Gotowa bibliografia na temat „Mouse papillary muscle”

We compared the contractile performance of papillary muscle from a mouse model of hypertrophic cardiomyopathy [α-cardiac actin ( ACTC) E99K mutation] with nontransgenic (non-TG) littermates. In isometric twitches, ACTC E99K papillary muscle produced three to four times greater force than non-TG muscle under the same conditions independent of stimulation frequency and temperature, whereas maximum isometric force in myofibrils from these muscles was not significantly different. ACTC E99K muscle relaxed slower than non-TG muscle in both papillary muscle (1.4×) and myofibrils (1.7×), whereas the rate of force development after stimulation was the same as non-TG muscle for both electrical stimulation in intact muscle and after a Ca2+ jump in myofibrils. The EC50 for Ca2+ activation of force in myofibrils was 0.39 ± 0.33 μmol/l in ACTC E99K myofibrils and 0.80 ± 0.11 μmol/l in non-TG myofibrils. There were no significant differences in the amplitude and time course of the Ca2+ transient in myocytes from ACTC E99K and non-TG mice. We conclude that hypercontractility is caused by higher myofibrillar Ca2+ sensitivity in ACTC E99K muscles. Measurement of the energy (work + heat) released in actively cycling heart muscle showed that for both genotypes, the amount of energy turnover increased with work done but with decreasing efficiency as energy turnover increased. Thus, ACTC E99K mouse heart muscle produced on average 3.3-fold more work than non-TG muscle, and the cost in terms of energy turnover was disproportionately higher than in non-TG muscles. Efficiency for ACTC E99K muscle was in the range of 11–16% and for non-TG muscle was 15–18%.

We recently demonstrated the abundant presence of cardiomyocytes in the wall of thoracic veins of adult mouse and rat. The highly differentiated morphology and myofilament protein contents of the venous cardiomyocytes suggested contractile functions. Here we further investigated the contractility of mouse and rat azygos venous rings compared with that of atrial strips and ventricular papillary muscle. 5-Bromo-4-chloro-indolyl-galactopyranoside (X-gal) staining of transgenic mouse vessels expressing lacZ under a cloned cardiac troponin T promoter demonstrated that the venous cardiomyocytes are discontinuous from atrial myocardium and aligned in the wall of thoracic veins perpendicular to the vessel axis. Histological sections displayed sarcomeric striations in the venous cardiomyocytes, which indicate an encirclement orientation of myofibrils in the vessel wall. Mechanical studies found that the rings of mouse and rat azygos vein produce strong cardiac type twitch contractions when stimulated with electrical pacing in contrast to the weak and slow smooth muscle contractions induced using 90 mM KCl. The twitch contraction and relaxation of mouse azygos veins further exhibited a cardiac type of β-adrenergic responses. Quantitative comparison showed that the contractions of venous cardiomyocytes are slightly slower than those of atrium muscle but significantly faster than those of ventricular papillary muscle. These novel findings indicate that the cardiomyocytes abundant in the wall of rodent thoracic veins possess fully differentiated cardiac muscle phenotype despite their anatomical and functional segregations from the heart.

Limb-girdle muscular dystrophy 2i (LGMD2i) is a dystroglycanopathy that compromises myofiber integrity and primarily reduces power output in limb muscles but can influence cardiac muscle as well. Previous studies of LGMD2i made use of a transgenic mouse model in which a proline-to-leucine (P448L) mutation in fukutin-related protein severely reduces glycosylation of α-dystroglycan. Muscle function is compromised in P448L mice in a manner similar to human patients with LGMD2i. In situ studies reported lower maximal twitch force and depressed force-velocity curves in medial gastrocnemius (MG) muscles from male P448L mice. Here, we measured Ca2+-activated force generation and cross-bridge kinetics in both demembranated MG fibers and papillary muscle strips from P448L mice. Maximal activated tension was 37% lower in MG fibers and 18% lower in papillary strips from P448L mice than controls. We also found slightly faster rates of cross-bridge recruitment and detachment in MG fibers from P448L than control mice. These increases in skeletal cross-bridge cycling could reduce the unitary force output from individual cross bridges by lowering the ratio of time spent in a force-bearing state to total cycle time. This suggests that the decreased force production in LGMD2i may be due (at least in part) to altered cross-bridge kinetics. This finding is notable, as the majority of studies germane to muscular dystrophies have focused on sarcolemma or whole muscle properties, whereas our findings suggest that the disease pathology is also influenced by potential downstream effects on cross-bridge behavior.

