David F. Goldspink

The effect of hypokinesia and hypodynamia on protein turnover and the growth of four skeletal muscles of the rat.

An animal suspension model has been used to simulate the weightlessness experienced during space travel. This procedure results in a reduction in the normal shortening (i.e. hypokinesia) and force generation functions of hind limb muscles (i.e. hypodynamia). The ensuing muscle atrophy was studied over 12 days in different muscle types. Slow muscles (e.g. the soleus) underwent a more pronounced atrophy than intermediate (i.e. gastrocnemius) and fast phasic muscles (e.g. extensor digitorum longus). In all muscle types inactivity resulted in a smaller accumulation of DNA and losses of RNA and protein after 5 days. The latter arose from a decrease in the rate of protein synthesis (measured in vivo) and an increase in protein breakdown. Increased specific activities of cathepsins B and D also supported the view that there is an increased proteolysis after hypokinesia and hypodynamia.When the inactive soleus was simultaneously held in a lengthened (stretched) state the atrophy was prevented through a large increase in the fractional rate of protein synthesis. Protein degradation remained elevated with stretch, thereby slowing the growth of these muscles relative to those in pair-fed, ambulatory controls. The much smaller atrophy of the tibialis anterior and extensor digitorum longus muscles in suspended only limbs represented an underestimate of the true atrophic effects of hypokinesia and hypodynamia. In this model gravity pulls the suspended foot into a plantar flexed position, thereby permanently stretching and protecting such flexor muscles. When this influence of stretch was removed a greater atrophy ensued, mainly due to the loss of the stretch-induced stimulation of protein synthesis. Despite this, the inactive fast-twitch muscles still exhibited less atrophy than the gastrocnemius and soleus muscles.

The effects of aging and chronic dietary restriction on whole body growth and protein turnover in the rat

Changes in whole body growth, nucleic acids, and protein turnover have been studied in conjunction with ageing and chronic dietary restriction. Normal developmental changes between weaning and senescence included progressive decreases in the fractional rates of growth, protein synthesis, and protein breakdown; the decline in the synthetic rate correlating with decreases in the ribosomal capacity. Dietary intervention was imposed at weaning and involved pair feeding to 50% of the ad libitum food intake. Although this regime slowed whole body growth by retarding the developmental decline in protein turnover, growth was extended into the second and third years of life. The dietary-induced increase in longevity resulting from a retardation of the ageing process(es) appears therefore to be associated with an enhanced turnover of proteins during the major portion of the life span of dietary restricted rats.

The effect of chronic and acute dietary restriction on the growth and protein turnover of fast and slow types of rat skeletal muscle

Changes in the growth and protein turnover of the anterior tibialis and soleus muscles were studied in response to acute and chronic dietary restriction (50% of ad libitum intake) between 3 and 149 weeks post partum. The effect of long-term dietary restriction from weaning to senescence was to retard the growth and normal developmental of the two types of skeletal muscle. This was evident from measurements of various parameters of growth, i.e. total protein, RNA and DNA and protein/DNA-P, which were reduced by approximately 50% when compared with age-matched controls. These decreases, however, were not accompanied by a decline in the fractional rate of synthesis (%/day) or ribosomal activity (mg protein/day per mg RNAP). The slowing down of the age-related decline in muscle growth has been attributed to a reduction in RNA capacity (RNA/protein), with similar responses in the fast- and slow-twitch skeletal muscles. The initial effects of piecemeal feeding of this restricted diet on the two types of muscle were also monitored. Short term starvation effects, i.e. 24 hr after feeding a reduced ration, were measured on the protein content and RNA/protein of both the anterior tibialis and soleus muscles; both parameters were unchanged within 24 hr. In contrast, a rapid and significant decline in the ribosomal synthetic activity (mg/d per mg RNAP), and a corresponding fall in the fractional rate of synthesis, occurred within 24 hr of feeding.

The role of passive stretch and repetitive electrical stimulation in preventing skeletal muscle atrophy while reprogramming gene expression to improve fatigue resistance.

The effects of mechanical stimuli on preserving muscle mass while transforming them into slow, fatigue resistant muscles have been studied in the rabbit. When combined, stretching and electrical stimulation (10 Hz) Induce rapid and marked growth of muscles. This procedure also more rapidly activates the transformation process(es) than when either stretching or electrical stimulation (10 Hz) are used alone. Stretch by itseif is also anabolic causing useful lengthening of muscles and preventing collagen accumuiation. In contrast, muscle inactivity leads to rapid atrophy, fiber shortening and reduced muscle compliance. We believe these findings have important implications to cardiomyoplasty.

