Cytochemical and biochemical glucose 6-phosphatase activity in skeletal muscle cells of mice

The role of histidine dipeptides on postmortem acidification of broiler muscles with different energy metabolism

It is generally held that the content of several free amino acids and dipeptides is closely related to the energy-supplying metabolism of skeletal muscles. Metabolic characteristics of muscles are involved in the variability of meat quality due to their ability to influence the patterns of energy metabolism not only in living animal but also during postmortem time. Within this context, this study aimed at establishing whether the concentration of histidine dipeptides can affect muscle postmortem metabolism, examining the glycolytic pathway of 3 chicken muscles (pectoralis major, extensor iliotibialis lateralis, and gastrocnemius internus as glycolytic, intermediate, and oxidative-type, respectively) selected based on their histidine dipeptides content and ultimate pH. Thus, a total of 8 carcasses were obtained from the same flock of broiler chickens (Ross 308 strain, females, 49 d of age, 2.8 kg body weight at slaughter) and selected immediately after evisceration from the line of a commercial processing plant. Meat samples of about 1 cm³ were excised from bone-in muscles at 15, 60, 120, and 1,440 min postmortem, instantly frozen in liquid nitrogen and used for the determination of pH, glycolytic metabolites, buffering capacity as well as histidine dipeptides content through ¹H-NMR. Overall results suggest that glycolysis in leg muscles ceased already after 2 h postmortem, whereas in breast muscle continued until 24 h, when it exhibited significantly lower pH values (P < 0.05). However, considering its remarkable glycolytic potential, pectoralis major muscle should have exhibited a greater and faster acidification, suggesting that its higher (P < 0.05) histidine dipeptides' content might have prevented a potentially stronger acidification process. Accordingly, breast muscle also showed greater (P < 0.05) buffering ability in the pH range 6.0–7.0. Therefore, anserine and carnosine, being highly positively correlated with muscle's buffering capacity (P < 0.001), might play a role in regulating postmortem pH decline, thus exerting an effect on muscle metabolism during prerigor phase and the quality of the forthcoming meat. Overall results also suggest that total histidine dipeptides content along with muscular ultimate pH represent good indicators for the energy-supplying metabolism of chicken muscles.

Low levels of glucose-6-phosphate hydrolysis in the sarcoplasmic reticulum of skeletal muscle: involvement of glucose-6-phosphatase

Glucose-6-phosphate hydrolysis was measured in a fraction obtained from rabbit fast-twitch skeletal muscle and corresponding to total sarcoplasmic reticulum, as well as in three subfractions containing longitudinal tubules, terminal cisternae or both structures. In all cases the levels of hydrolysis measured both in native and disrupted membranes were approximately 60-100 times lower than the microsomal glucose-6-phosphatase activity of the corresponding livers. In contrast to liver microsomes, most (up to 80%) of the glucose-6-phosphate hydrolysing activity in muscle sarcoplasmic reticulum membranes was not inactivated by pH 5.0 pre-incubation indicating that it was not catalysed by the specific glucose-6-phosphatase enzyme. Osmotically induced changes in light-scattering intensity of sarcoplasmic reticulum vesicles revealed that, in contrast to liver microsomes, sarcoplasmic reticulum vesicles were not selectively permeable to glucose-6-phosphate as mannose-6-phosphate was also permeable and in addition they were poorly permeable to glucose. Immunoblot experiments using antibodies raised against the glucose-6-phosphatase enzyme, and liver endoplasmic reticulum glucose and Pi translocases, failed to detect the presence of these protein components in sarcoplasmic reticulum membranes. Southern blot analysis of reverse transcriptase-polymerase chain reaction products from rat muscle revealed that glucose-6-phosphatase mRNA is present in muscle. Quantification of Northern blot analysis of liver and muscle mRNA indicated that muscle contains less than 2% of the amount of glucose-6-phosphate mRNA found in corresponding livers. We conclude that very low levels of specific glucose-6-phosphatase (e.g. as in liver; E.C. 3.1.3.9) are present in muscle sarcoplasmic reticulum and that the muscle and liver glucose-6-phosphatase systems have several different properties.