Blebbistatin is a powerful inhibitor of actin-myosin interaction in isolated contractile proteins. To examine whether blebbistatin acts in a similar manner in the organized contractile system of striated muscle, the effects of blebbistatin on contraction of cardiac tissue from mouse were studied. The contraction of paced intact papillary muscle preparations and shortening of isolated cardiomyocytes were inhibited by blebbistatin with inhibitory constants in the micromolar range (1.3–2.8 μM). The inhibition constants are similar to those previously reported for isolated cardiac myosin subfragments showing that blebbistatin action is similar in filamentous myosin of the cardiac contractile apparatus and isolated proteins. The inhibition was not associated with alterations in action potential duration or decreased influx through L-type Ca2+ channels. Experiments on permeabilized cardiac muscle preparations showed that the inhibition was not due to alterations in Ca2+ sensitivity of the contractile filaments. The maximal shortening velocity was not affected by 1 μM blebbistatin. In conclusion, we show that blebbistatin is an inhibitor of the actin-myosin interaction in the organized contractile system of cardiac muscle and that its action is not due to effects on the Ca2+ influx and activation systems.

AbstractObjectiveStem cells hold promise for treating hair loss. Here an in vitro mouse model was developed using outer root sheaths (ORSs) isolated from hair follicles for studying stem cell-mediated dermal papillary regeneration.MethodsUnder sterile conditions, structurally intact ORSs were isolated from hair follicles of 3-day-old Kunming mice and incubated in growth medium. Samples were collected daily for 5 days. Stem cell distribution, proliferation, differentiation, and migration were monitored during regeneration.ResultsCell proliferation began at the glass membrane periphery then spread gradually toward the membrane center, with the presence of CD34 and CD200 positive stem cells involved in repair initiation. Next, CD34 positive stem cells migrated down the glass membrane, where some participated in ORS formation, while other CD34 cells and CD200 positive cells migrated to hair follicle centers. Within the hair follicle matrix, stem cells divided, grew, differentiated and caused outward expansion of the glass membrane to form a dermal papillary structure containing alpha-smooth muscle actin. Neutrophils attracted to the wound site phagocytosed bacterial and cell debris to protect regenerating tissue from infection.ConclusionIsolated hair follicle ORSs can regenerate new dermal papillary structures in vitro. Stem cells and neutrophils play important roles in the regeneration process.

Peroxisome proliferator-activated receptor (PPAR)-α deletion induces a profound decrease in MnSOD activity, leading to oxidative stress and left ventricular (LV) dysfunction. We tested the hypothesis that treatment of PPAR-α knockout (KO) mice with the SOD mimetic tempol prevents the heart from pathological remodelling and preserves LV function. Twenty PPAR-α KO mice and 20 age-matched wild-type mice were randomly treated for 8 wk with vehicle or tempol in the drinking water. LV contractile parameters were determined both in vivo using echocardiography and ex vivo using papillary muscle mechanics. Translational and posttranslational modifications of myosin heavy chain protein as well as the expression and activity of major antioxidant enzymes were measured. Tempol treatment did not affect LV function in wild-type mice; however, in PPAR-α KO mice, tempol prevented the decrease in LV ejection fraction and restored the contractile parameters of papillary muscle, including maximum shortening velocity, maximum extent of shortening, and total tension. Moreover, compared with untreated PPAR-α KO mice, myosin heavy chain tyrosine nitration and anion superoxide production were markedly reduced in PPAR-α KO mice after treatment. Tempol also significantly increased glutathione peroxidase and glutathione reductase activities (∼ 50%) in PPAR-α KO mice. In conclusion, these findings demonstrate that treatment with the SOD mimetic tempol can prevent cardiac dysfunction in PPAR-α KO mice by reducing the oxidation of contractile proteins. In addition, we show that the beneficial effects of tempol in PPAR-α KO mice involve activation of the glutathione peroxidase/glutathione reductase system.