The effects of ageing and chronic dietary restriction on in vivo hepatic protein synthesis in the rat

Hepatic growth and protein synthesis in vivo was studied with age in ad libitum-fed and dietary restricted rats in which the mean and maximum lifespan was significantly extended. Livers from underfed rats showed significantly lower DNA, RNA and protein contents, and total protein synthesis. The fractional rate of synthesis although initially depressed by restricted feeding, showed no consistent trend with age when compared with control values. The lower fractional rate of synthesis observed in livers from dietary restricted rats at 7 weeks of age is attributable to a significant decrease in ribosomal capacity, with no effect on ribosomal activity being evident. Liver tissue from rats fed ad libitum demonstrated a progressive loss of translational efficiency with age which was delayed by chronic dietary restriction.

The influence of chronic dietary intervention on protein turnover and growth of the diaphragm and extensor digitorum longus muscles of the rat.

Changes in weight, protein, RNA and DNA contents of the E.D.L. and diaphragm muscles were studied in conjunction with aging and chronic dietary restriction. Between weaning and senescence both muscles exhibited progressive decreases in their fractional rates of growth, protein synthesis and protein breakdown; these rates being age for age higher in the diaphragm. Dietary restriction (50% of ad libitum food intake) from weaning onwards retarded muscle growth, particularly at the early stages (i.e. 4 weeks) after its implementation. Here the suppression of protein synthesis was due to the combined effects of piece meal feeding and long term reductions in food intake. Later, muscle sizes and total, but not fractional, synthetic rates were consistently decreased by chronic dietary intervention. The onset of the ageing atrophy may also be delayed by underfeeding. The changes in these 2 muscles have been compared to those in the whole animal and other striated muscles, as previously reported by the authors.

Maternal Diabetes in Rats: I. Effects on Placental Growth and Protein Turnover

The developmental growth of the rat placenta was investigated between days 14 and 21 of gestation in normal control, gestational-diabetic, established-diabetic, and insulin-maintained-diabetic mothers. While established–diabetic mothers were hyperglycemic for 2 wk before and throughout the pregnancy, gestational-diabetic mothers were only hyperglycemic for the second half of pregnancy. Daily insulin replacements successfully restored normoglycemia. The wet weight and protein content of control placentas increased linearly between days 14 and 21. Although placentas from diabetic animals were initially smaller, placentomegaly was found at full term. Placental glycogen concentrations were also markedly increased in all diabetic animals. These changes were largely prevented by insulin replacement. The changes in placental size during normal development and in association with the diabetic state were explained by measuring placental rates of protein turnover (in vivo). In normal placentas, protein synthetic and degradative rates progressively declined over the last week of gestation. Because synthesis rates were unchanged in placentas of diabetic mothers, it appears that the differences in placental size primarily arise from alterations in protein degradation.

Protein turnover and cathepsin B activity in several individual tissues of foetal and senescent rats

The fractional rate of protein synthesis has been measured in vivo, and compared in the whole body and 12 major individual tissues of foetal and senescent rats. This synthetic rate was found to decrease in most tissues with increasing animal age. The rate of protein degradation was also determined and compared with cathepsin B activity within each tissue; both protein turnover and the endopeptidase activity decreased with ageing. Age-related changes in each tissues contributions to the protein mass and synthetic rates of the whole animal are also summarized and related to developmental variations in physiological function.

Protein Turnover and Growth of the Rat Brain from the Foetus to Old Age

Abstract: Growth of the rat brain was studied between 16 days of foetal life and old age (105 weeks). Developmental changes in cerebral RNA, DNA, and protein contents are described. The age‐related decline in brain growth rates correlates with progressive decreases in the fractional rates of protein synthesis (from 58 to 6.8% per day) and break down (from 36.4 to 4.1% per day).

The effects of denervation on protein turnover of the soleus and extensor digitorum longus muscles of adult mice.

1. Changes in protein turnover of the soleus and EDL muscles of adult mice have been studied 1, 7 and 80 days after denervation. 2. Increased rates of protein degradation 7 and 80 days post-denervation correlated with the atrophy and loss of protein from these muscles. 3. Rates of protein synthesis in the EDL decreased 24 hr after nerve section. However, these synthetic rates increased again to become higher in the 7 day denervated muscles compared with their controls. These latter anabolic changes are inconsistent with the concept of a denervated muscle being inactive. 4. These findings have been compared with a similar study on muscles of growing rats. Any passive stretching of the denervated muscles by continued bone growth appears unlikely to be a crucial factor explaining the increased rates of protein synthesis 7 days after denervation.