Biomechanical Properties of the Sarcolemma and Costameres of Skeletal Muscle Lacking Desmin

Intermediate filaments (IFs), composed primarily by desmin and keratins, link the myofibrils to each other, to intracellular organelles, and to the sarcolemma. There they may play an important role in transfer of contractile force from the Z-disks and M-lines of neighboring myofibrils to costameres at the membrane, across the membrane to the extracellular matrix, and ultimately to the tendon (“lateral force transmission”). We measured the elasticity of the sarcolemma and the connections it makes at costameres with the underlying contractile apparatus of individual fast twitch muscle fibers of desmin-null mice. By positioning a suction pipet to the surface of the sarcolemma and applying increasing pressure, we determined the pressure at which the sarcolemma separated from nearby sarcomeres, Pseparation, and the pressure at which the isolated sarcolemma burst, Pbursting. We also examined the time required for the intact sarcolemma-costamere-sarcomere complex to reach equilibrium at lower pressures. All measurements showed the desmin-null fibers to have slower equilibrium times and lower Pseparation and Pbursting than controls, suggesting that the sarcolemma and its costameric links to nearby contractile structures were weaker in the absence of desmin. Comparisons to earlier values determined for muscles lacking dystrophin or synemin suggest that the desmin-null phenotype is more stable than the former and less stable than the latter. Our results are consistent with the moderate myopathy seen in desmin-null muscles and support the idea that desmin contributes significantly to sarcolemmal stability and lateral force transmission.

Dynamic Control of Contractile Force in Engineered Heart Tissue

<p>Three-dimensional engineered heart tissues (EHTs) derived from human induced pluripotent stem cells (iPSCs) have become an important resource for both drug toxicity screening and research on heart disease. A key metric of EHT phenotype is the contractile force with which the tissue spontaneously beats. It is well-known that cardiac muscle contractility – its ability to do mechanical work – depends on tissue prestrain (preload) and external resistance (afterload). Objectives: Here, we demonstrate a technique to control both preload and afterload dynamically while monitoring contractile force exerted by EHTs. Methods: We developed an apparatus that uses real-time feedback control to monitor and regulate EHT forces. The system is comprised of a pair of high-speed piezoelectric actuators that can strain the EHT scaffold and a fast optical measurement tool to provide EHT contractile force feedback while monitoring tissue strain. Results: The system was used to regulate the effective stiffness of the scaffold. When controlled to have effectively isometric boundary conditions, EHTs exerted a contractile force that was almost twice as large as that observed under auxotonic conditions. Conclusion: These experimental results demonstrate that EHT contractility can be increased through feedback control to regulate boundary stiffness. Significance: The work advances our understanding of the role that mechanical environment plays in EHT contractility. This could be used to help study or alter EHT phenotype and potentially EHT maturation through controlled mechanical conditioning.</p>

Dynamic Control of Contractile Force in Engineered Heart Tissue

<p>Three-dimensional engineered heart tissues (EHTs) derived from human induced pluripotent stem cells (iPSCs) have become an important resource for both drug toxicity screening and research on heart disease. A key metric of EHT phenotype is the contractile force with which the tissue spontaneously beats. It is well-known that cardiac muscle contractility – its ability to do mechanical work – depends on tissue prestrain (preload) and external resistance (afterload). Objectives: Here, we demonstrate a technique to control both preload and afterload dynamically while monitoring contractile force exerted by EHTs. Methods: We developed an apparatus that uses real-time feedback control to monitor and regulate EHT forces. The system is comprised of a pair of high-speed piezoelectric actuators that can strain the EHT scaffold and a fast optical measurement tool to provide EHT contractile force feedback while monitoring tissue strain. Results: The system was used to regulate the effective stiffness of the scaffold. When controlled to have effectively isometric boundary conditions, EHTs exerted a contractile force that was almost twice as large as that observed under auxotonic conditions. Conclusion: These experimental results demonstrate that EHT contractility can be increased through feedback control to regulate boundary stiffness. Significance: The work advances our understanding of the role that mechanical environment plays in EHT contractility. This could be used to help study or alter EHT phenotype and potentially EHT maturation through controlled mechanical conditioning.</p>

Effects of Steganotaenia Araliacae Root Extract on Contractile Function of Isolated Rat Ileum

