Distribution and muscle-sparing effects of clenbuterol in hindlimb-suspended rats.

Based on their anabolic properties in skeletal muscles, beta-adrenergic agonists are of interest as potential countermeasures to microgravity-induced skeletal muscle atrophy. The levels of clenbuterol (Cb), a beta(2)-adrenergic agonist, in both plasma and skeletal muscle were higher in hindlimb-suspended rats than in their nonsuspended Cb-treated controls. Cb treatment was shown to help maintain the body weight in suspended rats, while reducing the amount of mesenteric fat. However, hindlimb suspension attenuated Cb's lipolytic effects. In skeletal muscle, the magnitude of response to unloading and Cb treatment followed a general regional pattern and was muscle and type specific. The highest magnitude of response to unloading was in predominantly slow-twitch muscles, and the least responsive were the predominately fast-twitch muscles.

Beta-agonist-induced alterations in organ weights and protein content: comparison of racemic clenbuterol and its enantiomers.

Clenbuterol is a relatively selective beta2-adrenergic partial agonist that has bronchodilator activity. This drug has been investigated as a potential countermeasure to microgravity- or disuse-induced skeletal muscle atrophy because of presumed anabolic effects. The purpose of this study was to: 1) analyze the anabolic effect of clenbuterol's (-)-R and (+)-S enantiomers (0.2 mg/kg) on muscles (cardiac and skeletal) and other organs; and 2) compare responses of enantiomers to the racemate (0.4 mg/kg and 1.0 mg/kg). Male Sprague Dawley rats were treated with: a) racemic clenbuterol (rac-clenbuterol, 0.4 or 1.0 mg/kg); b) enantiomers [clenbuterol (-)-R or (+)-S]; or c) vehicle (1.0 mL/kg buffered saline). Anabolic activity was determined by measuring tissue mass and protein content. HPLC teicoplanin chiral stationary phase was used to directly resolve racemic clenbuterol to its individual enantiomers. In skeletal muscle, both enantiomers had equal anabolic activity, and the effects were muscle- and anatomic region-specific in magnitude. Although the enantiomers did not affect the ventricular mass to body weight ratio, clenbuterol (+)-S induced a small but significant increase in ventricular mass. Both clenbuterol enantiomers produced significant increases in skeletal muscle mass, while being less active in producing cardiac ventricular muscle hypertrophy than the racemic mixture.

Motor-unit territories in the masseter muscle of infant pigs.

In adult miniature pigs (Sus scrofa) fibres of the masseter contract differentially, helping to produce the successive movements of the mandible during the chewing cycle. In infant pigs, however, most fibres of the masseter contract simultaneously. One hypothesis to explain the ontogenetic change in contraction pattern is that the infant masseter is neurologically immature, with large overlapping motor units incapable of producing differential contractions. This hypothesis was tested by mapping the territories of motor units in the masseters of piglets. Filaments from the masseteric nerve were stimulated repetitively; muscle fibres belonging to the stimulated motor units were identified by their failure to react for glycogen. Just as in older pigs, motor-unit territories were found to be very restricted, occupying only a small portion of total muscle volume. Thus, neural organization does not appear to be immature in piglets. An alternative hypothesis, that the ontogenetic change in activity pattern results from growth changes in the anatomy of the masseter, may be a more likely explanation.

Neural organization of the masseter muscle in the pig.

The neural organization of the pig masseter, an architecturally and functionally compartmentalized muscle, was investigated by using dissection, glycogen depletion, evoked electromyography, and counts of axon numbers at various levels along the masseteric nerve. The masseteric nerve enters the muscle as two rostral branches, which also supply the zygomatico-mandibularis, and a more caudal main branch, which soon divides into four terminal nerves with variable distributions. Stimulation of filaments containing roughly 50 extrafusal motor axons resulted in glycogen depletion of 5-20% of the muscle fibers in very small subvolumes of the masseter; the affected subvolumes were delimited by perimysium. Electromyography after stimulation of various branches of the nerve confirmed the distributions deduced from anatomy and further indicated that axons do not branch between the rostral and main nerve branches but may occasionally do so among the more distal terminal branches of the main branch. The proximal trunk of the masseteric nerve contains about 3,500 myelinated fibers with a bimodal size distribution. Approximately 1,000 of the larger fibers were estimated to be extrafusal motor axons. Along the proximal trunk of the nerve, fibers were constantly rearranged; coupled with the observation that the locations of motor unit territories were usually not related to the position of the stimulated axons within the nerve, this suggests that the nerve trunk is not strictly ordered somatotopically.

Phenoxyethanol as a nontoxic preservative in the dissection laboratory.

