Joseph F. Y. Hoh

Electrophoretic analysis of multiple forms of rat cardiac myosin: Effects of hypophysectomy and thyroxine replacement

Electrophoretic analysis in pyrophosphate gels of intact myosin of adult rat myocardium revealed the presence of five distinct components, two in atrial myosin (A1, A2) and three in ventricular myosin (V1, V2, V3). Analysis of Ca2+-activated myosin ATPase activity in the gels revealed that A1, A2 and V1 had about the same specific activity; V3 had the lowest activity, while that of V2 was intermediate. At 3 weeks of age, ventricular myosin was exclusively V1, there being a slow age-dependent shift in myosin distribution toward the adult pattern. Hypophysectomy of juvenile rats caused a shift towards V3, which by 45 days after operation accounted for 90% of total myosin. This shift was prevented by daily administration of 5 μg of thyroxine. When chronically hypophysectomized rats were similarly treated, there was a rapid shift of myosin components towards V1. These changes in distribution of myosin components were associated with appropriate changes in Ca2+-activated ATPase activity of electrophoretically purified ventricular myosin as measured in the gel. Sodium dodecyl sulphate gel analysis of purified myosin showed little or no contaminants. For both V1 and V3 rich myosins, the ratio of the two ventricular light chains (V-LC1, V-LC2) and the ratio of light chains to heavy chains are consistent with the model that each intact molecule contains two each of V-LC1 and V-LC2. Sodium dodecyl sulphate electrophoresis of purified atrial myosin revealed two types of light chains: A-LC1 (mol. wt 27 000, not resolved from V-LC1) and A-LC2 (mol. wt 22 000), the latter being distinct from V-LC2.

Expression of extraocular myosin heavy chain in rabbit laryngeal muscle

SummaryThe intrinsic laryngeal muscles of mammals are functionally heterogeneous, some of these muscles (e.g. the thyroarytenoid) contract extremely rapidly, like extraocular muscle, whilst others (e.g. the cricothyroid) contract as fast as limb fast muscle. The extraordinarily rapid contraction speed of extraocular muscles is associated with a fast myosin not found in limb muscles. In this work we explored the possibility that the thyroarytenoid muscle may also express this extraocular-specific fast myosin by raising a monoclonal antibody (mab 4A6) against its heavy chain. Electrophoretic separation of native isomyosins revealed that both the extraocular and the thyroarytenoid have two similar bands migrating ahead of bands found in limb fast or cricothyroid myosins. These two bands bound mab 4A6. The thyroarytenoid muscle can be divided into two divisions, a vocalis division which is important in phonation and an external division which functions in closing the glottis. Fibres in the vocalis are heterogeneous, some stain with mab 4A6, whilst others stain with mabs against limb myosin heavy chains. Fibres in the external division stain almost homogeneous with mab 4A6. The immunohistochemical staining pattern in the cricothyroid muscle resembled that of fast limb muscle: no fibres stained with mab 4A6. Thus, the high speed of contraction of the thyroarytenoid is associated with the same myosin heavy chain found in extraocular muscles, this characteristic is presumably an evolutionary adaptation for rapid closure of the glottis to enhance airway defense mechanisms.

Adrenaline increases the rate of cycling of crossbridges in rat cardiac muscle as measured by pseudo-random binary noise-modulated perturbation analysis.

