Changes in myosin heavy chain stoichiometry in pig tracheal smooth muscle during development

Correlation of tracheal smooth muscle function with structure and protein expression during early development

With increased survival of premature infants, understanding the impact of development on airway function and structure is imperative. Airway smooth muscle plays a primary role in the modulation of airway function. The purpose of this study is to correlate the functional maturation of airway smooth muscle during the perinatal period with structural alterations at the cellular, ultrastructural, and molecular levels. Length-tension and dose-response analyses were performed on tracheal rings acquired from preterm and term newborn lambs. Subsequent structural analyses included isolated airway smooth muscle cell length, electron microscopy, and myosin heavy chain isoform expression measurements. Functionally the compliance, contractility, and agonist sensitivity of the tracheal rings matured during preterm to term development. Structurally, isolated cell lengths and electron microscopic ultrastructure were not significantly altered during perinatal development. However, expression of myosin heavy chain isoforms increased significantly across the age range analyzed, correlating with the maturational increase in smooth muscle contractility. In conclusion, the developmental alterations in tracheal function appear due, in part, to enhanced smooth muscle myosin heavy chain expression.

RNA interference targeting embryonic myosin heavy chain isoform inhibited mRNA expressions of phenotype markers in rabbit cultured vascular smooth muscle cells

To investigate whether the knockdown of SMemb gene expression induces phenotypic modulation of vascular smooth muscle (VSM) cells toward a contractile type, we constructed a siRNA targeting the 3' untranslated region (UTR) of SMemb gene (SMemb-siRNA). The SMemb-siRNA was introduced into cultured rabbit VSM cells for 48 h at 37 degrees C by the lipofection method. The mRNA expressions were estimated by comparative reverse transcription-polymerase chain reaction (RT-PCR). SMemb-siRNA significantly decreased the ratio of SMemb to glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mRNA expression in a dose-dependent manner (P < 0.01): 0 nM, 0.90 +/- 0.08; 100 nM, 0.43 +/- 0.07. Immunofluorescence and immunoblot analyses demonstrated that SMemb-siRNA markedly decreased SMemb protein expression to 56% +/- 7.8% (P < 0.01). Other MHC isoform (SM1 and SM2) mRNA expressions were not changed. The relative mRNA expressions of other phenotype markers (plasminogen activator inhibitor (PAI)-1 and beta-actin) were significantly decreased by SMemb-siRNA to 71% +/- 7.5% and 61% +/- 7.5%, respectively (P < 0.01). Expression of smooth muscle (SM) alpha-actin protein and cell proliferation was not changed by SMemb-siRNA. Thus, SMemb gene might be involved in the transcription of PAI-1 and beta-actin, but not involved in SM alpha-actin and cell proliferation in cultured VSM.

Urinary bladder contraction and relaxation: physiology and pathophysiology

The detrusor smooth muscle is the main muscle component of the urinary bladder wall. Its ability to contract over a large length interval and to relax determines the bladder function during filling and micturition. These processes are regulated by several external nervous and hormonal control systems, and the detrusor contains multiple receptors and signaling pathways. Functional changes of the detrusor can be found in several clinically important conditions, e.g., lower urinary tract symptoms (LUTS) and bladder outlet obstruction. The aim of this review is to summarize and synthesize basic information and recent advances in the understanding of the properties of the detrusor smooth muscle, its contractile system, cellular signaling, membrane properties, and cellular receptors. Alterations in these systems in pathological conditions of the bladder wall are described, and some areas for future research are suggested.

The carboxyl-terminal isoforms of smooth muscle myosin heavy chain determine thick filament assembly properties

The alternatively spliced SM1 and SM2 smooth muscle myosin heavy chains differ at their respective carboxyl termini by 43 versus 9 unique amino acids. To determine whether these tailpieces affect filament assembly, SM1 and SM2 myosins, the rod region of these myosin isoforms, and a rod with no tailpiece (tailless), were expressed in Sf 9 cells. Paracrystals formed from SM1 and SM2 rod fragments showed different modes of molecular packing, indicating that the tailpieces can influence filament structure. The SM2 rod was less able to assemble into stable filaments than either SM1 or the tailless rods. Expressed full-length SM1 and SM2 myosins showed solubility differences comparable to the rods, establishing the validity of the latter as a model for filament assembly. Formation of homodimers of SM1 and SM2 rods was favored over the heterodimer in cells coinfected with both viruses, compared with mixtures of the two heavy chains renatured in vitro. These results demonstrate for the first time that the smooth muscle myosin tailpieces differentially affect filament assembly, and suggest that homogeneous thick filaments containing SM1 or SM2 myosin could serve distinct functions within smooth muscle cells.

