but do not, fit easily together. One conception, called here separation-of-functions, stresses the need to keep the three government powers in different departments; the other conception, called here stresses the need to balance the departments of government through the creation and maintenance of tension and competition among them. The two conceptions are conflated or treated as if they easily relate to one another. Muddling the two conceptions together, however, is a mistake because they are distinct and in some ways in tension. Identifying a connection between the two ideas proves fruitless. The consensus, for instance, suggests that separating government functions leads to balance-of-power, but that connection does not hold up to close examination. And treating separation-of-functions as a way of achieving balance-of-power does more than fail; it ignores the independent reasons - reasons unrelated to balance-of-power - one might wish to separate government functions. Finally, the two conceptions suggest different, often irreconcilable, doctrinal concerns. The lesson of this Article, then, is that the consensus must be abandoned in favor of a new set of ideas about separation of powers. The Article takes the first step toward a new consensus by extracting the two distinct and sometimes conflicting conceptions from the muddle of the present consensus. While the latter steps of constructing a new consensus are not completed here, the challenges of those steps are clarified by this effort.

The Real Separation in Separation of Powers Law

This Article argues that contemporary separation of powers commentary is misconceived. Despite the disagreement that dominates the commentary, a closer look at that debate reveals a surprise: commentators subscribe to a consensus about separation of powers. Once exposed, however, that consensus turns out to be underdeveloped, confused, and possibly incoherent. This Article, first, identifies the latent consensus about separation of powers, and, second, critically examines the consensus. The Article argues that the present consensus must be abandoned or refashioned in some as-yet-undeveloped way. Separation of powers commentary is conventionally thought to be dominated by a contest between adherents of or methodologies. A closer look at that debate, however, reveals that the description is mistaken; it disguises a robust consensus about separation of powers that one finds at the base of every approach to separation of powers - formalist and functionalist alike. That consensus is civics-class familiar: it calls for dispersal of three government functions among three separate government institutions and equipping each institution with select powers to protect itself and police the other departments. Having exposed the consensus, this Article critically examines it. The consensus simultaneously embraces two different substantive conceptions that are assumed to, but do not, fit easily together. One conception, called here separation-of-functions, stresses the need to keep the three government powers in different departments; the other conception, called here stresses the need to balance the departments of government through the creation and maintenance of tension and competition among them. The two conceptions are conflated or treated as if they easily relate to one another. Muddling the two conceptions together, however, is a mistake because they are distinct and in some ways in tension. Identifying a connection between the two ideas proves fruitless. The consensus, for instance, suggests that separating government functions leads to balance-of-power, but that connection does not hold up to close examination. And treating separation-of-functions as a way of achieving balance-of-power does more than fail; it ignores the independent reasons - reasons unrelated to balance-of-power - one might wish to separate government functions. Finally, the two conceptions suggest different, often irreconcilable, doctrinal concerns. The lesson of this Article, then, is that the consensus must be abandoned in favor of a new set of ideas about separation of powers. The Article takes the first step toward a new consensus by extracting the two distinct and sometimes conflicting conceptions from the muddle of the present consensus. While the latter steps of constructing a new consensus are not completed here, the challenges of those steps are clarified by this effort.

The presence of an extractable substance in the CSF of humans with cerebral vasospasm after subarachnoid haemorrhage that correlates with phosphatase inhibition

The cellular events leading to cerebral vasospasm after subarachnoid haemorrhage are poorly understood, although an increase in smooth muscle myosin light chain phosphorylation has been observed. This study set out to determine if phosphatase inhibition may be involved in the pathological maintenance of tension observed during vasospasm. We found that 1 nM okadaic acid, a type 2A protein phosphatase inhibitor, elicited an increase in rate of O 2 consumption in the porcine carotid artery similar to that by cerebrospinal fluid (CSF) from vasospastic patients (CSF V, n=5) (control 0.23±0.03, CSF V 0.84±0.16 and okadaic acid 0.85±0.02 μmol min −1 g dwt −1). It was also observed that phosphatase inhibition with 1 nM okadaic acid significantly slowed relaxation after a stretch in a similar fashion to CSF V haemorrhage. CSF from vasospastic subarachnoid haemorrhage patients, but not from those without vasospasm, contains an extractable substance which modulates myosin light chain phosphorylation in vitro. A phosphatase preparation obtained from the porcine carotid artery dephosphorylated 63±2% of the phosphorylated (MLC 20) substrate in vitro, and non-vasospastic CSF treated enzyme dephosphorylated 60±2.6%. Okadaic acid inhibited phosphatase dephosphorylated only 7.5±1% of the substrate where CSF V treated enzyme dephosphorylated 22±2.8% of the substrate. We conclude that inhibition of smooth muscle phosphatase may be involved in the mechanisms associated with cerebral vasospasm after subarachnoid haemorrhage.

