Control of enzyme activity levels by serum and hydrocortisone in neonatal rat heart cells cultured in serum-free medium

Growth and development in a heliox incubator environment: a long-term safety study

BACKGROUND

Neonates exposed to mechanical ventilation may develop bronchopulmonary dysplasia (BPD). BPD neonates exhibit a 25-30% increase in energy expenditure which may decrease the rate of growth and development. Heliox has been shown to improve pulmonary function and may decrease energy expenditure. We hypothesized that heliox would provide a safe environment for sustained growth and development.

OBJECTIVE

To assess the safety of the heliox environment we observed developmental milestones; recorded changes in weight, total length, limb length and head circumference; measured blood chemistries; compared primary organ and muscle weights, and analyzed muscle enzymatic activity.

DESIGN/METHODS

Four-day-old rabbit pups (n = 27) were randomized into control (21% O(2); 79% N(2)) or heliox (21% O(2); 79% He) groups, then raised for 14 days at 26.7 degrees C and 50% relative humidity. Pups were euthanized on day 14, blood drawn and primary organs, diaphragm and gastrocnemius weighed and snap-frozen.

RESULTS

All pups thrived in both environments, achieving expected developmental milestones. There were no physiologically significant group differences in weight, growth factors, tissue weight, blood chemistry or muscle enzyme activity.

CONCLUSIONS

No observed long-term differences in growth or development. RESULTS demonstrated that long-term heliox exposure is safe in this rabbit model. These data suggest that heliox administration may provide time for pulmonary improvement in the BPD population, warranting appropriate clinical trials.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

The role of phosphometabolites in cell proliferation, energy metabolism, and tumor therapy

A common characteristic of tumor cells is the constant overexpression of glycolytic and glutaminolytic enzymes. In tumor cells the hyperactive hexokinase and the partly inactive pyruvate kinase lead to an expansion of all phosphometabolites from glucose 6-phosphate to phosphoenolpyruvate. In addition to the glycolytic phosphometabolites, synthesis of their metabolic derivatives such as P-ribose-PP, NADH, NADPH, UTP, CTP, and UDP-N-acetyl glucosamine is also enhanced during cell proliferation. Another phosphometabolite derived from P-ribose-PP, AMP, inhibits cell proliferation. The accumulation of AMP inhibits both P-ribose-PP-synthetase and the increase in concentration of phosphometabolites derived from P-ribose-PP. In cells with low glycerol 3-phosphate and malate-aspartate shuttle capacities the inhibition of the lactate dehydrogenase by low NADH levels leads to an inhibition of glycolytic ATP production. Several tumor-therapeutic drugs reduce NAD and NADH levels, thereby inhibiting glycolytic energy production. The role of AMP, NADH, and NADPH levels in the success of chemotherapeutic treatment is discussed.

Neonatal rat heart cells cultured in simulated microgravity

In vitro characteristics of cardiac cells cultured in simulated microgravity are reported. Tissue culture methods performed at unit gravity constrain cells to propagate, differentiate, and interact in a two-dimensional (2D) plane. Neonatal rat cardiac cells in 2D culture organize predominantly as bundles of cardiomyocytes with the intervening areas filled by nonmyocyte cell types. Such cardiac cell cultures respond predictably to the addition of exogenous compounds, and in many ways they represent an excellent in vitro model system. The gravity-induced 2D organization of the cells, however, does not accurately reflect the distribution of cells in the intact tissue. We have begun characterizations of a three-dimensional (3D) culturing system designed to mimic microgravity. The NASA-designed High-Aspect Ratio Vessel (HARV) bioreactors provide a low shear environment that allows cells to be cultured in static suspension. HARV-3D cultures were prepared on microcarrier beads and compared to control-2D cultures using a combination of microscopic and biochemical techniques. Both systems were uniformly inoculated and medium exchanged at standard intervals. Cells in control cultures adhered to the polystyrene surface of the tissue culture dishes and exhibited typical 2D organization. Cells cultured in HARVs adhered to microcarrier beads, the beads aggregated into defined clusters containing 8 to 15 beads per cluster, and the clusters exhibited distinct 3D layers: myocytes and fibroblasts appeared attached to the surfaces of beads and were overlaid by an outer cell type. In addition, cultures prepared in HARVs using alternative support matrices also displayed morphological formations not seen in control cultures. Generally, the cells prepared in HARV and control cultures were similar; however, the dramatic alterations in 3D organization recommend the HARV as an ideal vessel for the generation of tissuelike organization of cardiac cells in vitro.

Serum factor induces selective increase in Na-channel expression in cultured skeletal muscle

We have examined effects of horse serum (HS) and various fractions (1 million-1M, 300K, 100K, and 30K nominal molecular weight limit) obtained by ultrafiltration on expression of TTX-sensitive Na-channels and on activities of the Na-K pump and glucose transport systems in cultured myotubes obtained from 1-2-day-old neonatal rat pups. Five-day-old cells were transferred to serum-free medium with no hormone or growth factor supplements (DMEM) for 24 hr and then treated with the various serum fractions for 48 hr. Measurements were made of specific [3H]-saxitoxin (STX) binding, action potential properties, 86Rb-uptake and 2-deoxyglucose (2-DG) uptake. HS significantly increased all parameters compared to DMEM (increases in STX-binding, 69%; Rb-uptake, 65%; 2-DG uptake, 93%). Results of treatment with the separate fractions showed that the 300K fraction caused a significantly greater increase in STX-binding than either HS or the other fractions. In contrast, the increases in Rb and 2-DG uptakes induced by the different fractions were not different from that obtained with HS. We conclude that serum contains a factor that selectively increases expression of TTX-sensitive Na-channels in skeletal muscle.