Smooth-muscle myosin heavy-chain SM-B isoform expression in developing and adult rat lung

The smooth-muscle cells composing the vasculature and airways of the lung display a variety of contractile protein phenotypes. To date, however, it has remained unclear how these phenotypes might contribute differentially to contractile activity. To address this issue, we made monospecific rabbit polyclonal antibodies against the difference peptide for the SM-B smooth-muscle myosin heavy chain (SMMHC) and used these to investigate the distribution of the SM-B isoform in lung. SM-B has a seven-amino acid insert in the head region that is known to result in a higher actin-activated adenosine triphosphatase activity and in vitro motility. During development, reactivity is first seen in the trachea and bronchi of saccular lung at the time of birth, when other SMMHC isoforms also are present. Immunoreactivity spreads distally through the airways as development proceeds, reaching the level of alveolar septae in the adult. Although the smaller vessels of the pulmonary vasculature react strongly with the SM-B antibody, reactivity is infrequently observed in large pulmonary vessels. Adult tracheal smooth muscle is highly and more uniformly reactive, commensurate with its relatively high maximal velocity of shortening. The differential expression of the SM-B isoform in vascular and airway smooth muscles demonstrated in this study may provide the molecular basis for functional differences between these smooth-muscle cell types and may provide one mechanism for adapting contractility in response to physiologic stresses in the lung.

Cross-bridge cycling in smooth muscle: a short review

This review is focused on the cross-bridge interaction of the organized contractile system of smooth muscle fibres. By using chemically skinned preparations the different enzymatic reactions of actin-myosin interaction have been associated with mechanical events. A rigor state has been identified in smooth muscle and the binding of ATP causes dissociation of rigor cross-bridges at rates slightly slower than those in skeletal muscle, but fast enough not to be rate-limiting for cross-bridge turn over in the muscle fibre. The release of inorganic phosphate (Pi) is associated with force generation, and this process is not rate-limiting for maximal shortening velocity (Vmax) in the fully activated muscle. The binding of ADP to myosin is strong in the smooth muscle contractile system, a property that might be associated with the generally slow cross-bridge turn over. Both force and Vmax are modulated by the extent of myosin light chain phosphorylation. Low levels of activation are considered to be associated with the recruitment of slowly cycling dephosphorylated cross-bridges which reduces shortening velocity. The attachment of these cross-bridge states in skinned smooth muscles can be regulated by cooperative mechanisms and thin filament associated systems. Smooth muscles exhibit a large diversity in their Vmax and the individual smooth muscle tissue can alter its Vmax under physiological conditions. The diversity and the long-term modulation of phenotype are associated with changes in myosin heavy and light chain isoform expression.

Lung smooth muscle differentiation

The vascular and visceral smooth muscle tissues of the lung perform a number of tasks that are critical to pulmonary function. Smooth muscle function often is compromised as a result of lung disease. Though a great deal is known about regulation of smooth muscle cell replication and cell and tissue contractility, much less is understood regarding the phenotype of the contractile protein machinery of lung smooth muscle cells. This review focuses on the expression of cytoskeletal and contractile proteins of lung vascular and airway smooth muscle cells during development, in the adult and during vascular and airway remodeling. Emphasis is placed on the expression of the heavy chain of smooth muscle myosin, as well as the regulation of its gene. Important areas for future research are discussed.