In an effort to rid the dissection room of irritating and potentially health-threatening toxic chemicals, we have modified the phenoxyethanol technique for long-term preservation of embalmed cadavers. The new methods employ faster, less toxic embalming and reduced or eliminated phenoxyethanol immersions. Our results are comparable with or improved over those previously described and demonstrate that phenoxyethanol is an excellent, easily manageable alternative preservative to standard formaldehyde/phenol-based embalming fluids.

Development of the masseter muscle and oral behavior in the pig.

During mastication the adult pig masseter contracts with a complex pattern involving a wave of electromyographic (EMG) activity moving from the ventro-rostral corner to the dorso-caudal corner. The present study was undertaken to ascertain the ontogeny of that contraction pattern. Anatomical measurements were made on masseters from fetal, infant, and juvenile pigs. EMG activity from different parts of the masseter was recorded along with oral movements in infant and juvenile pigs as they suckled, drank, and chewed on food and non-food objects. The basic arrangement of muscle fibers and tendinous aponeuroses was found to be the same in all ages. The longest and most vertical fibers were found rostrally and ventrally, whereas the shortest and most horizontal fibers were found caudally and dorsally. The length of fasciculi decreased with age, relative to muscle weight. Variance in length among different parts of the muscle increased with age. Fetal masseters were oriented generally more horizontally than the masseters of older animals, except that the dorso-caudal corner, usually the most horizontal portion, is not developed in fetuses. The contraction patterns within the infant masseter were less complex than those of older animals; only the dorso-caudal corner was distinct. The further development of intramuscular differences in activity may be associated with the increasing anatomical complexity of the masseter, which augments its functional capabilities.

Facial morphology and vibrissal movement in the golden hamster.

The major cranial vibrissae in the golden hamster can be moved in complex ways that suggest they are served by a finely controlled motor system. Movements are hypothesized to be the products of differential blood flow and pressure regulation in the sinus surrounding each vibrissal follicle, contractions of the striated facial muscles, and elastic rebound in the connective tissues. The vasculature contributes hydrostatic forces that erect the vibrissae slightly and distort their connective tissue bedding, rigidify the vibrissal capsules, thus forming firm bases of attachment for certain facial muscles, and theoretically provide a pressure plate around the follicle, important in lowering the firing thresholds of receptor endings. The facial muscles supply the major forces in erection and protraction of the vibrissae by acting on both the capsules and the connective tissue bedding. The connective tissues are organized into capsular and extracapsular systems that serve to stabilize the vibrissae and return them to initial rest positions. The slight movements of the genal vibrissa are the effects of vascular and connective tissue dynamics, the musculature being uninvolved. Wide angle movements of the supraorbital vibrissae are products of the vasculature and connective tissues, plus contractions of the Mm. orbicularis oculi and frontalis. Mystacial vibrissal movement is quite complex. The vasculature supplies a small degree of capsular erection and mystacial pad distortion, but primarily rigidifies the capsules. The bulk of erection and protraction is produced by the M. nasolabialis profundus (NLP) and the vibrissal capsular muscles (VCM). The NLP distorts the mystacial pad; the VCM tilt the capsules relative to the pad. Retraction is mainly accomplished by elastic rebound in the pad, this being aided in its extreme degrees by the Mm. nasolabialis and maxillolabialis. The Mm. nasolabialis superficialis and buccinator pars orbicularis oris help to spread the vibrissae into a dorsoventral fan and stabilize the mystacial pad during whisking.

Morphological basis of the feeding mechanics in the shingle-back lizard Trachydosaurus rugosus (Scincidae, Reptilia).

This report details certain morphological aspects of the feeding system of the lizard Trachydosaurus rugosus, an opportunistic omnivore, as a first step toward a functional characterization of its masticatory system. The skull is relatively solid and internally well braced; its anterodorsal elements are tightly tied to the integument and covering osteoderms. There is potential for intracranial kinesis and streptostyly. At small gapes, mandibular movements seem to be restricted to relatively simple, hingelike actions by a series of mechanical stops. The dentition features a progression of smaller to larger teeth posteriorly along the tooth row. The jaw adductor musculature is massive; other jaw muscles are relatively simple. The external adductor mass is particularly noteworthy in that it is subdivided into four mechanical units by a complex internal tendon tract (the coronoid aponeurosis). The internal adductor is composed of two separate gross muscles, pseudotemporalis (PST) and pterygoideus (PT). Each of these is subdivided into two main units by aponeurotic sheets, the PST by parts of the coronoid aponeurosis and the PT by a separate series. The form of the aponeurotic system in Trachydosaurus confounds the separation and identification of the adductor muscles and their component parts along the lines of traditional nomenclature, and underscores the need for separating criteria based on homology from those reflecting morphological and possibly functional divisions.