Age-related changes in fibre number, fibre size, fibre type composition and adenosine triphosphatase activity in rat soleus muscle

To study the aging of muscle fibres in red skeletal muscle, fibre number, fibre diameter and fibre type composition in the soleus muscle of male rats of 3, 12 and 24 months old were examined. The total number of muscle fibres remained unchanged, while average diameter increased slightly with increasing age. The staining intensity of myosin adenosine triphosphatase (ATPase) activity in the fibres decreased with advancing age. Therefore, observation on the basis of myosin ATPase histochemistry alone is not adequate to study the aging of muscle fibres. In the muscles of 24 month-old animals, four fibre types were recognized; 1) many (52%) type I-O fibres showing weak ATPase and succinate dehydrogenase (SDH) reactions with slight subsarcolemmal aggregates of diformazan (SAD); 2) some (33%) type M fibres showing weak ATPase and intense SDH reactions with marked SAD; 3) a few (12%) type O fibres showing weak ATPase and intense SDH reactions without SAD; and 4) very few (4%) type IIA fibres. Histochemical and morphometric results suggest that type I-O, type M and type O fibres are derived from type I, type I and type IIA fibres, respectively. Furthermore, no transitional fibres from type IIA to type I were observed. Therefore, age-related changes in fibre type composition in the muscle cannot be explained by the simple idea that most type IIA fibres are transformed into type I fibres.

Neuronal regulation of substrate cycle between glucose 6-phosphate and glucose in brown adipose tissues of cold-exposed mice

The increase in both glucose 6-phosphatase and hexokinase activities in brown adipose tissues of cold-exposed mice probably relates to thermogenesis by the substrate cycle between glucose 6-phosphate and glucose (Watanabe et al.: Anatomical Record 219:39-44, 1987). To clarify the factors causing the simultaneous increase, we examined biochemically the effects of uni- or bilateral surgical denervation of brown adipose tissues, of adrenalectomy, or of streptozotocin injection on the increase in the two activities in the tissues of cold-exposed mice. Further, the effects of denervation on the increase were also examined histochemically. The simultaneous increase in the two activities was inhibited in the denervated tissues of cold-exposed animals in biochemical and histochemical experiments. However, the increase in the activities was not inhibited in the tissues of animals exposed to cold after adrenalectomy or streptozotocin injection. The results suggest strongly that the activation of the substrate cycle in brown adipose tissues of cold-exposed mice is caused by a transmitter released from sympathetic nerve endings, probably norepinephrine.

Significance of increase in glucose 6-phosphatase activity in brown adipose cells of cold-exposed and starved mice

Cytochemical and biochemical glucose 6-phosphatase (G6Pase) activity was examined in brown adipose tissues of normal, cold-exposed, or starved mice. In addition, G6Pase activity in white adipose tissue and hexokinase activity in brown and white adipose tissues were biochemically measured. In normal animals, the reaction product for G6Pase activity was localized in the endoplasmic reticulum and nuclear envelope of brown adipose cells. The amount of the reaction product increased in cold-exposed or starved animals. Biochemical G6Pase activity (259.7 +/- 48.5 ng Pi/min/mg protein) in brown adipose tissues of normal animals was higher when the value was compared with values of other organs. Biochemical G6Pase and hexokinase activities increased rapidly in brown adipose tissues of cold-exposed animals, and a close relation was found between activities of the two enzymes. In brown adipose tissues of animals starved for 3 days, biochemical G6Pase activity increased, but hexokinase activity did not change. In white adipose tissues of normal, cold-exposed, or starved animals, G6Pase activity was very low, although the enzyme activity increased slightly in animals starved for 3 days. The results show that the high G6Pase activity in brown adipose cells probably relates to thermogenesis in cold-exposed animals and may be concerned with glucose release into the blood in starved animals.