Myosin isoform shifts and decreased reactivity in hypoxia-induced hypertensive pulmonary arterial muscle

The principal stimulus that evokes pulmonary hypertension is chronic alveolar hypoxia. Pulmonary hypertension is associated with remodeling of the vessel walls, involving hypertrophy and hyperplasia of pulmonary arterial smooth muscle (PASM) and a concomitant increase in the deposition of connective tissue, resulting in increased wall thickness. The purpose of the present study was to determine the effect of hypoxia-induced hypertension on the structure and function of PASM. Experiments were designed to determine whether hypoxia-induced pulmonary hypertension is associated with alterations in PASM: 1) reactivity to a variety of agonists, 2) contractile protein proportions and isoforms, and 3) structural properties. Young adult male rats were made hypoxic by lowering the fraction of inspired O2 (10%) for 14 days. Pulmonary arterial segments were isolated and dose-response curves to various agonists (high K+, norepinephrine, serotonin, angiotensin II, and adenosine) were generated. Gel electrophoresis was used to measure changes in the relative amounts of actin or myosin and of myosin heavy chain (MHC) isoforms. Structural changes were correlated with the pharmacological and biochemical data. Hypoxia-induced pulmonary hypertension caused a general decreased reactivity, an increase in the proportion of nonmuscle to muscle MHC isoforms in PASM, and an increase in arterial wall thickness with PASM hypertrophy or hyperplasia.

Myosin heavy chain transitions during development. Functional implications for the respiratory musculature

The myosin heavy chain (MHC) exists as multiple isoforms that are encoded for by a family of genes. The respiratory musculature demonstrates muscle-specific and temporally-dependent changes in MHC isoform expression during maturation. Developmental expression of MHC isoforms correlate well with postnatal changes in actomyosin ATPase activity, specific force generation (P0/CSA), maximum unloaded velocity of shortening (V0) and and fatigue resistance. More specifically, as the expression of MHCneonatal declines and MHC2A, MHC2X, and MHC2B increase, actomyosin ATPase activity, P0/CSA, V0, and muscle fatigability increase. The increase in actomyosin ATPase activity with maturation is partially offset by a postnatal increase in oxidative capacity; however, as fatigue resistance declines with development it is apparent that the energy costs of contraction are not fully matched by an increase in energy production. Developmental transitions in smooth muscle MHC phenotype also occur although their functional importance remains unclear.

Myosin isoform heterogeneity in single smooth muscle cells

We review the current understanding of the myosin heavy chain (MHC) isoforms and show that the mRNA levels of smooth muscle (SM)1 and SM2 mimic the expressed levels of SM1 and SM2 protein. The reverse transcriptase-polymerase chain reaction technique has been shown to be sufficiently sensitive to examine SM-MHC expression at the single cell level. Most single smooth muscle cells isolated from adult rabbit carotid express both SM1 and SM2. However, expression of these SM-MHC isoforms at the cellular level is nonuniform and highly variable. This work provides a foundation for future investigations as to the possible unique functional characteristics of the SM-MHC isoforms, SM1 and SM2. This methodology may also prove useful when used with mechanical studies to determine the physiological significance of the alternatively spliced myosin isoforms, including the SM-MHC-head and LC17 isoforms.

Preferential differentiation of P19 mouse embryonal carcinoma cells into smooth muscle cells. Use of retinoic acid and antisense against the central nervous system-specific POU transcription factor Brn-2

Investigation of the molecular mechanisms that control smooth muscle cell (SMC) development and differentiation is a prerequisite in understanding the regulatory mechanisms of physiological and pathological SMC-associated vascular processes. The pluripotent murine embryonal carcinoma P19 cell, whose developmental potential resembles that of early embryonic cells, can develop into cell types derived from the neuroectoderm, mesoderm, and endoderm. In the present study, we have shown a unique strategy to enhance SMC differentiation in P19 cells. Under chemical induction of high concentrations of retinoic acid (1 micromol/L), P19 cells showed optimum differentiation into SMCs. Because the P19 cells thus induced also showed differentiation into neuronal cells, a strategy to block neuronal lineage differentiation was developed using a stable transformant antisense RNA construct against Brn-2, a neuronal lineage-specific POU-domain transcription factor; thus, by specifically inhibiting neuronal differentiation, enhanced SMC differentiation by P19 cells was attained. SMC expression was confirmed by immunohistochemical staining, RNA analysis (RNase protection assay), and protein analysis (Western blot) using SMC-specific markers (eg, SM1 and calponin) and alpha-smooth muscle actin. Our results show that the pathway of SMC differentiation may provide an in vitro system useful in the investigation of SMC regulatory mechanisms (eg, transcriptional regulation) and in the further understanding of SMC development and differentiation.