Background: Various parts of a small tree, Steganotaenia araliacaeare used as medicine in local traditional settings in Zambia to initiate and augment parturition although very little is documented about its physiological and pharmacological effects. Steganotaenia araliacaecold extract has been observed to cause contractions of uterus in rats but its effect on non-uterine muscle is unclear. The aim of this study was to establish the contractile effect of Steganotaenia araliacaecold extract on isolated rat ileum smooth muscle.Method: Animals were sacrificed by cervical dislocation. Abdominal incisions were made to expose and dissect three ileum segments from each rat. The ileum segments were immediately transferred and mounted in the organ bath containing Tyrode solution. The contractile effects of acetylcholine (a reference agonist) and Steganotaenia araliacaecold extract on ileum segments were investigated starting with the least effective doses, thereafter doubling the doses until maximal tissue response was observed. Antagonists that include atropine, indomethacin, mepyramine, ondansetron and nifedipine in the presence and absence of Steganotaenia araliacaecold extract were also investigated to establish the mechanism of actionResults: Steganotaenia araliacaecold extract increased the contractile force of isolated rat ileum in a dose-response manner but had no significant effects on the frequency of the spontaneous contractions. Pre-treating the tissue with atropine, indomethacin, mepyramine or ondansetron did not inhibit the contractile force of Steganotaenia araliacaecold extract, while pre-treating the tissue with nifedipine inhibited its contractile force by 100% (p<0.05)Conclusion: The cold root extract of Steganotaenia araliacae induced contractions on isolated rat ileum smooth muscle in a dose response manner by probable activation of calcium channels. It is possible that SAEᶜif used in high doses may cause severe abdominal cramps an effect that needs to be noted as it is being used in parturition.

Tibial lengthening in achondroplasia patients aged 6–9 years as the first stage of growth correction

Background Height increase and improvement of body proportions for achondroplasia patients normally require two or more stages of reconstructive treatment to be followed by rehabilitation between lengthening periods, and growth correction can take a significant part of life in the cohort of patients. What is the best age to start growth correction is an important question. The purpose of this paper was to present an argument for arranging the first stage of growth correction in achondroplasia patients aged 6–9 years based on the structural and functional muscle evaluation of tibiae to be lengthened. Material and methods Achondroplasia patients aged 6–9 years (n = 30) were examined preoperatively, during distraction, fixation and at 1.5 to 2 years of frame removal. Tibial lengthening was produced monofocally and bifocally. Contractile force of the dorsal and plantar flexion muscles of the foot was measured with dynamometer. Ultrasonography of tibial muscles was performed with HITACHI ultrasound imaging device (Japan). Results Achondroplasia patients aged 6–9 year who underwent tibial lengthening of at least 50% of the initial length developed neuropathy in 2.6 % of cases and soft tissue inflammation in 5.6 % of cases. Characteristic muscle striation of m. tibialis anterior and m. extensor digitorum longus appeared to restore at 1.5 to 2 years of tibial lengthening with clear contouring of the intermuscular septa and retained contractile force of the muscles. The contractile force restored to 96.15 % of preoperative level in the anterior tibial muscles, and to 101.92 % in the posterior muscles. Conclusion The comprehensive clinical, ultrasonographic and dynamometric evaluation of tibial muscles presented a good argument for tibial lengthening in achondroplasia patients aged 6–9 years. Regained muscle striation and spare capacity of m. tibialis anterior and extensor digitorum longus, the restored force of the anterior tibial muscles to 96.15 % of the preoperative level suggested the possibility for the next stage of growth correction.

Maintenance of contractile force of the hind limb muscles by the somato-lumbar sympathetic reflexes

This study aimed to clarify whether the reflex excitation of muscle sympathetic nerves induced by contractions of the skeletal muscles modulates their contractility. In anesthetized rats, isometric tetanic contractions of the triceps surae muscles were induced by electrical stimulation of the intact tibial nerve before and after transection of the lumbar sympathetic trunk (LST), spinal cord, or dorsal roots. The amplitude of the tetanic force (TF) was reduced by approximately 10% at 20 min after transection of the LST, spinal cord, or dorsal roots. The recorded postganglionic sympathetic nerve activity from the lumbar gray ramus revealed that both spinal and supraspinal reflexes were induced in response to the contractions. Repetitive electrical stimulation of the cut peripheral end of the LST increased the TF amplitude. Our results indicated that the spinal and supraspinal somato-sympathetic nerve reflexes induced by contractions of the skeletal muscles contribute to the maintenance of their own contractile force.

Critical role of V1a vasopressin receptor in murine parturition

The precise mechanisms of the reproductive physiological processes, such as labor initiation, are poorly understood. Oxytocin (OT) is one of the well-known uterotonics and is clinically adopted as a medication to facilitate childbirth. Vasopressin (VP), a posterior pituitary hormone similar to OT, has also been proposed to be involved in the reproductive physiology. In this study, we found that a total deficiency of V1a receptor subtype (V1aR) in mice resulted in a reduced number of pups, delayed labor initiation, and increased post-delivery hemorrhage compared with those in wild-type mice. Among the VP receptor subtypes, only V1aR was found to be expressed in the murine uterus and its distribution pattern was different from that of the oxytocin receptor (OTR); V1aR expression was mainly distributed in the circular myometrium, whereas OTR was strongly expressed in both the circular and longitudinal myometrium. The maximum contractile force of the circular myometrium, induced by VP or OT, was attenuated in the pregnant uterus of Avpr1a-deficient mice. Contrarily, while OT expression was decreased in the Avpr1a-deficient uterus, OTR expression was significantly increased. These results suggest that V1aR deficiency not only reduces the uterine contractile force but also perturbs the expression of genes responsible for the reproductive physiology. Therefore, V1aR is necessary to exert the maximum contraction of the circular myometrium to deliver pups. This study revealed an important role of V1aR in physiological contraction and term parturition in mice.