Physiological role of skeletal muscle glycogen in starved mice

To study the physiological role of skeletal muscle glycogen in starved animals, effects of starvation on glycogen and glycogen phosphorylase (EC 2.4.1.1.) activity were studied in muscle fibers (morphologic study) and in whole muscles (biochemical study) of the rectus femoris muscle of mouse. Glycogen content in the liver of the starved animals was also measured. PAS reaction, strong in muscle fibers of fed animals, became weak predominantly in type IIB fibers after 2 days and almost disappeared after 4 days of starvation. Glycogen particles, numerous in the sarcoplasm between myofibrils of muscle fibers, decreased markedly predominantly in type IIB fibers after 2 days and almost disappeared after 4 days. Phosphorylase a activity, undetected in fibers of fed mice, appeared weak in type IIB fibers and very weak in type IIA fibers after 2 days and became moderate in type IIB fibers and weak in type IIA fibers after 4 days. Muscle glycogen content did not differ by 16 hours from the values of corresponding fed animals. However, liver glycogen content had already decreased after 8 hours and markedly so after 12 hours. The results support our hypothesis-"skeletal muscle glycogen is used for maintaining the blood glucose level in starved mice" (Hirose et al.: Anat. Rec., 216:133-138, 1986)-and show that type IIB fibers play a main role in maintaining the glucose level and that muscle glycogen is utilized after depletion of liver glycogen.

Development of transferrin-positive oligodendrocytes in the rat central nervous system

Transferrin is the second most abundant plasma protein and functions to transport iron. It is an essential constituent in culture media for virtually all cells. In a recent study, we reported that transferrin (Tf) is specifically located in oligodendrocytes in the rat nervous system. This investigation examines immunohistochemically the development of Tf in the cerebral cortex, corpus striatum, and spinal cord. Tf is first seen in oligodendrocytes in the spinal cord white matter at 5 days of age. The immunoreactivity is confined to the white matter in the periphery of the spinal cord between 5 and 8 days of age. By 10-12 days of age, the number of immunoreactive oligodendrocytes in the spinal cord white matter increases considerably, corresponding to the onset of myelination. Tf-positive oligodendrocytes are first found in the gray matter at 15 days of age. By 30 days of age, the number and distribution of Tf-positive oligodendrocytes in both the brain and spinal cord have reached the adult pattern. The results of this study demonstrate a spatial and temporal association between Tf development and myelinogenesis. This suggests that part of the process of differentiation of oligodendrocytes includes the accumulation of Tf, perhaps in order to support the metabolic demands associated with the production and maintenance of myelin.

Significance of the increase in glucose 6-phosphatase activity in skeletal muscle cells of the mouse by starvation

The effects of starvation on glucose 6-phosphatase (G6Pase; EC 3.1.3.9., D-glucose 6-phosphate phosphohydrolase) and glycogen phosphorylase (EC 2.4.1.1.) activities, and on glycogen content, were studied in skeletal muscles (m. rectus femoris) of mice. In the muscle cells from fed animals, the cytochemical reaction product for G6Pase activity was observed in moderate amounts in the terminal cisternae of sarcoplasmic reticulum and in small amounts in the nuclear envelope, and was rare or absent in the intermyofibrillar sarcoplasmic reticulum. After 4 days of starvation, however, the reaction product became abundant in all of the terminal cisternae, intermyofibrillar sarcoplasmic reticulum, and nuclear envelope. Biochemical G6Pase and glycogen phosphorylase a (active form) activities were higher in the muscles of starved mice than in those of fed animals. The glycogen content decreased markedly in the muscles of starved mice. The results suggest that the role of the increased G6Pase in skeletal muscle cells of starved mice is to release glucose into the blood by hydrolyzing glucose 6-phosphate produced through the increased phosphorylase activity.

Glucose 6-phosphatase activity in pregnant and lactating mammary glands of the mouse

Glucose 6-phosphatase activity was studied in the secretory epithelial cell and other cell types composing alveoli of the mammary gland (cytochemical study) and in the whole mammary gland (biochemical study) of pregnant and lactating mice. The reaction product for the enzyme activity was seen in the endoplasmic reticulum and nuclear envelope in secretory epithelial cells from all animals examined (days 7 and 14 of pregnancy, and days 0, 3, 10, and 20 of lactation. The amounts of the reaction product appeared scarce at day 7 of pregnancy, moderate at day 14 of pregnancy and day 0 of lactation, and abundant at days 3 and 10 of lactation. The reaction product, however, became generally scarce at day 20 of lactation. Biochemical activity was relatively low at days 7 and 14 of pregnancy and days 0 and 20 of lactation, while it was high at days 3 and 10 of lactation. The increased activity is probably related to functions of secretory epithelial cells in the lactating gland.