Contractility and myosin heavy chain isoform patterns in developing tracheal muscle

Changes in airway smooth muscle reactivity with development may be caused by either modification of the excitation-contraction coupling system or alteration of the contractile apparatus. The mechanism responsible for the reported changes in reactivity was addressed in this study by examining airway smooth muscle contractility and myosin heavy chain isoform patterns as a function of post-neonatal development. Changes in length and force, in response to supramaximal electrical stimulation, were recorded simultaneously as functions of time for tracheal smooth muscle (TSM) strips from 8-week-old and 25-week-old male rabbits. Both the passive and active length-tension (L-T) curves as well as the force-velocity (F-V) curves for the two age groups of rabbit TSM were not significantly different indicating no changes in contractility during post-neonatal development in rabbits. This conclusion is surprising in light of reports of myosin heavy chain (MHC) isoform shifts in porcine trachealis during comparable periods of development. Therefore, MHC isoform ratios were compared by sodium dodecyl sulfate-polyacrylimide gel electrophoresis for tracheal smooth muscle from male rabbits of 8 and 25 weeks of age. Unlike the reported MHC isoform shifts in the pig tracheal muscle, the rabbit trachealis showed no difference in MHC isoform ratios between the two age groups compared in this study. In conclusion, no changes occur in contractility or MHC isoform patterns during post-neonatal development of rabbit tracheal smooth muscle. Therefore, reported changes in airway muscle reactivity are likely due to changes in receptors or in second messenger systems rather than to changes in the contractile apparatus.

Embryonic chicken gizzard: smooth muscle and non-muscle myosin isoforms

Antibodies to smooth muscle and non-muscle myosin allow the development of smooth muscle and its capillary system in the embryonic chicken gizzard to be followed by immunofluorescent techniques. Although smooth muscle development proceeds in a serosal to luminal direction, angiogenetic cell clusters develop independently at the luminal side close to the epithelial layer, and the presumptive capillaries invade the developing muscle in a luminal to serosal direction. The smooth muscle and non-muscle myosin heavy chains in this avian system cannot be separated by SDS polyacrylamide gel electrophoresis and do not show isoform specificity in immunoblotting, unlike the system found in mammals. Only two myosin heavy chains with M(r) of 200 and 196 kDa were separable and considerable immunological cross-reactivity was found between the denatured myosin isoform heavy chains.

Human smooth muscle myosin heavy chain isoforms as molecular markers for vascular development and atherosclerosis

Smooth muscle myosin heavy chains (MHCs) exist in multiple isoforms. Rabbit smooth muscles contain at least three types of MHC isoforms: SM1 (204 kD), SM2 (200 kD), and SMemb (200 kD). SM1 and SM2 are specific to smooth muscles, but SMemb is a nonmuscle-type MHC abundantly expressed in the embryonic aorta. We recently reported that these three MHC isoforms are differentially expressed in rabbit during normal vascular development and in experimental arteriosclerosis and atherosclerosis. The purpose of this study was to clarify whether expression of human smooth muscle MHC isoforms is regulated in developing arteries and in atherosclerotic lesions. To accomplish this, we have isolated and characterized three cDNA clones from human smooth muscle: SMHC94 (SM1), SMHC93 (SM2), and HSME6 (SMemb). The expression of SM2 mRNA in the fetal aorta was significantly lower as compared with SM1 mRNA, but the ratio of SM2 to SM1 mRNA was increased after birth. SMemb mRNA in the aorta was decreased after birth but appeared to be increased in the aged. To further examine the MHC expression at the histological level, we have developed three antibodies against human SM1, SM2, and SMemb using the isoform-specific sequences of the carboxyl terminal end. Immunohistologically, SM1 was constitutively positive from the fetal stage to adulthood in the apparently normal media of the aorta and coronary arteries, whereas SM2 was negative in fetal arteries of the early gestational stage. In human, unlike rabbit, aorta or coronary arteries, SMemb was detected even in the adult. However, smaller-sized arteries, like the vasa vasorum of the aorta or intramyocardial coronary arterioles, were negative for SMemb. Diffuse intimal thickening in the major coronary arteries was found to be composed of smooth muscles, reacting equally to three antibodies for MHC isoforms, but reactivities with anti-SM2 antibody were reduced with aging. With progression of atherosclerosis, intimal smooth muscles diminished the expression of not only SM2 but also SM1, whereas alpha-smooth muscle actin was well preserved. We conclude from these results that smooth muscle MHC isoforms are important molecular markers for studying human vascular smooth muscle cell differentiation as well as the cellular mechanisms of atherosclerosis.