The contractile apparatus and mechanical properties of airway smooth muscle.

The functional properties of airway smooth muscle are fundamental to the properties of the airways in vivo. However, many of the distinctive characteristics of smooth muscle are not easily accounted for on the basis of molecular models developed to account for the properties of striated muscles. The specialized ultrastructural features and regulatory mechanisms present in smooth muscle are likely to form the basis for many of its characteristic properties. The molecular organization and structure of the contractile apparatus in smooth muscle is consistent with a model of force generation based on the relative sliding of adjacent actin and myosin filaments. In airway smooth muscle, actomyosin activation is initiated by the phosphorylation of the 20 kDa light chain of myosin; but there is conflicting evidence regarding the role of myosin light chain phosphorylation in tension maintenance. Tension generated by the contractile filaments is transmitted throughout the cell via a network of actin filaments anchored at dense plaques at the cell membrane, where force is transmitted to the extracellular matrix via transmembrane integrins. Proteins bound to actin and/or localized to actin filament anchorage sites may participate in regulating the shape of the smooth muscle cell and the organization of its contractile filament system. These proteins may also participate in signalling pathways that regulate the crossbridge activation and other functions of the actin cytoskeleton. The length-dependence of active force and the mechanical plasticity of airway smooth muscle may play an important role in determining airway responsiveness during lung volume changes in vivo. The molecular basis for the length-dependence of tension in smooth muscle differs from that in skeletal muscle, and may involve mechano-transduction mechanisms that modulate contractile filament activation and cytoskeletal organization in response to changes in muscle length. The reorganization of contractile filaments may also underlie the plasticity of the mechanical response of airway smooth muscle. Changes in the structural organization and signalling pathways of airway smooth muscle cells resulting form alterations in mechanical forces in the lung may be important factors in the development of pathophysiological conditions of chronic airway hyperresponsiveness.

Time-resolved two-dimensional X-ray diffraction study of the effect of shortening on activation of contracting skeletal muscle.

In twitch contractions of frog skeletal muscle, the isometric tension peaks when intracellular calcium has fallen to near-resting levels. To understand the mechanism of this delayed tension maintenance in the context of calcium regulation, the time course of the tropomyosin movement on actin was monitored by recording the intensity of the 2nd actin layer lines in a time-resolved two-dimensional X-ray diffraction study. The intensity rose ahead of tension, reflecting the tropomyosin movement from its "off" to "on" positions, but it fell with a time course similar to that of tension. Muscle shortening applied at the tension peak was followed by a poor recovery of tension, and accelerated the fall of the reflection intensity. The results suggest that the force-generating myosin heads retain the tropomyosin in its "on" position after the fall of intracellular calcium, and their shortening-induced detachment makes the tropomyosin return to its "off" position, thereby preventing myosin reattachment to actin.

Dictyostelium myosin IK is involved in the maintenance of cortical tension and affects motility and phagocytosis.

Dictyostelium discoideum myosin Ik (MyoK) is a novel type of myosin distinguished by a remarkable architecture. MyoK is related to class I myosins but lacks a cargo-binding tail domain and carries an insertion in a surface loop suggested to modulate motor velocity. This insertion shows similarity to a secondary actin-binding site present in the tail of some class I myosins, and indeed a GST-loop construct binds actin. Probably as a consequence, binding of MyoK to actin was not only ATP- but also salt-dependent. Moreover, as both binding sites reside within its motor domain and carry potential sites of regulation, MyoK might represent a new form of actin crosslinker. MyoK was distributed in the cytoplasm with a significant enrichment in dynamic regions of the cortex. Absence of MyoK resulted in a drop of cortical tension whereas overexpression led to significantly increased tension. Absence and overexpression of MyoK dramatically affected the cortical actin cytoskeleton and resulted in reduced initial rates of phagocytosis. Cells lacking MyoK showed excessive ruffling, mostly in the form of large lamellipodia, accompanied by a thicker basal actin cortex. At early stages of development, aggregation of myoK null cells was slowed due to reduced motility. Altogether, the data indicate a distinctive role for MyoK in the maintenance and dynamics of the cell cortex.

