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Biomedical subjects

J B Li

Publications and source records attributed to J B Li.

At least 55 records · Page 3Linked to original sources
PubMed1980

Effects of glucocorticoids on peptide chain initiation in heart and skeletal muscle.

Treatment of rats for 5 days with cortisone acetate reduced synthesis of skeletal muscle protein by 56% compared to untreated controls, but had no effect on protein synthesis in heart. The reduction in synthesis in skeletal muscle was accounted for by both a loss of tissue RNA and development of a block in peptide-chain initiation. Activity of an eIF-2-like initiation factor decreased in psoas muscle of hormone-treated rats in proportion to the loss of RNA. Peptide-chain initiation, RNA content, and initiation factor activity were unaffected in heart muscle. In skeletal muscle of rats treated for 4 hr with dexamethasone, peptide-chain initiation was inhibited, whereas tissue RNA content and initiation factor activity were unchanged. These experiments suggested that total activity of eIF-2 did not always correlate with the rate of initiation, but that there did appear to be a relationship between initiation factor activity and tissue RNA content. Purification of eIF-2 from bovine heart muscle was undertaken in order to directly investigate the mechanism by which glucocorticoids modify eIF-2-activity and control peptide-chain initiation.

Animals↗
PubMed1978

Protein turnover in rat skeletal muscle: effects of hypophysectomy and growth hormone.

The role of growth hormone in regulating protein turnover was examined in a perfused preparation of rat skeletal muscle. The perfused muscle maintained in vivo levels of ATP and creatine phosphate and exhibited constant rates of oxygen consumption and protein synthesis. Hypophysectomy reduced the rate of protein synthesis, the concentration of RNA, and the efficiency of protein synthesis in gastrocnemius muscle to 30, 46, and 66 percent of normal, respectively. In vivo treatment of hypophysectomized (hypox) rats with bovine growth hormone (250 microgram/day for 5 days) resulted in small increases in protein synthesis and RNA, whereas synthesis/RNA was returned to near normal. Elevation of ribosomal subunits in psoas muscle indicated an inhibition of peptide-chain initiation in hypox rats that was reversed by in vivo growth hormone treatment. Thus, hypox rats exhibited both a decreased capacity and a decreased efficiency of protein synthesis. Growth hormone replacement primarily increased efficiency of protein synthesis. The rate of protein degradation and the activity of cathepsin D in gastrocnemius muscle were decreased by hypophysectomy. Growth hormone treatment had no significant effect on degradation.

Animals↗
PubMed1978

Effects of thyroxine on protein turnover in rat skeletal muscle.

The effects of thyroxine (T4) on protein turnover in skeletal muscle were studied using normal, thyroidectomized (thyrex), and hypophysectomized (hypox) rats. Thyrex rats had a depressed growth rate that was accompanied by 50% reductions in the level of RNA and the rate of protein synthesis in gastrocnemius muscle, as determined in the perfused hemicorpus. Protein synthetic efficiency (protein synthesis per unit RNA) was decreased by 18%. Daily treatment of thyrex rats with T4 at different dose levels for up to 16 days led to improved growth rates, elevated RNA concentrations, and increased protein synthesis rates. The primary effect of T4 was to increase the protein synthetic capacity of muscle. Protein degradation, determined in the perfused hemicorpus, and activity of a lysosomal protease, determined in unperfused muscle, were reduced in the thyrex condition. Treatment of thyrex rats with T4 increased protein degradative rates, but not protease activity. Hypox rats, which also exhibited depressed skeletal muscle protein synthesis, responded to T4 and combined T4 and growth hormone with marked improvements in protein synthesis.

Animals↗
PubMed1977

Regulation by insulin of amino acid release and protein turnover in the perfused rat hemicorpus.

Net changes in the concentrations of 18 amino acids in perfusate and skeletal muscle were followed during perfusion of hemicorpus preparations from fed rat. Perfusate levels of 16 amino acids showed little change from their initial concentrations during the 1st h, but increased dramatically during the 2nd and 3rd h. Aspartate and glutamate levels decreased continuously throughout the perfusion. Release of alanine and glutamine accounted for approximately 50% of the total change in perfusate amino acids. The increase in perfusate amino acids was derived from net breakdown of muscle proteins and not from leakage from the intracellular pool as evidenced by elevated concentrations of intracellular amino acids in perfused muscle. Addition of insulin to the perfusate did not change the pattern of amino acid release during the 1st h of perfusion. However, during the 2nd and 3rd h the hormone completely prevented the net release of most amino acids and maintained intracellular concentrations of most amino acids at levels found in upperfused tissue. Effects of time of perfusion and insulin on amino acid release were accounted for by changes in the rate of protein turnover. Protein synthesis in gastrocemius and psoas muscles in control perfusions decreased after 1 h to approximately 50% of the initial rate. This decrease was accompanied by a 2-fold increase in the level of ribosomal subunits, indicating development of a block in peptide chain initiation. Addition of insulin maintained the initial rate of synthesis and the in vivo level of ribosomal subunits, demonstrating that the hormone prevented the block in peptide chain initiation from forming. Addition of insulin after 2 h reversed the perfusion-induced block in initiation. Synthesis of the specific muscle protein myosin was increased 45% over the control rate in the presence of insulin. Insulin also produced a 50% decrease in the rate of protein degradation during the 2nd and 3rd h of perfusion. A similar effect was noted when protein synthesis was inhibited by addition of cycloheximide. Higher concentrations of insulin were required to maximally inhibit protein degradation than to increase protein synthesis. Involvement of lysosomal proteases in the effect of insulin on protein degradation was evaluated by measuring cathepsin D activity in psoas muscle homogenates. "Free" enzyme activity increased as a result of perfusion while addition of insulin maintained this activity at the unperfused level. Neither perfusion nor insulin had any effect on total cathepsin D activity. Alterations in protein degradation and lysosomal enzyme activity were not due to changes in levels of adenine nucleotides, GTP, or creatine phosphate.