Early maturation of force production in pig tracheal smooth muscle during fetal development

The contractility of airway smooth muscle is fully established at late term at birth but its responsiveness during fetal life has not been defined. In this study, the contractile force of airway smooth muscle to acetylcholine (ACh), K+ depolarizing solution, and electrical field stimulation (EFS) was measured in tracheas from small fetal pigs. Contraction to either agonist and to EFS was detectable in fetuses of as low as 9 g body weight, which corresponds to approximately 36 days of gestation. Isometric force increased progressively with age, reaching 4.1 +/- 0.4 mN for K+ and 5.8 +/- 0.5 mN for ACh (10(-4) M) at 600 g fetal weight (90 days). However, when normalized for cross sectional area of smooth muscle, the stress was essentially the same from 17- to 600-g fetuses. (K+: 17 g = 74.4 +/- 10.6 mN/mm2, 600 g = 89.3 +/- 13.0 mN/mm2; ACh [10(-4) M]: 17 g = 76.3 +/- 16.0 mN/mm2, 600 g = 127.0 +/- 13.0 mN/mm2). The sensitivities of the various groups to ACh were not significantly different (e.g., EC50: 30 g = 4.0 +/- 0.2 x 10(-6) M, 600 g = 3.7 +/- 1.1 x 10(-6) M). EFS produced frequency-dependent contractile responses in all groups. With increasing fetal size, there was a corresponding increase in force. When this force was normalized to a maximum ACh response (10(-4) M), there was no significant difference between groups of fetuses. Histologic examination showed the major tissue components of the trachea were present in fetuses above 7 g. Immunocytochemistry detected myosin, caldesmon, and filamin in the smooth muscle from fetuses of 7 g and above, showing that contractile and actin-binding proteins were present from a very early age. It is concluded that smooth muscle contractile function is well developed very early in fetal life in pigs.

A novel myosin heavy chain isoform in vascular smooth muscle

Previous studies demonstrated two myosin heavy chain isoforms in vascular smooth muscles with SDS-polyacrylamide gel electrophoresis; MHC1 (204 kDa) and MHC2 (200 kDa). We report the existence of a novel myosin heavy chain isoform, MHC3 (196 kDa), which was exclusively contained in inferior vena cava. Equal amount of MHC1 and MHC2 was observed in aorta and pulmonary artery, respectively. However, inferior vena cava contained only MHC3. Proteolytic artifact was refuted by immunoblotting of tissue homogenates without purification, or SDS-polyacrylamide gel electrophoresis of myosin bands isolated by pyrophosphate gel electrophoresis. Furthermore, alpha-chymotryptic cleavage of MHC1, MHC2, and MHC3 displayed different peptide maps, indicating the primary structural difference among all three isoforms.

Contractile and cytoskeletal proteins in smooth muscle during hypertrophy and its reversal

Hypertrophy of rat urinary bladder smooth muscle was induced by partial urethral obstruction. Bladder weight increased from 70 to 240 mg after 10 days and to 700 mg after 7 wk. Removal of the obstruction after 10 days caused a regression of bladder weight to 130 mg. The relative volume of smooth muscle in the bladder wall increased during hypertrophy. The concentration of myosin in the smooth muscle cells decreased in 10-day hypertrophied bladders, whereas the concentration of actin was unchanged. The actin-myosin ratio was 2.3 in controls, 3.3 in 10-day obstructed bladders, and 2.9 in 7-wk obstructed bladders. After removal of obstruction, the ratio was normalized. Two isoforms of myosin heavy chains were identified (SM1 and SM2). The relative amount of SM2 decreased during hypertrophy. The relative proportion of actin isoforms (alpha, beta, and gamma) was altered toward more gamma and less alpha. These changes were reversible upon removal of the obstruction. Desmin was the dominating intermediate filament protein. The concentration of desmin and filamin increased in the hypertrophic bladders. The increased desmin-actin and filamin-actin ratios in obstructed bladders were normalized after removal of the obstruction. The results suggest that the turnover of contractile and cytoskeletal proteins is fast and can be regulated in response to changes in the functional demands in smooth muscle.

