The response of hind-limb muscles of the weanling rat to undernutrition and subsequent rehabilitation

The effect of acute dietary restriction on muscle fibre number in weanling rats

1. Male Sprague-Dawley rats were allocated at 100 g into either an ad lib.-fed control group or a food-restricted group. The restricted group was fed for 9 d at 25% of ad lib. intake. Controls were killed at a body-weight of 100 g and 29 d of age. 2. The effects of food restriction on muscle weight, fibre number, fibre diameter, DNA, and protein were examined in three skeletal muscles, the soleus, plantaris and extensor digitorum longus (EDL). 3. Acute dietary restriction caused body- and muscle-weight loss and a decrease in both the number and cross-sectional area of muscle fibres in each of the muscles. 4. The restriction halted growth-related increases in DNA in all muscles and decreased the protein:DNA value in the plantaris and EDL. 5. These results indicate that present theories describing cellular development are not adequate to define growth potential or growth retardation of skeletal muscle.

Changes in skeletal muscle cellularity in starved and refed young rats

1. All food was withdrawn from male weanling rats until a 40% loss of body-weight was attained. Another group of animals was treated similarly and then refed a stock diet until the original body-weight was attained. 2. The body-weight loss caused a significant reduction in the weight of the heart, kidney, liver and epididymal fat pads. Refeeding produced a return to the control weight of the heart and kidney, an increase in the weight of the liver and a deficit in the weight of the epididymal fat pads. 3. Body-weight loss caused a decrease in the weight of the three different muscles studied, and in the number and diameter of the fibres in each muscle. Refeeding restored the weight and cellularity of two of the three muscles to that of the control animals. The soleus muscle was heavier in the refed animals when compared to controls due to an increased fibre diameter. 4. It is concluded that the decrease in the number and diameter of muscle fibres during starvation in the rat can be restored on refeeding a stock diet.

Comparison of the chemical and biochemical composition of thirteen muscles of the rat after dietary protein restriction

1. The objective of this study was to determine whether the chemical and biochemical changes induced by muscle wasting caused by dietary protein restriction are different in various skeletal muscles. 2. Rats were fasted for 3 d and then fed on a 10 g protein/kg diet for 21 d. Thirteen muscles from the trunk, forelimb, and hind-limb regions were analysed for muscle weight, and the content of water, fat, cellular and extracellular protein, DNA and RNA. Results were compared to values for an 'initial' control group killed at the start of the experiment. 3. Weight loss was greatest in trunk muscles and least in the distal forelimb muscles. Water content decreased in most muscles, but increased in three forelimb muscles. A significant loss of lipid was found in the gastrocnemius, while the biceps brachii gained lipid. Changes in lipid content of the muscles did not form a distinctive pattern. 4. All muscles except the distal forelimb muscles lost a significant amount of cellular protein, while all muscles except the diaphragm gained extracellular protein. 5. DNA content was unchanged in all muscles. The value for cellular protein:DNA was significantly reduced in the rectus abdominis and the diaphragm. A significant loss of RNA was found in all muscles; the percentage change was greatest in trunk muscles and least in the distal forelimb muscles. The values for RNA:protein and RNA:DNA were significantly lower in all muscles except two distal forelimb muscles. 6. With the exception of the water and lipid content of the muscles, the directions of the changes in the experimental animals were the same for all muscles. The results suggested, however, that the magnitude of changes in certain chemical and biochemical indices of composition may depend to some extent on the anatomical location of the muscle: trunk muscles tended to show the greatest percentage change, while the distal forelimbs changed the least.

Stability of rat skin collagen during recovery from under-nutrition

The growth of young rats was arrested for 6 weeks from 48 h after receiving an injection of L-[5-3H]proline. The 3H in the hydroxyproline of the newly synthesized skin collagen remained steady during under-nutrition and did not decrease during the subsequent recovery period. It was concluded that in this animal model the renewed growth did not induce degradation of the pre-existing collagen fibres.

Polyamines and their biosynthetic decarboxylases in various tissues of the young rat during undernutrition

1. Male weanling rats were maintained at a constant body-weight by feeding them reduced amounts of the normal diet for various periods up to 4 weeks. Control male rats were allowed free access to the normal diet and some were killed at the beginning of the experiment and others at the same ages as the experimental rats. 2. After killing by cervical dislocation the rats had their liver, quadriceps muscles and spleen removed. The tissues were weighed and the activities of the enzymes ornithine decarboxylase (ODC; EC 4.1.1.17) and S-adenosylmethionine decarboxylase (SAMD; EC 4.1.1.50) assayed in each tissue. In the liver the content of the polyamines (spermidine and spermine) and putrescine was also measured. 3. The liver and quadriceps muscles showed an over-all maintenance of weight during undernutrition, but the spleen lost weight during the first 7 d of undernutrition and then remained constant. The weight of the liver increased by approximately 50% following the daily maintenance feed, but returned to its prefeeding value by 24 h after feeding. 4. During the first 7 d of undernutrition ODC activity decreased in all three tissues, and remained fairly constant thereafter. In the liver there were marked increases in the activity of ODC during the first 4 h after the daily feed, but the activity then decreased to prefeeding values. SAMD activity tended to remain normal in the liver, decreased initially and then returned to normal in the quadriceps muscles, and remained normal initially and then decreased in the spleen. Hepatic SAMD activity showed no consistent response to the daily feed, but quadriceps SAMD activity increased significantly between 1 and 8 h after feeding. 5. Hepatic putrescine content remained constant during undernutrition whilst spermine increased slightly and was then maintained above normal for liver size. Hepatic spermidine content decreased initially and then remained constant. Putrescine increased slightly in response to the daily feed and spermidine increased considerably. Spermine content was unaffected by the daily feed. 6. It is suggested that the response of polyamine synthesis in the various tissues is primarily dependent upon the way in which nutrients are made available to the tissues. The maintenance of spermine content in the liver at the expense of spermidine may be related to differential changes in the nucleic acids.

Polyamines and their biosynthetic decarboxylases in various tissues of the young rat during recovery from undernutrition

1. Weanling male and female rats were undernourished for 4 weeks and then rehabilitated by allowing ad libitum feeding. 2. During rehabilitation polyamine-biosynthetic enzymes were examined in the liver, spleen and quadriceps and gastrocnemius muscles. 3. During the first few hours of rehabilitiation there was a marked increase in liver weight, accompanied by a very marked increase in ornithine decarboxylase activity. Increases in the activity of this enzyme in other tissues did not occur until between 2 and 7 days of rehabilitation, at which time there were further increases in enzyme activity in the liver. 4. S-Adenosylmethionine decarboxylase activity also showed marked fluctuations in activity in all the tissues examined. 5. Hepatic putrescine and spermidine concentrations also varied during rehabilitation, but permine concentration remained relatively constant. Both spermine and spermidine were at normal concentrations in the liver from the 10th days of rehabilitation onwards. 6. In all of the tissues examined there were marked sex differences in the parameters studied, particularly in splenic and muscular ornithine decarboxylase activity. 7. In the tissues of the male rats, changes in polyamine synthesis paralled changes in nucleic acid and protein synthesis.