Biomedical subjects

H M Cheng

Publications and source records attributed to H M Cheng.

At least 145 records · Page 8Linked to original sources

PubMed1986

Early vitreous changes in experimental proliferative vitreoretinopathy.

Proton magnetic resonance imaging (MRI) was employed to obtain information on early vitreal changes preceding ophthalmoscopically visible proliferative vitreoretinopathy. Rabbits were injected close to the posterior pole with a suspension of 250 000 cultured homologous fibroblasts. The MRI was carried out using a 1.4-tesla (T) superconducting imager (at a proton frequency of 61.4 MHz). The images were obtained over a span of six days, prior to any detection of proliferative vitreoretinopathy with ophthalmoscopy. As early as two days after injection, an area of increased spin-spin relaxation time (T2) corresponding to the vitreal injection site became visible. The MRI observations paralleled in vitro changes in proton relaxation times (T1 [spin-lattice relaxation time] and T2) after addition of beta-N-acetylglucosaminidase to the vitreous. Our data suggest that hyaluronate disruption due to the activity of fibroblastic enzymes may result in fibroblast dispersion and movement in the vitreous and that MRI can provide early signs of vitreal changes that lead to retinal detachment.

Animals↗

PubMed1986

The effect of high glucose and oxidative stress on lens metabolism, aldose reductase, and senile cataractogenesis.

Diabetic cataractogenesis, a multifactorial process, was examined with nuclear magnetic resonance (NMR). P-31 NMR spectroscopic studies showed substantial alteration of both energy and membrane metabolism in the diabetic lens. Findings from a C-13 NMR spectroscopic determination of the sorbitol pathway flux in lenses incubated in 35.5 mmol/L glucose revealed that (1) one-third of total glucose consumed was channeled through this pathway, and (2) the turnover rate of NADPH to NADP was 3,000%/hr. Furthermore, a competition for NADPH between aldose reductase and glutathione reductase was demonstrated. It is important to note that all metabolic changes in hyperglycemic/diabetic lenses can be prevented by aldose reductase inhibitors, eg, sorbinil.

Adenosine Triphosphate↗

PubMed1986

Proton NMR spectroscopy of glucose consumption by cultured lens epithelial cells.

Proton NMR spectroscopy was performed on media collected from cultured lens epithelial cells of the rabbit eye incubated with Krebs-Ringer's solution containing 5.5mM 13C-glucose (labeled at the C-1 position). Comparing proton resonance intensities of the lactate-methyl group of the C-3 carbon (both 12C and 13C) enabled us to quantify the hexose monophosphate shunt (HMPS) activity. Our results showed that the epithelial cells remained stable in both lactate production and HMPS activity for at least 8 hours. In addition, although tBHP (1mM) resulted in an increase of glucose flow through the HMPS, the rate of lactate production was not affected. In contrast, KCN (2mM) caused a 72% increase of lactate production and a slight (6%) decrease of glucose consumption through the HMPS.

Animals↗

PubMed1986

The effect of oxidation on sorbitol pathway kinetics.

The rapid conversion of glucose to sorbitol by aldose reductase and the consequent hyperosmolarity of the cytoplasm has been shown to be the primary cause of the so-called "sugar" or "osmotic" cataract in many animal lenses. It is not as clear, however, that hyperosmolarity is the principal factor in the etiology of cataracts in human diabetic subjects. In fact, the comparatively low activity of aldose reductase in the human lens as compared with several animal lenses, and the osmotically insignificant levels of sorbitol pathway products (sorbitol and fructose), suggest that hyperosmolarity, per se, may not be as important a factor in human cataract formation as it is in animals. We present evidence that the flux of glucose and sorbitol through the rat lens is markedly reduced by oxidative stress (0.1 mM H2O2). Sorbitol accumulation is reduced by 114%, sorbitol turnover is reduced by 78%, sorbitol production is reduced by 90%, fructose accumulation is reduced by 60%, and fructose turnover is reduced by 76% in the presence of 36 mM glucose. H2O2 does not affect glucose turnover, the glucose rate constant, or the ATP level significantly at 36 mM glucose, but at 5.5 mM glucose, 0.2 mM H2O2 leads to a rapid loss of ATP that can be prevented by 0.04 mM sorbinil, an aldose reductase inhibitor. These results suggest that inhibition of aldose reductase by sorbinil renders rat lenses better able to cope with oxidative stress. In the absence of an aldose reductase inhibitor, elevating ambient glucose may render a lens less able to scavenge oxidants by diverting NADPH into sorbitol production.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphate↗

PubMed1986

The accumulation of myoinositol and rubidium ions in galactose-exposed rat lens.