Developmental changes in actin and myosin heavy chain isoform expression in smooth muscle

Smooth muscle cells express isoforms of actin and myosin heavy chains (MHC). In early postnatal animals the nonmuscle (NM) actin and MHC isoforms in vascular (aorta) smooth muscle were present in relatively high percentages. More than 30% of the MHC and 40% of the actin isoforms were NM. The relative percentage of the NM isoforms decreased significantly as the animals reached maturity, with NM MHC less than 10% and NM actin less than 30% of the totals. Concurrent with this decrease in NM isoforms was an increase in the smooth muscle (SM) isoforms. The relative changes and time frame in which these changes occurred were very similar for the actin and MHC isoforms. In arterial tissue there were species differences for changes with development in the two SM MHC isoforms (SM1 and SM2). The ratio of SM1:SM2 in young rat aorta was approximately 0.5, while this same ratio was approximately 3 in young swine carotid. Both adult rats and swine had a SM1:SM2 MHC ratio of approximately 1.2. Rat bladder smooth muscle showed no significant change in NM vs SM ratio between young and old rats, while the SM1:SM2 ratio decreased from 2.7 to 1.7 between these age groups. The shifts in alpha and beta actin were similar to those in the vascular tissue, but of much smaller magnitude.

Different ratio of myosin heavy chain isoforms in arterial smooth muscle of spontaneously hypertensive rats

The relative proportion of the two putative heavy chains of smooth muscle myosin (MHC1 and MHC2) was determined in the caudal and femoral arteries of spontaneously hypertensive rats (SHR) and normotensive (WKY) rats at 16 weeks of age. The heavy chain polypeptides with Mr 204,000 and 200,000 were resolved electrophoretically under denaturing conditions in porous polyacrylamide gels. Both proteins reacted strongly with a monoclonal antibody (2C4) to smooth muscle MHC. In caudal arteries the ratio of MHC1/MHC2 was 3.1:1 in SHR rats compared with 1.8:1 in WKY rats (p less than 0.005) and similarly in femoral arteries, 2.8:1 vs 1.5:1 (p less than 0.001). In the portal vein there was no significant difference, 1.7:1 vs 1.5:1. The possibility that the higher MHC ratio in the SHR is the genetically mediated defect in arterial smooth muscle cells leading to the hypertension is discussed as an alternative to the elevated systemic blood pressure causing the altered MHC ratio.

Myosin heavy-chain isoforms in adult and developing rabbit vascular smooth muscle

Two monoclonal antibodies specific for smooth muscle myosin (designated SM-E7 and SM-A9) and one monoclonal anti-(human platelet myosin) antibody (designated NM-G2) have been used to study myosin heavy chain composition of smooth muscle cells in adult and in developing rabbit aorta. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis and Western blotting experiments revealed that adult aortic muscle consisted of two myosin heavy chains (MCH) of smooth muscle type, named MHC-1 (205 kDa), and MHC-2 (200 kDa). In the fetal/neonatal stage of development, vascular smooth muscle was found to contain only MHC-1 but not MHC-2. Non-muscle myosin heavy chain, which showed the same electrophoretic mobility as the slower migrating MHC, was expressed in an inverse manner with respect to MHC-2, i.e. it was detectable only in the early stages of development. The distinct pattern of smooth and non-muscle myosin isoform expression during development may be related to the different functional properties of smooth muscle cells during vascular myogenesis.