Heart failure (HF) as a result of myocardial infarction (MI) is a major cause of fatality worldwide. However, the cause of cardiac dysfunction succeeding MI has not been elucidated at a sarcomeric level. Thus, studying the alterations within the sarcomere is necessary to gain insights on the fundamental mechansims leading to HF and potentially uncover appropriate therapeutic targets. Since existing research portrays regulatory light chains (RLC) to be mediators of cardiac muscle contraction in both human and animal models, its role was further explored In this study, a detailed characterisation of the physiological changes (i.e., isometric force, calcium sensitivity and sarcomeric protein phosphorylation) was assessed in an MI mouse model, between 2D (2 days) and 28D post-MI, and the changes were related to the phosphorylation status of RLCs. MI mouse models were created via complete ligation of left anterior descending (LAD) coronary artery. Left ventricular (LV) papillary muscles were isolated and permeabilised for isometric force and Ca2+ sensitivity measurement, while the LV myocardium was used to assay sarcomeric proteins’ (RLC, troponin I (TnI) and myosin binding protein-C (MyBP-C)) phosphorylation levels and enzyme (myosin light chain kinase (MLCK), zipper interacting protein kinase (ZIPK) and myosin phosphatase target subunit 2 (MYPT2)) expression levels. Finally, the potential for improving the contractility of diseased cardiac papillary fibres via the enhancement of RLC phosphorylation levels was investigated by employing RLC exchange methods, in vitro. RLC phosphorylation and isometric force potentiation were enhanced in the compensatory phase and decreased in the decompensatory phase of HF failure progression, respectively. There was no significant time-lag between the changes in RLC phosphorylation and isometric force during HF progression, suggesting that changes in RLC phosphorylation immediately affect force generation. Additionally, the in vitro increase in RLC phosphorylation levels in 14D post-MI muscle segments (decompensatory stage) enhanced its force of isometric contraction, substantiating its potential in HF treatment. Longitudinal observation unveils potential mechanisms involving MyBP-C and key enzymes regulating RLC phosphorylation, such as MLCK and MYPT2 (subunit of MLCP), during HF progression. This study primarily demonstrates that RLC phosphorylation is a key sarcomeric protein modification modulating cardiac function. This substantiates the possibility of using RLCs and their associated enzymes to treat HF.

Both surface pH (pHs) and intracellular pH (pHi) were measured using single- and double-barreled pH-sensitive microelectrodes in isolated sheep cardiac Purkinje strands, rabbit and cat papillary muscle, and mouse and rat soleus muscle. Superfusion of the preparations with a relatively low buffered solution (containing 5 mM HEPES buffered to control pH) causes surface acidosis that correlates with efflux of metabolically produced acids in the unstirred layer of fluid surrounding the tissue. Acidification of the surface layer induces a slower acid change of pHi and depresses the rate of proton extrusion following an imposed intracellular acid load. In cardiac preparations, the lowering of pHi correlates with depression of twitch tension. Transient changes of pHs and pHi are seen when a weak acid or base is suddenly added to, or removed from the superfusion solution. Indirect evidence of the presence of carbonic anhydrase in the extracellular surface layer is obtained from analysis of transient pHs changes in presence and absence of acetazolamide.

The overall aim of this Thesis was to characterise the energetic properties of the mouse papillary muscle as this preparation could become a useful model to study alterations of energetic aspects of cardiac pathologies and heart-focussed genetic changes. Measurements of resting and active metabolism of the papillary muscles were made in vitro using the myothermic technique. In the first study the mechanism underlying impaired contractility of post-ischaemic rat papillary muscle was investigated. The rat preparation is well established and was used to develop protocols and approaches that could later be used as the basis for studies with mouse papillary muscle. The muscles were exposed to simulated ischaemia for 60 min and change in energetics was studied 30 min into the reperfusion phase. The work output was reduced to 66 ± 3% of the pre-ischaemia value and the enthalpy output decreased to 71 ± 3% of pre-ischaemia value. However, there was no change in either initial, 19 ± 3%, or net mechanical efficiency, 9.0 ± 0.9%. These data, in combination with studies of Ca2+ handling, suggests that the reduced work output was caused by attachment of fewer cross-bridges in each twitch, but with no change in work generated by each cross-bridge. The following two studies involved characterisation of the energetics of the mouse papillary muscle and included measurements of resting and active metabolism. The resting metabolic rate varied with muscle size but the mean initial value was tilda 25 mW g-1 and the estimated steady value tilda 5 mW g-1 . The resting metabolic rate declined exponentially with time towards a steady value, with a time constant of 18 ± 2 min. There was no alteration in isometric force output during this time. The magnitude of resting metabolism depended inversely on muscle mass, more than doubled following a change in substrate from glucose to pyruvate and was increased 2.5-fold when the osmolarity of the bathing solution was increased by addition of 300 mM sucrose. Addition of 30 mM BDM affected neither the time course of the decline in metabolic rate nor the eventual steady value. The energy requirements associated with contractile activity were tilda7 mJ g-1 twitch-1 at a contraction frequency of 1 Hz. The enthalpy output was not affected by changing substrate from glucose to pyruvate but did decrease with an increase in temperature. The enthalpy output was partitioned into force-dependent and force-independent components using BDM to selectively inhibit cross-bridge cycling. The force-independent enthalpy output was 18.6 ± 1.9% of the initial enthalpy output. Muscle initial efficiency was &tilda32% and net efficiency tilda 17% when shortening at a realistic velocity. The enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca 2+ into the SR for every three cross-bridge cycles. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction. The suitability of the mouse papillary muscle as a model to study ischaemia and reperfusion damage was also assessed. This preparation is excellent for studying muscle specific changes in work and enthalpy output; however, due to the long-term instability and variability amongst preparations, the suitability of this preparation in prolonged experiments remains uncertain.

