Effect of age on the response of four muscles of the rat to denervation

Muscle hypertrophy models: applications for research on aging

Muscle hypertrophy is an adaptive response to overload that requires increasing gene transcription and synthesis of muscle-specific proteins resulting in increased protein accumulation. Progressive resistance training (P(RT)) is thought to be among the best means for achieving hypertrophy in humans. However, hypertrophy and functional adaptations to P(RT) in the muscles of humans are often difficult to evaluate because adaptations can take weeks, months, or even years before they become evident, and there is a large variability in response to P(RT) among humans. In contrast, various animal models have been developed which quickly result in extensive muscle hypertrophy. Several such models allow precise control of the loading parameters and records of muscle activation and performance throughout overload. Scientists using animal models of muscle hypertrophy should be familiar with the advantages and disadvantages of each and thereby choose the model that best addresses their research question. The purposes of this paper are to review animal models currently being used in basic research laboratories, discuss the hypertrophic and functional outcomes as well as applications of these models to aging, and highlight a few mechanisms involved in regulating hypertrophy as a result of applying these animal models to questions in research on aging.

Mechanical induction in limb morphogenesis: the role of growth-generated strains and pressures

Morphogenesis is regulated by intrinsic factors within cells and by inductive signals transmitted through direct contact, diffusible molecules, and gap junctions. In addition, connected tissues growing at different rates necessarily generate complicated distributions of physical deformations (strains) and pressures. In this Perspective we present the hypothesis that growth-generated strains and pressures in developing tissues regulate morphogenesis throughout development. We propose that these local mechanical cues influence morphogenesis by: (1) modulating growth rates; (2) modulating tissue differentiation; (3) influencing the direction of growth; and (4) deforming tissues. It is in this context that we review concepts and experiments of cell signaling and gene expression in various mechanical environments. Tissue and organ culture experiments are interpreted in light of the developmental events associated with the growth of the limb buds and provide initial support for the presence and morphological importance of growth-generated strains and pressures. The concepts presented are used to suggest future lines of research that may give rise to a more integrated mechanobiological view of early embryonic musculoskeletal morphogenesis.

The effects of age on atrophy and recovery in denervated fiber types of the rat nasolabialis muscle

This study investigates the effects of advancing age on responses of nasolabialis muscle fibers to denervation and reinnervation. The nasolabialis is innervated by the facial nerve and is responsible for the whisking movement of the animal's large vibrissae. In young adult (3-month) and middle-aged (15-month) rats the muscle on one side of the head was denervated by crushing the facial nerve. At specific days postcrush, animals were sacrificed and thick sections of muscle were incubated to demonstrate cytochrome oxidase activity, a mitochondrial enzyme, which differentiated between red, white, and intermediate fiber types. The rate and extent of atrophy and recovery were evaluated using light microscope morphometric methods for which transverse fiber areas were measured and compared to fibers on the contralateral control side. There was an age-related delay in the time of functional return since older animals resumed normal whisking behavior 6 days later than the younger animals. In both age groups, white and intermediate fibers atrophied to the greatest extent and red fibers showed least atrophy. Despite the different responses of the fiber types to denervation, there was no age difference in the maximum degree of fiber atrophy within each fiber type. Age differences did occur in the rate of the denervation response since the middle-aged fibers consistently showed a more rapid significant atrophy than the young adult fibers. During recovery, older fibers may be limited in their ability to attain the size of fibers on the control side. The results indicate that through middle age, the process of advancing age increases the susceptibility of the nasolabialis muscle to denervation but does not alter the maximum extent of atrophy. The ability to recovery to normal fiber size, at least 2 months after denervation, is also age-related.

Different pattern of recovery of fast and slow muscles following nerve injury in the rat

The sciatic nerve was crushed in 5-6-day-old rats and the time course of recovery and changes in physiological and morphological properties of reinnervated fast and slow muscles was compared. The maximal tetanic tension developed by the reinnervated muscles was recorded at different times from about 18 days of age, when functional recovery was first seen, until 2 months. The maximal indirectly elicited tetanic tension of the reinnervated slow soleus muscle gradually increased from 55% of normal at 18 days to 75% of normal at 2 months. In contrast, the tension of the reinnervated fast muscle extensor digitorum longus (e.d.l.) fell sharply from 70% of normal at 18 days to 40% at 21 days and remained at that level till the end of the study. The total number of muscle fibres in control, reinnervated and denervated e.d.l. muscles was counted. At 18 days the number of fibres in the reinnervated e.d.l. was similar to normal but by 1 month it had fallen to one-third. This decrease did not take place in permanently denervated muscles until at least 35 days. Loss of fibres in the reinnervated soleus was small. During the early stages of reinnervation the contraction and relaxation of the fast muscles was very prolonged. By 1 month the time taken to reach peak twitch tension had decreased to normal values but the relaxation was still slower and remained so for several months. The study of fatigue resistance showed that at 18 days the reinnervated fast muscles were as fatigable as normal muscles from animals of the same age. The fatigability of normal muscles increased with age to adult levels, but the reinnervated muscles became more fatigue resistant and remained so. Our findings suggest that fast muscles become selectively impaired after nerve injury at 6 days because they lose a large number of fibres after reinnervation.