The mechanism of action of adrenaline on cardiac contractility in rat papillary muscles containing V1 and V3 isomyosins was analyzed during barium-activated contractures at 25± C by frequency domain analysis using pseudo-random binary noise-modulated perturbations. The analysis characterizes a frequency (fmin) at which dynamic stiffness of a muscle is a minimum, a parameter that reflects the rate of cycling of crossbridges. We have previously shown that fmin for V1- and V3-containing papillary muscles were 2.1±0.2 Hz (mean±SD) (n = 10) and 1.1±0.2 Hz (n = 8), respectively, and that these values were independent of the level of activation. The present studys goal was to determine whether the inotropic action of adrenaline was associated with an increased rate of crossbridge cycling. The results show that a saturating dose of adrenaline increased fmin in V1 hearts by 49 ±2% (n = 11). The action on V3 hearts was significantly less; the increase in fmin was 26 ±2% (n = 6). The increase in fminfor V1 hearts was shown to be sensitive to the β-blocking agent propranolol. These results suggest that adrenaline significantly increases the rate of crossbridge cycl ing by a β-receptor-mediated mechanism. We conclude that the increased contractility of the heart in the presence of adrenaline arises not only from more complete activation of the contractile proteins but also from the increased rate at which each crossbridge can transduce energy.

Influence of V1 and V3 isomyosins on the mechanical behaviour of rat papillary muscle as studied by pseudo-random binary noise modulated length perturbations

SummaryExperiments were done on (1) four-week-old rats, containing biochemically verified V1 only, and (2) thyroidectomized adult rats, treated with propylthiouracil, verified to contain V3 only. Contracture tension was induced in isolated papillary muscles either by high potassium solution or 0.5 mmol 1−1 Ba2+. Small amplitude length perturbations with peak-to-peak value not exceeding 0.15% L0 were applied to the activated muscle. Both the applied length perturbations and the corresponding resulting force changes were analysed by computer for dynamic stiffness and phase values. In order to reduce data acquisition time, pseudo-random binary noise length changes, rather than the conventional sinusoidal length changes, were used. The plot of the dynamic stiffness against frequency displays a minimum, akin to a resonance phenomenon. The frequency,fmin, at which this resonance occurs, reflects crossbridge kinetics. It was found that thefmin1 values for the two types of papillary muscles differed by a factor of two. Experiments were also done on chemically skinned muscles containing V1 or V3 isomyosin activated by different concentrations of either barium or calcium ions. It was found thatfmin values of skinned fibres were higher than those obtained from intact fibres. However, for each type of muscle thefmin was independent of the activator used as well as the level of activation. The ratio offmin for V3 to that for V1 remained the same as for intact preparations. We conclude that the difference in mechanical parameters did not arise from a possible difference in excitation-contraction coupling mechanism, but rather is a difference in the dynamic properties of the two types of crossbridges.

Energetic consequences of thyroid-modulated shifts in ventricular isomyosin distribution in the rat.

SummaryHeat production (measured myothermically), force development and isomyosin distribution were measured in left ventricular papillary muscles from adult male rats in three thyroid states: hyperthyroid (T3), euthyroid (C) and hypothyroid (Tx). Rats were rendered hyperthyroid by daily injections of tri-iodothyronine and hypothyroid by radioisotopic thyroidectomy. Papillary muscle performance was measured both for trains of isometric twitches and for brief (2 s) tetani achieved by increasing the Ca2+ concentration and adding caffeine to the bathing solution. Resting metabolic rate was uninfluenced by thyroid state. Heat-stress relations were determined for both twitches and tetani by altering muscle length. Tx muscles showed an elevated stress-independent or activation heat (intercept of the heat-stress relation), a depressed stress-dependent heat (slope of the heat-stress relation), and greatly enhanced peak twitch and tetanic (Smax) stresses. When normalized for Smax, the maximal rates of tetanic stress development and heat production were depressed in the Tx group. In the T3 group, only the normalized maximal rate of tetanic stress development was significantly increased. The lack of significant effects on other mechanical and energetic parameters probably reflects an under-dosing of animals in this tri-iodothyronine-treated group, an interpretation supported by the modest change in isomyosin distribution resulting from the treatment regimen used.Separate isomyosin analyses of papillary muscles and their associated ventricles yielded excellent correlation demonstrating the suitability of papillary muscles as a model of ventricular wall tissue. By experimentally manipulating the thyroid state, the distribution of the three ventricular isomyosins were correspondingly altered with a shift toward a greater and lesser proportion of high activity myosin ATPase in the hyperthyroid and hypothyroid groups respectively. The average proportions of the myosin heavy chain associated with high actin-activated myosin ATPase were 86, 74 and 6% for groups T3, C and Tx respectively. The measured changes in papillary muscle energetics correlate well with these thyroid-induced changes in isomyosin distribution and can be explained in terms of altered crossbridge dynamics.

