Contrast efficacy of novel phase convertible nanodroplets for safe CEUS imaging.

Microbubble contrast agents in ultrasound/echocardiography are used to increase the echogenicity of the target tissues, thereby raising the contrast resolution of the resultant image. Recently, the trend has shifted toward the development of phase-convertible nanodroplets as ultrasound contrast agents due to their promising theragnostic potential by switching capability at the active site. Herein, we fabricated pre-PGS- perfluoropentane phase convertible nanodroplets and checked their in vitro and in vivo enhancement and safety profile. For this, we performed experiments on 20 male Wistar rats and 2 dogs. Biochemical assays of both rats and dogs included complete blood profiles, liver function tests, and renal function tests. For rat vitals, monitoring and histopathological analysis were also performed. Converted nanodroplets showed excellent contrast enhancement, better than Sonovue upon in vitro testing, with an enhancement time of up to 14 min. In vivo, experiments showed comparable opacification of the ventricles of both rats and dogs. All biochemical assays remained within the normal range during the study period. The histopathological analysis did not show any signs of drug-induced toxicity, showing the safety of these nanodroplets. Pre-PGS-PFP nanodroplets hold great potential for use in echocardiography and abdominal imaging in both human and veterinary applications after clinical trials.

Genetic, structural and pharmacological characterization of polymannuronate synthesized by algG mutant indigenous soil bacterium Pseudomonas aeruginosa CMG1421.

AIMS: It was aimed to study the genetic, structural and pharmacological characteristics of polymannuronate synthesized by Pseudomonas aeruginosa CMG1421. METHODS AND RESULTS: Synthesis was analysed by transmission electron microscopy, FT/IR, 1 H-NMR and gel permeation chromatography followed by in vitro bioassays. Colony PCR followed by sequence analysis was employed for screening of structural genes. FT/IR analysis indicated the presence of hydroxyl, carboxyl and O-acetyl groups linked to mannuronate. 1 H-NMR analysis indicated M-M bond characteristics for mannuronic acid residues. The average relative molecular weight was found in range of 20 000-250 000 Da. The amplified DNA fragments were identified as 16S rRNA, algD, alg8, alg44, algG, algE and algX genes showing 99-100% homology with those of P. aeruginosa. However, in algG there were transition mutations of adenine and cytosine at nucleotide position 766 and 769, and 878 and 881 respectively. Polymannuronate and its oligomannuronates respectively showed moderate and significant antioxidant, anti-inflammatory, anti-obesity and antidiabetic activities. CONCLUSIONS: Alginate synthesized by ∆algG mutant P. aeruginosa CMG1421 was bioactive and solely consists of acetylated d-mannuronates. SIGNIFICANCE AND IMPACT OF THE STUDY: We investigated biocompatible, nonimmunogenic and nontoxic pharmacological agents for treatment and attenuation of degenerative, inflammatory, autoimmune disease, and metabolic disorders such as obesity and diabetes.

Combination of needle aspiration and core needle biopsy: A new technique of stereotactic biopsy.

AIM: The study aims at describing the results of using a new technique to acquire the tissue sample in stereotactic biopsy of brain lesions. MATERIALS AND METHODS: The study was performed in 19 patients over a period of 5 years in which we used the new technique, i.e., Abrar and Afzal technique (AT) of obtaining tissue biopsy. It is a combination of core tissue biopsy and needle aspiration techniques. The technique was devised to acquire greater amount of tissue for pathologic study. RESULTS: While we could give pathologic diagnosis in 18 patients out of 19 (94.7%), in one patient, the tissue sample revealed only inflammatory cells and definitive diagnosis could not be reached. There was no significant morbidity or any mortality in the series. CONCLUSION: Abrar and Afzal technique is a reasonably accurate technique of acquiring larger tissue sample in stereotactic brain biopsy without any additional risks. It can be done with little modification of the conventional equipment available with the stereotactic system.

Production, characterization, in vitro and ex vivo studies of babchi oil-encapsulated nanostructured solid lipid carriers produced by a hot aqueous titration method.