When rat lens is incubated in 30 mM galactose overnight, the extent of accumulation of rubidium ions (Rb) and myoinositol (MI) are affected, as well as the Na-K ATPase activity. Rb accumulation and Na-K ATPase activity are only slightly affected compared to the dramatic drop in MI accumulation. These changes are completely abolished by sorbinil, which blocks polyol formation, or by rendering the galactose medium hypertonic to offset the osmotic effect of polyol formation. On the other hand, the addition of excess MI to the galactose medium had no effect on correcting these changes. The results obtained are consistent with the polyol-osmotic theory of sugar cataract formation.

Aldehyde Reductase↗

PubMed1985

Sorbitol/fructose metabolism in the lens.

The function of the sorbitol pathway, both as a secondary energy source and as an osmotic counterbalancing force, was examined. Rat lenses were incubated in media containing fructose as the primary exogenous energy source, and 31P nuclear magnetic resonance (NMR) spectra were accumulated. Fructose was found to be a sub-optimal but usable substrate for glycolysis. The utilization of fructose was further confirmed by a 14C fructose tracer study, using high-pressure liquid chromatography. Thus the intralenticular pool of sorbitol + fructose could serve as a secondary energy source during severe hypoglycemia in the diabetic lens. However, fructose is not a physiologically significant alternative to glucose. 13C NMR spectroscopy was employed to determine the kinetics of sorbitol/fructose accumulation in lenses incubated in 35.5 mM 13C1-glucose, and the sorbitol/fructose consumption after the preincubated lenses were transferred to media containing no glucose. Based on these kinetic studies, we concluded that the sorbitol pathway cannot generate sorbitol/fructose fast enough to offset increased osmotic pressure from high glucose levels in the aqueous humor of the diabetic eye. The contribution of osmotic equivalents from sorbitol + fructose, however, cannot be ignored.

Adenosine Triphosphate↗

PubMed1985

Antibody reactivity against trophoblast and trophoblast products.

Sensitive immunoassays have been applied to WHO reference bank sera from fertile and infertile women in order to assess any naturally occurring antibody reactive with isolated human placental trophoblast membranes or two separate trophoblast protein products (hCG and SP1). A very low incidence of antibody reactive with solubilised trophoblast membrane was detected, and no significant antibody to either hCG or SP1 could be detected. Infertile states represented within this serum bank appear unlikely to involve adverse immune reactions to trophoblast.

Antibodies↗

PubMed1985

Study of vitreous liquifaction by NMR spectroscopy and imaging.

The vitreous gel is primarily composed of collagen, hyaluronic acid, and water (98-99%) and can break down into a liquid state devoid of collagen. The liquifaction of vitreous gel that occurs with age and in certain other disease states is believed to be important in the pathogenesis of retinal tears and detachments. The authors report measurements of water proton relaxation times and water proton nuclear magnetic resonance (NMR) imaging to study the process of vitreous liquifaction in extracted vitreous and in the intact bovine eye. A comparison is made between macroscopic viscosity, longitudinal (T1) and transverse (T2) relaxation times obtained on an NMR spectrometer and proton NMR images. Vitreous liquifaction as measured by a decrease in macroscopic viscosity resulted after treatment of vitreous with collagenase and to lesser degree with hyaluronidase. A shortening of relaxation times accompanied the drop in viscosity. An area of brightness or increased proton signal intensity corresponding to a focus of liquifaction was seen by NMR imaging only after injection with collagenase. The authors believe NMR imaging to be a useful new diagnostic modality by which vitreous liquifaction can be studied in the intact eye. The vitreous provides a new model for studying changes in proton relaxation times of protein solutions in biologic systems.

Animals↗

PubMed1984

Nuclear magnetic resonance imaging of the vitreous body.

Imaging with proton nuclear magnetic resonance is a valuable new tool for studying the vitreous body of the eye. It is particularly suited for the detection of vitreal liquefaction and intraocular hemorrhage because of the dependence of the signal on the physical environment of water. Conversely, the vitreous body provides a new model for studying changes in proton relaxation times of protein solutions in biological systems.

Animals↗

PubMed1984

Effect of glutathione deprivation on lens metabolism.