Serum-free, chemically defined medium to evaluate the direct effects of growth factors and inhibitors on proliferation and function of neonatal rat cardiac muscle cells in culture

Neonatal rat cardiac myocytes were isolated and cultured to evaluate the effects of growth factors and inhibitors on proliferation, survival, and functions in a serum-free medium. Insulin and transferrin in MCDB 107 nutrient medium elicited DNA and protein synthesis in cells on a fibronectin-coated culture surface in serum-free medium. Insulin was most effective on both DNA and protein synthesis in serum-free culture conditions. The serum-free, hormone-supplemented medium eliminated the contamination of noncardiac myocytes and supported the long-term survival (over 18 d) of cardiac myocytes. Dexamethasone was required to induce optimal contractility with or without insulin and transferrin. Serum contained both negative and positive effectors of DNA and protein synthesis of the cardiac myocytes. Concentrations of serum (above 5%) inhibited DNA and protein synthesis. Low density lipoprotein (LDL) accounted in part for the inhibitory activity. The serum-free culture system provides a useful model to elucidate the role of hormones, growth factors, and drugs in heart cell regeneration and function.

Hemin increases aerobic capacity of cultured regenerating skeletal myotubes

Regeneration of damaged, mature muscle occurs by differentiation of satellite cells. In culture, satellite cell myoblasts proliferate, align, and fuse to form cross-striated, contracting myotubes. The biochemical changes and the factors that regulate differentiation in satellite cells have not been investigated previously. We report here that no significant differences in glucose uptake rate or glucose oxidation rate were observed between regenerating myoblasts and myotubes, whereas the aerobic oxidation of palmitic acid increased 7.3-fold between these differentiation states. Specific activities of enzymes of critical importance in aerobic metabolism or in production of ATP were increased 2- to 3.5-fold during fusion. Addition of 20 microM hemin to regenerating muscle cultures potentiated the aerobic capacity as evidenced by a 23.6% increase in palmitate oxidation rate. Hemin also increased the specific activities of all nonheme enzymes investigated with the exception of phosphofructokinase. This augmentation of aerobic metabolism together with the time frame of active muscle differentiation suggests a complex role for hemin in myogenesis.

Increased aerobic glucose oxidation by cAMP in cultured regenerated skeletal myotubes

Previous studies of embryonic rat skeletal muscle cultures suggested that there was a correlation between intracellular adenosine 3',5'-cyclic monophosphate (cAMP) concentration and activities of enzymes of oxidative energy metabolism. We investigated the ability of agents that elevate intracellular cAMP by three different mechanisms (activation of adenylate cyclase, inhibition of phosphodiesterase, and analogues) to alter not only the activities of 11 glycolytic and mitochondrial enzymes but also the rate of flux through aerobic glucose oxidation in intact myotubes derived from regenerating rat muscle satellite cells. The enzyme activities were not consistently altered when cAMP was elevated, with the exception of the electron transport enzyme, NADH cytochrome c reductase, whose activity was elevated by exposure of the myotubes to cholera toxin (110% of control), 3-isobutyl-1-methylxanthine (112%), caffeine (119%), and 8-bromoadenosine 3',5'-cyclic monophosphate (120%). The rate of flux of aerobic glucose oxidation was elevated by all agents (116-157% of control) except cholera toxin. This study allowed us to compare the metabolic characteristics of myotube cultures derived from satellite cells with those from embryonic muscle, from a previous study. Despite differences between these two models, together, the data present strong evidence that an increase in intracellular cAMP can cause an increase in oxidative capacity.

Glucocorticoid stimulation of metabolism and glycerol-3-phosphate dehydrogenase activity in cultured heart cells

The direct effects of the glucocorticoids hydrocortisone and corticosterone on myocardial metabolism were studied in cultured heart cells by assessing several parameters previously unreported. Hormone and growth factor concentrations were carefully controlled by using a serum-free medium, which also allowed maintenance of cells in the absence of glucocorticoids. Heart cell beating rate, glucose uptake rate, and CO2 evolution from radioactively labeled glucose were increased by the addition of 0.03 microM corticosterone to the medium of cells maintained in culture for 11 days. There were no further changes in these parameters as steroid concentration was increased to 14.43 microM. The activity of NAD-linked sn-glycerol-3-phosphate dehydrogenase (EC 1.1.1.8) was increased by both corticosteroids and was dose dependent between 0.06 and 1.44 microM corticosterone. The difference between glycerol-3-phosphate dehydrogenase activity in cells maintained with hydrocortisone as compared to cells maintained without hydrocortisone increased with days in culture. The protein and DNA contents of dishes maintained with corticosteroid were depressed, demonstrating an inhibitory effect on cellular replication. Glucocorticoids have numerous direct effects on cardiac cell metabolism, and the nature of these effects suggests that secondary responses of the cell to chronic exposure are significant.