An aqueous dispersion of solid fat nanoparticles of babchi oil (BOSLN) was prepared by means of the hot water titration method. Surface morphology was determined by HR-TEM which revealed a fairly spherical shape of the formulations. Further they were evaluated for in vitro drug release characteristics and ex vivo skin permeation profile, zeta potential and particle diameter, rheological measures and droplet size distribution. Highest values for steady state flux (Jss), permeability coefficient (Kp) and enhancement ratio (Er) were observed for formulation, BOSLN3 comprised of oil [10% v/v; BO (3.33%), CAT (6.67%)], Tween 80 (9.25% v/v), transcutol-P (28.75% v/v) and distilled water (53% v/v). These results suggest that the studied SLN might be promising vehicles for babchi oil in the management of psoriasis.

Clinical profile, treatment, and visual outcome of ampiginous choroiditis.

PURPOSE: To report the clinical profile and management of patients diagnosed to have ampiginous choroiditis in a tertiary care referral centre in India. METHODS: Retrospective cohort study. Twenty-six eyes of 16 patients were included in the study, which was diagnosed as choroiditis, serpiginous choroiditis, and acute posterior multifocal placoid pigment epitheliopathy (APMPPE) or ampiginous choroiditis. Those who were initially diagnosed as having other forms of choroiditis were later classified as having ampiginous choroiditis clinically. Systemic steroids and immunosuppressives were the mainstay of therapy. RESULTS: There was a male preponderance (7:3). Age at presentation ranged from 22 years to 57 years with a (median 34 years); 81% had bilateral involvement and 35% had recurrences. Vision improved or maintained in 24 eyes, whereas it deteriorated in 2 eyes due to subretinal fibrosis and macula involvement, respectively. Resolution of lesions and improvement or stability of vision can occur with administration of timely steroids and immunosuppressive therapy. Regular follow-up is necessary to monitor the disease progression, recurrences, and involvement of the other eye. CONCLUSION: Ampiginous choroiditis is a separate disease entity due to its distinct clinical features. It is a disease with multiple relapses, which can be effectively controlled with a combination of immunosuppressive therapy, and a good visual acuity can be maintained on long-term follow-up.

Enhanced anti-inflammatory effects of celecoxib from a transdermally applied nanoemulsion.

The aim of the present study was to evaluate the enhanced anti-inflammatory effects of celecoxib (CXB) from a transdermally applied nanoemulsion. The anti-inflammatory effects of an optimized nanoemulsion formulation were compared with those of conventional CXB gel and nanoemulsion gel on carrageenan-induced paw edema in rats. These tests were compared using the Dunnett test of one-way analysis of variance (ANOVA). The % inhibition value after 24 h application was significant for optimized formulation C2 (85.4%) compared with CXB gel and nanoemulsion gel (p < 0.05). These results suggest that nanoemulsions can be successfully used to enhance the anti-inflammatory effects of CXB.

Effect of Labrasol on self-nanoemulsification efficiency of ramipril nanoemulsion.

The purpose of the present investigation was to evaluate the capacity of Labrasol as surfactant for self-nanoemulsification efficiency of ramipril nanoemulsion formulation. Based on the solubility profile of ramipril, Sefsol-218, Labrasol and Carbitol were selected as oil phase, surfactant and cosurfactant, respectively. Based on the stability profile of ramipril, standard buffer solution of pH 5.0 was selected as an aqueous phase for the development of ramipril nanoemulsion formulation. Nanoemulsion formulations of ramipril were developed using an aqueous phase titration method. Pseudoternary phase diagrams were constructed to identify the nanoemulsion region. Selected formulations were subjected to different thermodynamic stability tests using centrifugation, heating cooling cycles and freeze thaw cycles. The formulations which were stable at thermodynamic stability tests were taken for self-nanoemulsification efficiency test. No creaming, cracking, coalescence or phase inversion was observed on most of the formulations upon thermodynamic stability tests. All the formulations passed self-nanoemulsification tests in grade C, D and E but not in grade A and B. Because none of the formulation passed self-nanoemulsification efficiency test in grade A and B, it was concluded that Labrasol is not suitable as surfactant for oral or self nanoemulsifying drug delivery system of ramipril.

Skin permeation mechanism of aceclofenac using novel nanoemulsion formulation.