1-Chloro-2,4-dinitrobenzene (CDNB) was used to conjugate glutathione (GSH) through the catalysis of lens glutathione S-transferase without the untoward oxidative damage to the lens mediated by GSH oxidants. A 2 hr treatment of the rat lens with 1 mM CDNB resulted in a nearly total depletion of lens GSH with neither formation of GSSG nor glutathione-protein mixed disulfides. Rubidium uptake was found to decrease linearly with the loss of GSH; nevertheless, ionic imbalance did not commence until more than 30% cation pump activity was lost. Glycolytic rate dropped following CDNB treatment, due probably to a decline in demand for ATP by the deactivated cation pump. 31P-NMR studies confirmed the irreversible loss of ATP. CDNB depletion of GSH resulted in a two-fold increase in 14CO2 production from [14C]-1-glucose. Whereas oxidative stress resulted in a six-fold increase in glucose utilization through the hexose monophosphate shunt (HMPS), CDNB-treated lenses showed no such stimulation. This indicated that the residual GSH following CDNB treatment was insufficient for the activation of the glutathione peroxidase-reductase-HMPS mechanism and raised the possibility that the increased glucose utilization might be due to mechanisms other than the HMPS. These results indicate an intimate correlation between the GSH content and major metabolic functions in the lens.

Adenosine Triphosphate↗

PubMed1984

Altered phosphate metabolism in the intact rabbit lens under high glucose conditions and its prevention by an aldose reductase inhibitor.

Intact paired rabbit lenses were incubated in media containing 5.5 mM and 35.5 mM glucose (both at 290 +/- 3 mOsm) and examined by phosphorus-31 nuclear magnetic resonance spectroscopy. Lenses in 35.5 mM glucose exhibited an altered metabolic steady-state characterized by elevated alpha-glycerophosphate and depressed adenosine triphosphate concentrations. Time course studies revealed that these metabolic changes occurred chiefly during the initial 48 hr of incubation under high glucose conditions. The inclusion of an aldose reductase inhibitor in the medium prevented these changes in lenticular metabolism.

Adenosine Triphosphate↗

PubMed1984

Direct measurement of polyol pathway activity in the ocular lens.

A method to measure the polyol pathway metabolic flux in the intact rabbit lens by 13C nuclear magnetic resonance spectroscopy is described. In the lens exposed to 35.5 mM glucose, the polyol pathway accounts for 1/3 of the total glucose turnover. The high metabolic activity of the pathway suggests a significant alteration in the reduced to oxidized pyridine nucleotide ratio in the lens exposed to high glucose.

Animals↗

PubMed1984

Hexokinase of calf trabecular meshwork.

Although aqueous outflow is most likely a passive, nonenergy-dependent process, alterations in cellular function in the trabecular meshwork presumably are involved in the development of some types of glaucoma. Accordingly, it seems important to define both the normal and abnormal biochemistry of this tissue. The authors have chosen glycolysis as their starting point, concentrating on the regulatory enzymes, hexokinase, and, in a companion paper, phosphofructokinase. Hexokinase activity has been measured in the 100,000 X g supernatant of homogenates prepared from excised calf trabecular meshwork. Treatment of the homogenate with Triton X-100 before centrifuging caused a twofold increase in measurable activity. Electrophoresis on cellulose acetate revealed types I and II isoenzymes. Electrophoresis on starch gel further resolved type 1 into the adult and fetal subtypes. The principal isoenzyme type released into solution by Triton X-100 was type 1. The kinetic behavior of hexokinase was measured by varying the concentrations of glucose at saturating levels of ATP. Kms calculated from these plots were 7.15 X 10(-2) M, 1.78 X 10(-3) M, and 1.19 X 10(-4) M. Apart from the fetal form of type I hexokinase, the isoenzymes from trabecular meshwork resemble those of other ocular tissues and most extraocular tissues. The special role, if any, of the fetal isoenzyme in regulating glycolysis is not known. Possibly, it is a "kinetically adaptable" isoenzyme.

Animals↗

PubMed1983

Response of the lens to oxidative-osmotic stress.

Both aldose reductase and glutathione reductase share a common cofactor, NADPH. Glutathione reductase is preferentially activated due to its higher affinity for the cofactor. Since NADPH is primarily consumed by glutathione reductase, which in conjunction with glutathione peroxidase detoxifies H2O2 present in the aqueous humor, the cataractogenic role of sorbitol-induced osmotic pressure must therefore depend on the availability of NADPH for aldose reductase activity. We examined the response of the lens to an oxidative-osmotic double stress and found that the lens indeed produced 79% less sorbitol and 45% less fructose than a lens subjected to the osmotic stress alone. Morphological studies showed that photo-oxidation damaged the epithelium where the cation pump resided. However, with additional osmotic stress, the swelling of lens fibers in the posterior pole region became more pronounced, and cell nuclei deep in the lens nuclear bow were dislodged to the posterior pole. This could be explained by the slight but significant loss of K+ in the lenses under the double stress. Apparently, the slightly decreased 86Rb uptake (26% loss), caused by photooxidation could not maintain adequate ionic balance even though the stress from accumulation of sorbitol + fructose was sub-maximal. No disturbance in the glycolytic activity or to the 86Rb efflux was found in these lenses, however.