Transmitter release during normal and altered growth of identified muscle fibres in the crayfish

During growth of identified crayfish muscle fibres from a diameter of 20 to 400 micron, the excitatory junctional potential (e.j.p.) amplitude was found to be independent of diameter. Thus, e.j.p. amplitude was maintained during growth in spite of a 21-fold decrease in miniature excitatory junctional potential (m.e.j.p.) amplitude previously reported (Lnenicka & Mellon, 1983). The maintenance of e.j.p. amplitude was found to be partially due to a 5-fold increase in quantal release at 'active sites' during growth. In order to determine whether the increase in transmitter release can be regulated by the rate of muscle fibre growth, the rate of growth was experimentally reduced. By decreasing the resting length of the muscle during growth, the rate of increase in the diameter was reduced by approximately 50% compared with the contralateral control muscle fibres. The input resistance and the m.e.j.p. were appropriately larger in the smaller-diameter experimental fibres. However, e.j.p. amplitude in the experimental fibres was not significantly different from that in the contralateral control fibres. This was apparently due to the significantly smaller quantal release at active sites on the experimental fibres compared with control fibres. Thus, experimental alteration of the rate of muscle fibre growth results in regulation of transmitter release, suggesting that the muscle fibre may control the increase in transmitter release seen during normal growth.

Recovery of slow and fast muscles following nerve injury during early post-natal development in the rat

1. The sciatic nerve was crushed in 5-6-day-old rats and the recovery of function of slow and fast muscles was studied. The first signs of recovery of function were seen 10-12 days after the operation. 2. Maximal tetanic tension developed by the reinnervated muscles was recorded and taken as an indication of their recovery. Two months after nerve crush, slow soleus muscles developed only slightly less tension than the control unoperated soleus muscles. The reinnervated fast muscles tibialis anterior (t.a.) and extensor digitorum longus (e.d.l.) developed only about 50% of the tension of the unoperated controls. 3. The fast muscles never recovered, remaining weaker and smaller throughout the animals' life. 4. The number of muscle fibres in the reinnervated fast muscles was substantially reduced and their fibre composition altered in that they contained mainly muscle fibres with high levels of oxidative enzymes. 5. The reinnervated fast muscles became much more fatigue resistant than the unoperated controls. 6. The possibility that these changes are due to motoneurone death was examined. The motoneurones innervating the fast muscles were labelled by retrograde transport of HRP. No significant reduction in the number of motoneurones innervating the operated muscles was found. 7. These results show that nerve injury during early post-natal life causes permanent changes in fast muscles that are not caused by motoneurone death.

The effect of unilateral phrenicectomy on the rate of protein synthesis in rat diaphragm in vivo

The fractional rate of protein synthesis (ks) in the denervated rat-diaphragm has been measured in vivo by the continuous amino acid infusion technique at 1, 3, 5 and 10 days after nerve section, and compared with the rate determined in normal rats. Similar rates of protein synthesis, 14% per day, were found for both the left and right hemidiaphragms in the control animals. In the denervated rats, the rates of protein synthesis in the contralateral control hemidiaphragms were significantly increased as soon as 1 day after nerve section. This is considered to be evidence of a compensatory synthesis in the control tissues. In the denervated hemidiaphragm, the rate of protein synthesis had doubled by the third day after nerve section, but by the fifth day had fallen slightly to a value some 50% greater than that of the controls, and remained at this level for a further 5 days. Based on these measured values of protein synthetic rate, calculated estimates have been made of the rate of protein degradation necessary to account for the reported (Turner, L.V. and Manchester, K.L. (1972) Biochem. J. 128, 789-801) changes in mass of the denervated tissue. During the first three days after nerve section, the rate constant for degradation increased to more than twice the normal rate for skeletal muscle, and remained at this value throughout the peak of the hypertrophy.

Effects of denervation on the glycogen content and on the activities of enzymes of glucose and glycogen metabolism in rat diaphragm muscle

1. Changes in the content and concentration of glycogen and in the activity of a number of enzymes involved in glucose and glycogen metabolism were studied in the rat hemidiaphragm after unilateral denervation. 2. After nerve section the tissue hypertrophies; this hypertrophy is said to be confined to the smaller red fibres and not to the white. 3. The total hexokinase activity increases, whereas that of total glycogen phosphorylase decreases. The specific activity of phosphorylase a, determined after Halothane anaesthesia, remains fairly constant. 4. In fed animals the denervated tissue stores less glycogen, but in the early stages its glycogen content does not fall on starvation. 5. The effect of denervation on the specific activities of several other characteristically white-fibre enzymes are not consistent with the response of glycogen phosphorylase; the increase in content of glyceraldehyde 3-phosphate dehydrogenase and lactate dehydrogenase is thought to be related to proliferation of the sarcoplasmic reticulum. 6. The ratio of lactate dehydrogenase M/H subunits increases at the height of the hypertrophy, but then declines as the mass of the tissue falls. 7. The chronology of these changes in enzyme activities suggests a multiplicity of distinct responses after nerve section not consistent with any one model, either specific fibre development or reversion to de-differentiated, foetal-type metabolism.

Effects of denervation on the activities of some tricarboxylic acid-cycle-associated dehydrogenases and adenine-metabolizing enzymes in rat diaphragm muscle

1. The activity of several tricarboxylic acid-cycle-associated dehydrogenases, adenine-metabolizing enzymes and glutathione reductase and the content of myoglobin were measured in rat diaphragm muscle after unilateral nerve section. 2. Consistent with morphological disintegration of the mitochondria there was a rapid diminution in activity of NAD- and NADP-linked isocitrate dehydrogenase, malate dehydrogenase and glutamate dehydrogenase. 3. Creatine phosphokinase and adenylate kinase, by contrast, showed little change in activity; adenylate deaminase and glutathione reductase activities increased during the hypertrophic phase. The concentration of myoglobin at first declined, then increased again. 4. The distribution of enzymes between the left and right hemidiaphragms was found not to be uniform. 5. Activities of adenine-metabolizing enzymes in the diaphragm were as great as in white muscle. It is suggested that their reputedly lower activities in red muscle properly refer to muscle containing a high proportion of intermediate fibres, which is not the case with diaphragm. 6. The possible causes of the transient hypertrophy after nerve section are discussed.