The aim of the present study was to investigate the skin permeation mechanism of aceclofenac using a novel nanoemulsion formulation. An optimized oil-in-water nanoemulsion of aceclofenac was prepared by the spontaneous emulsification method. The optimized nanoemulsion contained 2% w/w aceclofenac, 10% w/w Labrafil, 5% w/w Triacetin, 35.33% w/w Tween 80, 17.66% w/w Transcutol P and 32% w/w distilled water. The skin permeation mechanism was evaluated by FTIR spectroscopy, DSC thermography, activation energy measurement and histopathological examination. FTIR spectra of skin treated with the nanoemulsion formulation indicated breaking of the hydrogen bond network at the head of ceramides. DSC thermograms indicated that intracellular transport could be a possible mechanism of permeation enhancement and that permeation occurred due to the extraction of SC lipids by the nanoemulsion. The significant decrease in activation energy for aceclofenac permeation across rat skin indicated that the SC lipid bilayers were significantly disrupted (p < 0.05). Photomicrography of skin showed disruption and extraction of lipid bilayers as distinct voids and empty spaces visible in the epidermal region. Overall these findings indicated that nanoemulsions can be successfully used to enhance skin permeation of drugs.

Differential effects of doxorubicin on atrial natriuretic peptide expression in vivo and in vitro.

Doxorubicin (Dox) is a potent anti-cancer agent with cardiotoxic side-effects but the mechanism of its cardiotoxicity and its effect on expression of the vasoactive atrial natriuretic peptide (ANP), an important marker for cardiac hypertrophy, are little understood. The present study examined Dox-induced changes in vivo in hearts of 6 mongrel dogs and 5 Sprague-Dawley rats and in vitro in cardiac cultures of neonatal rats. Quantitative RT-PCR analysis using gamma 32-p labeled primers for beta-actin, phospholamban (PLB) and ANP showed a selective 5-fold increase of ANP mRNA in Dox-treated dog hearts in comparison to controls. Similarly, northern analysis of GAPD, beta-actin, cardiac alpha-actin and ANP gave a selective 4.5-fold increase in ANP transcripts in Dox-treated rat hearts. On the other hand, there was a selective decrease (approximately 39%) of ANP transcripts in Dox-treated cardiac cultures relative to controls. Immunohistochemistry localized the ANP changes both in tissue sections and in cultures to the cardiomyocytes. The data clearly showed that Dox selectively increases ANP expression in dog and rat hearts in absence of cardiocyte hypertrophy but selectively decreases it in cardiac cultures. This differential effect of Dox on cardiocytes in vivo and in vitro should be a useful parameter for studies of transcriptional control of ANP expression.

Ventricular myosin light chain-2 gene expression in developing heart of chicken embryos.

Recent gene knock-out studies in mice have suggested that ventricular myosin light chain-2 (vMLC2) has a role in the regulation of cardiogenic development and that perturbation in expression of vMLC2 is linked to the onset of dilated cardiomyopathy. In an attempt to develop an avian model for such studies, we examined the expression pattern of vMLC2 in chicken embryos at various stages and analyzed the effect of antisense oligonucleotide-mediated interference of vMLC2 function in cultures of whole embryos. Our results showed vMLC2 to be a specific marker for ventricular chamber throughout chicken embryonic development and antisense vMLC2 treatment of primitive streak stage (stage 4) embryos to produce pronounced dilation of heart tube with severe deficiency in formation of striated myofibrils. Further studies with antisense mRNA techniques of whole embryo cultures should, therefore, be useful to evaluate the role of vMLC2 and other putative regulatory factors in cardiac myofibrillogenesis.

Anabolic-androgenic steroids induce apoptotic cell death in adult rat ventricular myocytes.

We tested whether exposure to anabolic-androgenic steroids (AASs) would induce apoptosis in adult rat ventricular myocytes in vitro. Myocytes were exposed to stanozolol (STZ), testosterone enanthate (TE) and testosterone (T) (0.1 micromol/L, 1 micromol/L, 10 micromol/L, and 100 micromol/L) for 20 h. The percentage of myocytes undergoing apoptosis was determined by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) and was found to be increased when compared to control myocytes at STZ 10 micromol/L 12 +/- 2% (mean +/- SD), STZ 100 micromol/L 42 +/- 3%; TE 1 micromol/L 11 +/- 2%, TE 10 micromol/L 21 +/- 3%, TE 100 micromol/L 62 +/- 2%; T 10 micromol/L 11 +/- 2%, T 100 micromol/L 40 +/- 3% (P < 0.001 vs. CTL 2 +/- 2%). The STZ-, TE- and T-induced dose-dependent apoptotic cell death was corroborated by a significantly increased DNA laddering in myocytes exposed to STZ and T > or = 10 micromol/L and TE > or = 1 micromol/L. Notably, STZ, TE, and T exposure markedly increased the expression of the pro-apoptotic oncogene Bax-alpha, as assessed by reverse transcription-polymerase chain reaction. Taken together, these results clearly show for the first time that AASs induce apoptotic cell death in a dose-dependent manner. This finding may have important implications in understanding the pathogenesis of ventricular remodeling, cardiomyopathy, and sudden cardiac death associated with AAS abuse.