Fiber Types in Rat Laryngeal Muscles and Their Transformations After Denervation and Reinnervation

The intrinsic laryngeal muscles cricothyroid (CT) and thyroarythenoid (TA) differ in myosin expression. CT expresses limb myosin heavy chains (MyHCs) and TA expresses an MyHC found in extraocular (EO) muscles, in addition to limb isoforms. We used immunohistochemical (IHC) analyses with highly specific monoclonal antibodies (MAbs) against various MyHCs to study muscle fiber types in rat CT and TA and to investigate whether nerves to laryngeal muscles control MyHC expression. CT was found to have the full complement of limb fiber types. TA had three major fiber types: 2b/eo, co-expressing 2B and EO MyHCs, 2x/2b, co-expressing 2X and 2B MyHCs, and 2x, expressing 2X MyHC. Type 2a and slow fibers were absent. TA consisted of two divisions: the external division (TA-X), which is homogeneously 2b/eo, and the vocalis division (TA-V), composed principally of 2x and 2b/eo fibers with a minority of 2x/2b fibers. TA-V had two compartments that differ in fiber type composition. At 4 weeks after cutting and re-uniting the recurrent laryngeal nerve (RLN), many 2b/eo fibers in the TA-X began to express 2X MyHC, while EO and 2B MyHC expression in these fibers progressively declined. By 12 weeks, up to 16.5% of fibers in the TA-X were of type 2x. These findings suggest that nerve fibers originally innervating 2x fibers in TA-V and other muscles have randomly cross-innervated 2b/eo fibers in the TA-X and converted them into 2x fibers. We conclude that CT and TA are distinct muscle allo-types and that laryngeal muscle fibers are subject to neural regulation.

Immunocytochemical and electrophoretic analyses of changes in myosin gene expression in cat limb fast and slow muscles during postnatal development

SummaryChanges in myosin synthesis during the postnatal development of the fast extensor digitorum longus (EDL) and the slow soleus muscles of the kitten were examined using immunocytochemical techniques supplemented by pyrophosphate gel electrophoresis and gel electrophoresis-derived enzyme linked immunosorbent assay (GEDELISA) of myosin isoforms. The antibodies used were monoclonals against heavy chains of slow and fast myosins and a polyclonal against foetal/embryonic myosin. In both muscles in the newborn kitten, there was a population of more mature fibres which stained strongly for slow but weakly for foetal/embryonic myosin. These fibres were considered to be primary fibres. They formed 4.8% of EDL fibres and 26% of soleus fibres at birth, and continued to express slow myosin in adult muscles. The less mature secondary fibres stained strongly for foetal/embryonic myosin, and these could be divided into two subpopulations; fast secondaries in which foetal/embryonic myosin was replaced by fast myosin, and slow secondaries in which the myosin was replaced by slow myosin. At 50 days the EDL had a large population of fast secondaries (83% of total fibres) and a small population of slow secondaries which gradually transformed into fast fibres with maturity. The vast majority of secondary fibres in the soleus were slow secondaries, in which slow myosin synthesis persisted in adult life. There was a restricted zone of fast secondaries in the soleus, and these gradually transformed into slow fibres in adult life. It is proposed that the emergence of primary fibres and the two populations of secondary fibres is myogenically determined.