Adenosine Diphosphate↗
PubMed1977

Effect of isoproterenol on amino acid levels and protein turnover in skeletal muscle.

The effect of isoproterenol on amino acid concentrations in perfusate and skeletal muscle was studied during a 3-h perfusion of the isolated rat hemicorpus. The beta-adrenergic agonist inhibited the accumulation of alanine, threonine, phenylalanine, tyrosine, lysine, arginine, leucine, and valine and increased the loss of glutamate, aspartate, serine, and isoleucine from the pool of free amino acids in perfusate and muscle. The loss of glutamate was accompanied by a greater accumulation of glutamine. Changes in alanine levels showed the greatest response with a net accumulation of 98 mumol in the controls becoming a net loss of 54 mumol in the experimentals. These changes in amino acid levels were accounted for in part by a 20% decrease in protein degradation. Protein synthesis was not affected by isoproterenol. In addition to an effect on degradation, it appeared that isoproterenol affected amino acid levels by increasing alanine utilization and causing formation of glutamine instead of alanine. Other effects of the drug included increased rates of lactate production, muscle glycogen breakdown, and oxygen consumption, whereas no effects were observed on ATP and creatine phosphate levels. Pyruvate content of muscle was maintained at a higher level in the presence of the drug than in control perfusions.

Alanine↗
PubMed1976

Effects of food deprivation on protein synthesis and degradation in rat skeletal muscles.

The effects of food deprivation on protein turnover in rat soleus and extensor digitorum longus (EDL) were investigated. Muscles were removed from fed or fasted growing rats, and protein synthesis and breakdown were measured during incubation in vitro. Rates of synthesis and degradation were higher in the dark soleus than in the pale EDL. One day after food removal protein synthesis and RNA content in the EDL decreased. On the 2nd day of fasting, rates of protein catabolism in this muscle increased. Little or no change in synthesis and degradation occurred in the soleus. Consequently, during fasting the soleus lost much less weight than the EDL and other rat muscles. In unsupplemented buffer or in medium containing amino acids, glucose, and insulin, the muscles of fasted rats showed a lower rate of protein synthesis expressed per milligram of tissue but not per microgram of RNA. Thus the decrease in muscle RNA on fasting was responsible for the reduced synthesis observed under controlled in vitro conditions. In vivo the reduction in muscle protein synthesis on fasting results both from a lower RNA content and lower rate of synthesis per microgram of RNA. Reduced supply of glucose, insulin, and amino acids may account for the lower rate of synthesis per microgram of RNA demonstrable in vivo.

Amino Acids↗
PubMed1975

Effects of insulin, glucose, and amino acids on protein turnover in rat diaphragm.

A simple method is described for measuring rates of protein synthesis and degradation in isolated rat diaphragm. Muscles incubated in Krebs-Ringer bicarbonate buffer showed a linear rate of synthesis for 3 hours. At the same time, the muscle released tyrosine and ninhydrin-positive material, primarily amino acids, at a linear rate. This release was not a nonspecific leakage of material from the intracellular pools, but reflected net protein degradation. Tyrosine was chosen for studies of protein turnover, since it rapidly equilibrates between intracellular pools and the medium, it can be measured fluorometrically, and it is neither synthesized nor degraded by this tissue. To follow protein degradation independently of synthesis, muscles were incubated in the presence of cycloheximide. Under these conditions, the amount of tyrosine in the intracellular pools was constant, while the muscle released tyrosine at a linear rate. This tyrosine release was used as a measure of degradation. This preparation was used to study the influence of various factors known to be important for muscle growth on protein synthesis and degradation. Similar effects were obtained with diaphragms of normal and fasted rats although the latter showed decreased synthesis and increased protein degradation. Insulin by itself not only stimulated synthesis but also inhibited protein degradation (even in the presence of cycloheximide). These two effects served to reduce the net release of tyrosine from muscle protein to comparable extents. Effects of insulin on synthesis and degradation were greater when glucose was also present in the medium. Glucose by itself inhibited protein degradation but in the absence of insulin glucose had no significant effect on synthesis. Nevertheless, glucose stimulated incorporation of radioactivive tyrosine into protein, but this effect was due to an increased intracellular specific activity. Unlike glucose neither beta-hydroxybutyrate or octanoic acid had any demonstrable effects on protein degradion. The addition of amino acids at plasma concentrations both promoted protein synthesis and inhibited degradation in the diaphragm. Five times normal plasma concentrations of the amino acids had larger effects. The three branched chain amino acids together stimulated synthesis and reduced degradation, while the remaining plasma amino acids did not affect either process significantly. Thus leucine, isoleucine, and valine appear responsible for the effects of plasma amino or isoleucine and valine together, also were able to inhibit protein degradation and promote synthesis.

Amino Acids↗