Modulation of MLC-2v gene expression by AP-1: complex regulatory role of Jun in cardiac myocytes.

Hypertrophic stimulation of cardiac myocytes results in rapid induction of a number of transcription factors, including members of the AP-1 family, which is followed by a programmed alteration in the pattern of gene expression. In the ventricular cardiocytes there is re-expression of the fetal atrial natriuretic factor (ANF) gene and upregulation of its myosin light chain-2 (MLC-2v). The mechanism(s) by which the induction ofAP-1 is coupled to the promoters of these target genes is largely unknown. In this report, we demonstrate that in transient co-transfection assay, c-Jun inhibited while Jun B stimulated the MLC-2v promoter activity. Mutant c-Jun recombinants, in which the activation domains were deleted, still remained inhibitory, but a specific mutation in the leucine zipper, which changes the alignment of Jun with its dimerization partner, caused a reversal of its effect on the target MLC-2v promoter. Based on these findings, we propose that in chicken cardiac myocytes, the regulation of MLC-2v promoter by Jun may occur via its interaction with other proteins, possibly of the leucine zipper family.

Beta-adrenergic receptor subtypes differentially affect apoptosis in adult rat ventricular myocytes.

BACKGROUND-Catecholamine-induced apoptosis is mediated by activation of the beta-adrenergic signaling pathway. We tested the hypothesis that beta(1)- and beta(2)-adrenergic receptor (AR) subtypes differentially affect apoptosis in adult rat ventricular myocytes in vitro. METHODS AND RESULTS-Myocytes were first exposed to norepinephrine (NE) alone (10 mcmol/L) or NE+atenolol (AT) (10 mcmol/L) for 12 hours. AT, a beta(1)-selective AR antagonist, abolished the NE-induced increase in nick end-labeling (TUNEL)-positive cells compared with control (NE, 33+/-3% versus control, 3+/-1%, P<0.0001; NE+AT, 4+/-2% versus control, 3+/-1%, P=0. 98). Annexin V staining, DNA laddering, and caspase activity determinations corroborated these results. Subsequent experiments under prazosin treatment established the apoptosis dose-response curves for the increasingly beta(2)-selective AR agonists isoproterenol (ISO) (beta(1) approximately beta(2)) and albuterol (ALB) (beta(2)>beta(1)). ISO and ALB induced significantly less apoptosis than NE (beta(1)>beta(2)) at equimolar concentrations as assessed by TUNEL staining [1 mcmol/L: NE (8+/-2%) approximately ISO (7+/-1%)>ALB (2+/-1%); 10 mcmol/L: NE (35+/-2%)>ISO (23+/-1%)>ALB (3+/-1%); 100 mcmol/L: NE (50+/-2%)>ISO (29+/-2%)>ALB (14+/-1%), P<0.0001 except for NE versus ISO at 1 mcmol/L with P=0.62]. ALB-induced apoptosis at 100 mcmol/L was abolished by AT (10 mcmol/L), indicating a beta(1)AR-mediated effect. Importantly, ICI 118551 (0.1 mcmol/L), a highly selective beta(2)AR antagonist, did not decrease the percentage of NE-, ISO-, and ALB-induced apoptosis. Reverse transcription-polymerase chain reaction studies revealed that AT completely reversed the beta-adrenergic signaling-induced changes in the Bcl-2-to-Bax ratio. CONCLUSIONS-These observations provide evidence that beta AR-mediated apoptotic death signaling is largely dissociated from beta(2)ARs and selectively mediated by beta(1)ARs in adult rat ventricular myocytes.

Norepinephrine-induced apoptosis is inhibited in adult rat ventricular myocytes exposed to volatile anesthetics.