Effect of cell migration on the maintenance of tension on a collagen matrix.

Although it is known that cells promote structural reorganization of the collagen architecture, how individual cells exert mechanical tension on the matrix is not clearly understood. In the present study we have investigated the mechanical interaction of individual corneal fibroblasts with a collagen matrix using an improved version of our previously described in vitro force-measurement system (Roy, P. et al. Exp. Cell Res. 232:106-117, 1997). The elastic distortion of the collagen matrix exerted by cells was temporally recorded and analyzed using a two-dimensional finite-element model to quantify the forces exerted on the matrix. Time-lapse videomicroscopy of serum-cultured cells on the matrix for up to 6 h revealed that individual fibroblasts generated measurable tension on the matrix during pseudopodial extension and slow retraction. Fast retraction, an event observed during active cell migration, was associated with dramatic release of tension on the matrix. An apparent inverse correlation was observed between cell translocation and maintenance of matrix tension. Additional experiments with cells under serum-free conditions revealed that these cells fail to generate any detectable tension on the matrix despite undergoing filopodial extension and retraction. Since serum-free cells do not form focal adhesions or stress fibers, these experimental data suggest that contractility of nonmotile cells, coupled with strong cell-matrix adhesion, is the most favorable mechanism of generating and maintaining tension on the extracellular matrix.

Transient Kinetic Analysis of the 130-kDa Myosin I (MYR-1 Gene Product) from Rat Liver: A MYOSIN I DESIGNED FOR MAINTENANCE OF TENSION?

The 130-kDa myosin I (MI(130)), product of the myr-1 gene, is one member of the mammalian class I myosins, a group of small, calmodulin-binding mechanochemical molecules of the myosin superfamily that translocate actin filaments. Roles for MI(130) are unknown. Our hypothesis is that, as with all myosins, MI(130) is designed for a particular function and hence possesses specific biochemical attributes. To test this hypothesis we have characterized the enzymatic properties of MI(130) using steady-state and stopped-flow kinetic analyses. Our results indicate that: (i) the Mg(2+)-ATPase activity is activated in proportion to actin concentration in the absence of Ca(2+); (ii) the ATP-induced dissociation of actin-MI(130) is much slower for MI(130) than has been observed for other myosins (-Ca(2+), second order rate constant of ATP binding, 1.7 x 10(4) M(-1) s(-1); maximal rate constant, 32 s(-1)); (iii) ADP binds to actin-MI(130) with an affinity of approximately 10 microM and competes with ATP-induced dissociation of actin-MI(130); the rate constant of ADP release from actin-MI(130) is 2 s(-1); (iv) the rates of the ATP-induced dissociation of actin-MI and ADP release are 2-3 times greater in the presence of CaCl(2), indicating a sensitivity of motor activity to Ca(2+); and (v) the affinity of MI(130) for actin (15 nM) is typical of that observed for other myosins. Together, these results indicate that although MI(130) shares some characteristics with other myosins, it is well adapted for maintenance of cortical tension.

A Self-Locking Knot for Lateral Fabellotibial Suture Stabilisation of the Cranial Cruciate Ligament Deficient Stifle in the Dog

SummaryMaintaining adequate tension when knotting large diameter monofilament sutures can be challenging, especially without surgical assistance. A self-locking knot is described for lateral fabellotibial suture stabilisation of the canine cranial cruciate ligament deficient stifle. The self-locking knot enables maintenance of tension during tying and the knot can be tied by an unassisted surgeon. The inherent ability of the knot to maintain tension on the fabellotibial suture is due to the first throw of the knot creating a locking-loop effect. Increasing the tension on the suture material is readily achieved by applying traction on the free ends and tightening the first throw of the knot to remove the slack created. When the suture is tightened sufficiently four additional square throws are applied to the knot.A self-locking knot is described for lateral fabellotibial suture stabilisation of the canine cranial cruciate ligament deficient stifle. The inherent ability of the knot to maintain tension on the suture material is due to the first throw of the knot creating a locking-loop effect. Tension on the knot can be readily adjusted and the knot may be tied by an unassisted surgeon.