Differential expression of myosin heavy chain isoforms between abductor and adductor muscles in the human larynx

OBJECTIVE: This study examines the differential expression of myosin heavy chain (MyHC) components in human laryngeal muscle groups. STUDY DESIGN: A battery of monospecific monoclonal antibodies in Western blots was used to determine expression of IIX, extraocular-specific (EOM), and IIB MyHCs for the thyroarytenoid (TA), vocalis (VOC), lateral cricoarytenoid (LCA), cricothyroid (CT), and posterior cricoarytenoid (PCA) muscles obtained from fresh cadaver specimens. RESULTS: Fast IIX MyHC was only expressed in the TA, VOC, and LCA muscles. Fast IIA and slow MyHCs were expressed in all laryngeal muscles including the CT and PCA. The CT with mixed phonatory and respiratory function and the PCA with respiratory function did not express IIX MyHC. The 2 MyHC isoforms associated with the highest speeds of contraction in rat laryngeal muscle, namely, the EOM MyHC and IIB MyHC, were not detected in human laryngeal muscles. Novel MyHC bands were not detected in SDS-PAGE gels or Western blots using a broad specificity MyHC antibody. CONCLUSION: The profile of MyHC expression in human laryngeal muscles differs from that observed in human extraocular and masticator muscles, and other mammalian species. Our data demonstrate that IIX MyHC expression is associated primarily with muscles affecting glottic closure and is absent in CT and PCA. SIGNIFICANCE: A higher percentage of IIX MyHC is expected to impart a high speed of shortening to the TA and LCA muscles. The absence of IIX MyHC in muscles with respiratory (PCA) and mixed respiratory/phonatory function (CT) further supports the inference that the physiologic difference between laryngeal muscles is reflected in the molecular composition of contractile protein.

Mechanism of action of endothelin in rat cardiac muscle: cross-bridge kinetics and myosin light chain phosphorylation

1 The molecular mechanism of inotropic action of endothelin was investigated in rat ventricular muscle by studying its effects on characteristics of isometric twitch, barium‐induced steady contracture and the level of incorporation of 32Pi into myosin light chain 2. 2 Exposure of rat papillary muscle to endothelin caused an increase in isometric twitch force but did not alter the twitch‐time parameters. 3 Endothelin did not significantly change the maximum contracture tension but did cause an increase in contracture tension at submaximal levels of activation, without changes in the tension‐to‐stiffness ratio and kinetics of attached cross‐bridges. Kinetics of attached cross‐bridges were deduced during steady contracture from complex‐stiffness values, and in particular from the frequency at which muscle stiffness assumes a minimum value, fmin. Endothelin did not alter fmin. 4 Endothelin caused an increase in the level of incorporation of 32Pi into myosin light chain 2 without a concurrent change in the level of incorporation of 32Pi into troponin I. 5 We conclude that the inotropic action of endothelin is not due to an increase in the kinetics of attached cross‐bridges, nor due to a change in the force per unit cross‐bridge, but may result from an increased divalent cation sensitivity caused by elevated myosin light chain 2 phosphorylation, resembling post‐tetanic potentiation in fast skeletal muscle fibres.

Insulin-like growth factor I slows the rate of denervation induced skeletal muscle atrophy

Loss of the nerve supply to skeletal muscle results in a relentless loss of muscle mass (atrophy) over time. The ability of insulin-like growth factor-1 to reduce atrophy resulting from denervation was examined after transection of the sciatic nerve in transgenic MLC/mIGF-1 mice that over-express mIGF-1 specifically in differentiated myofibres. The cross sectional area (CSA) of all types of myofibres and specifically type IIB myofibres was measured in tibialis anterior muscles from transgenic and wild-type mice at 28 days after denervation. There was a marked myofibre atrophy ( approximately 60%) in the muscles of wild-type mice over this time with increased numbers of myofibres with small CSA. In the muscles of MLC/mIGF-1 mice, over-expression of mIGF-1 reduced the rate of denervation induced myofibre atrophy by approximately 30% and preserved myofibres with larger CSA, compared to wild-type muscles. It is proposed that the protective effect of mIGF-1 on denervated myofibres might be due to reduced protein breakdown.