BACKGROUND: Volatile anesthetics are used to provide anesthesia to patients with heart disease under heightened adrenergic drive. The purpose of this study was to test whether volatile anesthetics can inhibit norepinephrine (NE)-induced apoptosis in cardiomyocytes. METHODS: Rat ventricular cardiomyocytes were exposed to NE (10 microm) alone or in the presence of increasing concentrations of isoflurane and halothane. RESULTS: Isoflurane at 1.6 minimum alveolar concentration (MAC) (4 +/- 2% [SD]) and halothane at 1.2 MAC (3 +/- 2%) abolished the percentage of cardiomyocytes undergoing NE-induced apoptosis (34 +/- 8%), as assessed by terminal deoxynucleotidyl transferase-mediated nick end labeling (TUNEL) (P < 0.0001). Lower concentrations of isoflurane and halothane markedly decreased the number of TUNEL-positive cells. Similarly, isoflurane at 1.6 MAC (5 +/- 3%) and halothane at 1.2 MAC (6 +/- 3%) prevented the increase in annexinV-staining cardiomyocytes (38 +/- 7%; P < 0. 0001). These findings were corroborated with a decreased quantity of NE-induced DNA laddering by volatile anesthetics. Halothane at 1.2 MAC abolished the increase in TUNEL-positive cardiomyocytes exposed to the dihydropyridine Ca2+-channel agonist BAY K-8644 (1 microm) (BAY K-8644 + halothane: 3 +/- 2% vsBAY K-8644: 34 +/- 6%; P < 0. 0001) and the Ca2+-ionophore 4-bromo-A23187 (1 microm) (4-bromo-A23187 + halothane: 2 +/- 2% vs4-bromo-A23187: 13 +/- 4%; P = 0.03). NE treatment increased caspase-9 activity to 197 +/- 62% over control myocytes (P < 0.0001), whereas no caspase-8 activation was detectable. This increase in caspase-9 activity was blocked by isoflurane at 1.6 MAC and halothane at 1.2 MAC. CONCLUSIONS: Volatile anesthetics offer significant protection against beta-adrenergic apoptotic death signaling in ventricular cardiomyocytes. The authors present evidence that this protection is mainly mediated through modulation of cellular Ca2+ homeostasis and inhibition of the apoptosis initiator caspase-9.

Identification of a novel cardiac lineage-associated protein(cCLP-1): A candidate regulator of cardiogenesis.

We describe the isolation and characterization of a cDNA clone, called cCLP-1, that is a candidate for the previously described early cardiac-specific transcription factor BBF-1. BBF-1 binds the MEF2 (or element B) binding site within the cardiac myosin light chain 2 (MLC2) gene promoter. We used the element B sequence as a probe to screen an expression library constructed from mRNA obtained from the presumptive heart-forming regions of stage 6 chicken embryos. This yielded the cCLP-1 cDNA clone. Gel-shift analysis of stage 6 embryonic chicken protein extracts suggests that a protein that is recognized by the anti-cCLP-1 antibody binds to the same element B binding site to which BBF-1 binds. cCLP-1 mRNA was detected early in chicken development, prior to cardiac fate assignment at stage 4. The expression pattern of cCLP-1, based on whole mount in situ hybridization, coincides remarkably well with the established morphogenetic field of early heart formation. The nuclear localization of cCLP-1 is phosphorylation-dependent, suggesting that cCLP-1 may be a member of that class of transcription factors whose activity is regulated by cytoplasm to nucleus transport. Taken together, these data suggest that cCLP-1 may encode a novel transcription factor whose expression pattern is in agreement with that of the cardiogenic precursor cells of the early chicken embryo.

(CTG)n repeats markedly inhibit differentiation of the C2C12 myoblast cell line: implications for congenital myotonic dystrophy.

Although the mutation for myotonic dystrophy has been identified as a (CTG)n repeat expansion located in the 3'-untranslated region of a gene located on chromosome 19, the mechanism of disease pathogenesis is not understood. The objective of this study was to assess the effect of (CTG)n repeats on the differentiation of myoblasts in cell culture. We report here that C2C12 myoblast cell lines permanently transfected with plasmid expressing 500 bases long CTG repeat sequences, exhibited a drastic reduction in their ability to fuse and differentiate into myotubes. The percentage of cells fused into myotubes in C2 C12 cells (53.4+/-4.4%) was strikingly different from those in the two CTG repeat carrying clones (1.8+/-0.4% and 3.3+/-0. 7%). Control C2C12 cells permanently transfected with vector alone did not show such an effect. This finding may have important implications in understanding the pathogenesis of congenital myotonic dystrophy.

Myotonic dystrophy: decreased levels of myotonin protein kinase (Mt-PK) leads to apoptosis in muscle cells.