Animals↗

PubMed1983

GSSG-reducing activity in lenses deficient in glucose-6-phosphate dehydrogenase.

Steroids that inhibit glucose-6-phosphate dehydrogenase (G6PD) were used to examine the correlation between the loss of GSSG-reducing activity and G6PD deficiency in the lens. The correlation was found to be nonlinear. In senile cataracts, which had lost 36% of NADPH-generating activity as compared to clear lenses, the estimated loss of GSSG reduction was only 20%. On the other hand, lenses with severe G6PD deficiency (i.e. 93% loss) retained at least 28% GSSG-reducing activity. The declined reducing activity, however, suggested a possible role of G6PD deficiency in cataract formation in young patients.

Adult↗

PubMed1983

Classification of human senile cataractous changes by the American Cooperative Cataract Research Group (CCRG) method. I. Instrumentation and technique.

The American Cooperative Cataract Research Group (CCRG) has adopted a system of classifying human cataractous changes that is based on separate and independent photographic documentation of opacification and nuclear color. This system has been extremely useful to the laboratory scientist who wishes to know the significance of associations between laboratory data and the extent or type of cataractous change. It has been applied to the analysis of nearly 2500 cataracts since 1976. This study presents the details of the instrumentation and technique of this new system and the results of classifying 2231 intracapsularly-extracted cataracts.

Aged↗

PubMed1981

Age and the control of glycolysis in the rat lens.

Previous studies with lens dispersions indicated that the rate-limiting step in glycolysis shifts from hexokinase (HK) in the young lens to phosphofructokinase (PFK) in older lenses. Because the concentrations of the complex controlling factor for these enzymes could not be reproduced reliably in homogenates, the question of age-related control of glycolysis was re-examined in intact lenses. Toward this end, the levels of several metabolites of glucose were measured in fresh and incubated clear lenses. Of the substrates measured per fresh lens, only one changed significantly with age; fructose diphosphate was increased. When lenses were incubated in 2 to 12 mM glucose, the lactate production per lens was not significantly different with age. Together these results suggested that the glycolytic mass of the lens was constant with age. In both young and older lenses, increases in glucose in the medium led to increases in both glucose and glucose-6-phosphate in the lens. The lack of corresponding increase in lactate production suggested that the regulatory step lay downstream from HK, probably at PFK. This finding was corroborated by evidence that the initial acceleration of lactate production by the addition of cyanide (the Pasteur effect) was accompanied by decreases in the substrates of PFK, glucose-6-phosphate and fructose-6-phosphate. A secondary disinhibition of HK, as indicated by decreased lens glucose, became apparent after longer incubation with cyanide. This suggested that after disinhibition of PFK, HK became rate-limiting until the level of glucose-6-phosphate fell enough to allow the disinhibition of the latter enzyme as well. Thus PFK seemed to be the primary regulatory step in aerobic glycolysis in lenses of rats from 1 to 12 months of age.

Aging↗

PubMed1981

Supplementing glucose metabolism in human senile cataracts.

Assay of the activities of hexokinase, phosphofructokinase, and pyruvate kinase showed that the first two declined in aging human lens cortex and all three enzymes retained constant activities in the epithelium throughout life. Moreover, both clear and cataractous aging lenses contained the same enzyme activities. ATP contents in cataracts, however, were lower than in clear lenses; in fact, after incubation at 37.5 degrees C in isotonic (290 to 300 mOsm), glucose-containing media, ATP was rapidly lost from cataracts (but not from clear lenses), suggesting excessive ATP expenditure in cataracts for osmotic balance. Cataracts incubated in media containing either glucose-6-phosphate or fructose-1, 6-diphosphate produced significantly higher ATP than with glucose in the media, indicating that glucose metabolism in human senile cataracts could be supplemented with hexose phosphates. Fructose-1, 6-diphosphate appeared to be more efficient than glucose-6-phosphate in preventing lens swelling during incubation.

Adenosine Triphosphate↗