The overall aim of this Thesis was to characterise the energetic properties of the mouse papillary muscle as this preparation could become a useful model to study alterations of energetic aspects of cardiac pathologies and heart-focussed genetic changes. Measurements of resting and active metabolism of the papillary muscles were made in vitro using the myothermic technique. In the first study the mechanism underlying impaired contractility of post-ischaemic rat papillary muscle was investigated. The rat preparation is well established and was used to develop protocols and approaches that could later be used as the basis for studies with mouse papillary muscle. The muscles were exposed to simulated ischaemia for 60 min and change in energetics was studied 30 min into the reperfusion phase. The work output was reduced to 66 ± 3% of the pre-ischaemia value and the enthalpy output decreased to 71 ± 3% of pre-ischaemia value. However, there was no change in either initial, 19 ± 3%, or net mechanical efficiency, 9.0 ± 0.9%. These data, in combination with studies of Ca2+ handling, suggests that the reduced work output was caused by attachment of fewer cross-bridges in each twitch, but with no change in work generated by each cross-bridge. The following two studies involved characterisation of the energetics of the mouse papillary muscle and included measurements of resting and active metabolism. The resting metabolic rate varied with muscle size but the mean initial value was tilda 25 mW g-1 and the estimated steady value tilda 5 mW g-1 . The resting metabolic rate declined exponentially with time towards a steady value, with a time constant of 18 ± 2 min. There was no alteration in isometric force output during this time. The magnitude of resting metabolism depended inversely on muscle mass, more than doubled following a change in substrate from glucose to pyruvate and was increased 2.5-fold when the osmolarity of the bathing solution was increased by addition of 300 mM sucrose. Addition of 30 mM BDM affected neither the time course of the decline in metabolic rate nor the eventual steady value. The energy requirements associated with contractile activity were tilda7 mJ g-1 twitch-1 at a contraction frequency of 1 Hz. The enthalpy output was not affected by changing substrate from glucose to pyruvate but did decrease with an increase in temperature. The enthalpy output was partitioned into force-dependent and force-independent components using BDM to selectively inhibit cross-bridge cycling. The force-independent enthalpy output was 18.6 ± 1.9% of the initial enthalpy output. Muscle initial efficiency was &tilda;32% and net efficiency tilda 17% when shortening at a realistic velocity. The enthalpy output decreased with increased contraction frequency but was independent of shortening velocity. On the basis of these values, it was calculated that the twitch energetics were consistent with ATP splitting by half the cross-bridges and the pumping of one Ca 2+ into the SR for every three cross-bridge cycles. The lack of influence of shortening velocity on energy cost supports the idea that the amount of energy to be used is determined early in a twitch and is not greatly influenced by events that occur during the contraction. The suitability of the mouse papillary muscle as a model to study ischaemia and reperfusion damage was also assessed. This preparation is excellent for studying muscle specific changes in work and enthalpy output; however, due to the long-term instability and variability amongst preparations, the suitability of this preparation in prolonged experiments remains uncertain.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Physiotherapy and Exercise Science
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