The pathogenesis of myotonic dystrophy (DM) and the function of the product of the DM gene, myotonin protein kinase (Mt-PK), and its relationship to the disease are uncertain. To gain insight into the function of Mt-PK we studied the effect of decreasing the levels of Mt-PK in cultured human myoblasts. Myoblasts were transfected with an anti-sense oligonucleotide (ODN) targeted to the translation initiation site of DM mRNA which resulted in about 76% reduction in the levels of Mt-PK protein. A large percentage (about 48 to 90%) of myoblasts transfected with this oligonucleotide (but only about 2 to 23% of myoblasts transfected with a control oligonucleotide) underwent apoptosis within 24 h. To further substantiate these results we delivered a specific antibody to Mt-PK into the myoblast cells using a lipid carrier to inhibit its function and show that this resulted in apoptosis in 57 to 72% of the cells within 24 h. These results suggest that decreased levels of Mt-PK may contribute to muscle pathology in DM by leading to apoptosis of muscle cells.

In situ hybridization analysis for expression of myogenic regulatory factors in regenerating muscle of mdx mouse.

Precise temporal and spatial coordination of expression of the myogenic regulatory factors (MRF) plays a critical role in the development of skeletal muscle. Whether this pattern is recapitulated postnatally during regeneration of mature muscle after injury is not known. The aim of this study was to determine the cellular distribution of mRNA expression of the various myogenic regulatory factors (MyoD, Myogenin, Myf-5 and Myf-6) during regeneration in mature skeletal muscle. We used the mdx mouse, an animal model for Duchenne muscular dystrophy, which undergoes necrosis and vigorous regeneration of muscle fibers, as a natural model to study muscle fibers at various stages of maturity ranging from satellite cells to mature muscle fibers. In situ hybridization analysis revealed that MyoD expression was the most widespread and was expressed in satellite cells and small regenerating muscle fibers surrounding necrotic fibers. Myogenin, Myf5, and Myf6 were expressed weakly in some immature fibers and none of the MRF's could be detected in mature muscle. These findings, taken together with previous studies, suggests that the pattern of expression of the various MRF's in regenerating skeletal muscle differs from that in developing muscle in embryos and that MyoD may play a critical role in the initiation and progression of events which lead to regeneration in mature skeletal muscle.

Interaction of a conserved peptide domain in recombinant human ventricular myosin light chain-2 with myosin heavy chain.

Recent work on molecular genetics of mammalian contractile proteins has provided valuable insights into the basis of the heterogeneity of muscle proteins and their regulated expression during development, yet information on the precise role(s) of light chains in actomyosin interaction and muscle function is still lacking. The selective increase in ventricular myosin light chain-2 (MLC2) in hypertrophied heart muscle has been implicated as a compensatory feature of myosin, but its relevance to myosin function is not known. To investigate the role of cardiac MLC2, we have isolated a full-length cDNA clone for human ventricular MLC2 and produced a full-length and N-terminal deleted MLC2 polypeptides in Escherichia coli using the bacterial expression vector pT7-7 system. The interaction of recombinant MLC2 with myosin heavy chain (MHC) and its subfragment-1 was studied using the full-length and truncated recombinant polypeptides. The results demonstrated that the bacterially produced full-length human cardiac MLC2 exchanges effectively with the native MLC2 and binds with specificity to MHC and to intact myofibrils. Domain mapping by deletion and in vitro exchange/competition analysis with a synthetic peptide suggests that a conserved central domain in MLC2 participates in the functional association of the two myosin subunits.

Evidence for new isoform of fast myosin heavy chain in rat skeletal muscle.

The purpose of this study was to investigate whether the previously demonstrated heterogeneity of myosin heavy chain (MHC) in type 2B fibers of the superficial portion of the lateral gastrocnemius (SLG) muscle of the rat may be due to presence of type 2D/X fibers. Immunohistochemical identification of MHC heterogeneity, histochemical identification of appendicular 2D/X muscle fibers, and immunoblotting of myosins were used. It was found that some, but not all, of the heterogeneity of rat SLG is correlated with the presence of type 2D/X fibers. Immunoblots with MHC from several muscles revealed the presence of at least two antigenically distinct isoforms of MHC with the same electrophoretic mobility as the "2B" band. These results show that a previously undetected type of MHC is expressed in rat skeletal muscle and raise the possibility that there may be as yet undetected MHC isoforms in human muscle which may be clinically important.