The distribution of heavy-chain isoforms of myosin in airways smooth muscle from adult and neonate humans

Changes in the expression of heavy chains of myosin during development determine the functional characteristics of striated muscles. The distribution of heavy-chain isoforms of smooth-muscle myosin was determined in the airways of adult and infant humans to see whether it might underlie the hyperreactivity of human airways. The protein bands corresponding to myosin were separated using SDS/polyacrylamide-gel electrophoresis (4% gels) and identified by immunoblotting using both monoclonal and polyclonal antibodies against smooth-muscle myosin and non-muscle myosin. The relative proportion of each heavy chain stained by Coomassie Blue was measured by densitometric scanning. Three major bands corresponding to myosin heavy-chain isoforms were found; the two slower migrating bands (MHC1 and MHC2) were smooth-muscle myosin, and the third band was non-muscle myosin. The MHC1/MHC2 ratio was 0.69:1 in adult bronchus, and in infant bronchus and trachea. This contrasted with the airway smooth muscle in pigs, which was run concurrently, where the smooth-muscle heavy-chain ratio changed with development [Mohammad & Sparrow (1988) FEBS Lett. 228, 109-112]. The non-muscle myosin heavy chain comprised 63% of the smooth-muscle myosin. In both adult and infant lungs an additional putative myosin heavy chain which migrated slightly more rapidly than non-muscle myosin heavy chain was identified using the monoclonal smooth-muscle myosin antibody BF 48. This was unique to the human species.

Quantitative and qualitative heterogeneity in smooth muscle myosin heavy chains

Previous studies have shown that smooth muscle myosin consists of two heavy chains (MHCs) of unequal molecular weight; however, it is not clear whether there are intermuscle, inter- and intraspecies differences in the MHCs. The purpose of these experiments was to quantitatively and qualitatively compare MHCs in different smooth muscles. Extracts of bovine aorta (BAo), dog saphenous vein (dSV) and femoral artery (dFA), and rat aorta (rAo), femoral artery (rFA), carotid artery (rCA), ileum (rGI) and uterus (rUt) were electrophoresed on 5% polyacrylamide-1% SDS gels. All tissues exhibited two MHCs with molecular weights of 207,000 (MHC1) and 204,000 (MHC2) daltons. In all cases the proportion of total MHC made up by MHC1 was greater than that by MHC2. Based on their relative proportions (MHC1:MHC2), the tissues fell into one of three groups: (1) 55:45 - rAo, rCA, dFA; (2) 60:40 - dSV, BAo, rGI; and (3) 65:35 - rUt, rFA. Group 1 differed significantly from group 3 in the proportion of each MHC. One dimensional peptide maps indicated that BAo, dSV and dFA were similar while subtle differences existed between rUt and rAo. Differences between rUt and rAo were also observed in their cross-reactivity to a monoclonal antibody to smooth muscle MHC, confirming the differences seen on peptide maps. These results indicate that there are intertissue and inter- and intraspecies differences in smooth muscle MHCs. The significance of these differences to muscle function remains to be determined.

Correlation between the distribution of smooth muscle or non muscle myosins and alpha-smooth muscle actin in normal and pathological soft tissues

The distribution of smooth muscle (SM) and non muscle myosins was compared with that of alpha-SM actin in various normal and pathological tissues and in cultured cells by means of indirect immunofluorescence using a monoclonal antibody specific for alpha-SM actin [anti-alpha sm-1, Skalli et al., 1986b] and two polyclonal antibodies raised against bovine aortic myosin (ABAM) and human platelet myosin (AHPM), respectively. In normal tissues ABAM stained vascular and parenchymal smooth muscle cells (SMC), myoepithelial cells and myoid cells of the testis in a pattern similar to that reported by other authors with antisera raised against non vascular SM myosin. Cells stained with ABAM were always positive for anti-alpha sm-1. In human and experimental atheromatous plaques, most cells were positive for AHPM; a variable proportion was also stained for ABAM plus anti-alpha sm-1. Myofibroblasts from rat granulation tissue, Dupuytren's nodule and stroma from breast carcinoma were constantly positive for AHPM and negative for ABAM; however, myofibroblasts from Dupuytren's nodule and breast carcinoma were anti-alpha sm-1 positive. Early primary cultures of rat aortic SMC were positive for ABAM and anti-alpha sm-1 and became negative for ABAM and positive for AHPM after a few days in culture. They remained positive for AHPM and anti-alpha sm-1 after passages; the staining of AHPM and anti-alpha sm-1 appeared to be colocalized along the same stress fibers. These results may be relevant for the understanding of SMC function and adaptation, and show that in non malignant SMC proliferation, alpha-SM actin represents a more general marker of SM origin than SM myosin.