Milena Sochor

The Effect of an Aldose Reductase Inhibitor (Sorbinil) on the Level of Metabolites in Lenses of Diabetic Rats

This study examined the effect of an aldose reductase inhibitor (Sorbinil, CP 45634,Pfizer, Sandwich, Kent, United Kingdom) on the metabolite profile of the lens during the first week after induction of diabetes with alloxan. The lens content of sorbitol, fructose, glycerol 3-phosphate, and glucose 6-phosphate was, respectively, 0.33 ± 0.03, 0.55 ± 0.05, 0.10± 0.01, and 0.074 ± 0.006 μmol/g (means ± SEM) in the control group rising to 12.2 ± 0.52, 3.20 ± 0.10, 0.76 ± 0.10, and 0.200 ± 0.009 in lenses from alloxan-diabetic rats. Sorbinil treatment (40 mg/kg) decreased the lens content of sorbitol to 0.60 ± 0.06, fructose to 0.85 ± 0.08, and glycerol 3-phosphate to 0.36 ± 0.03 μmol/g; glucose 6-phosphate remained unchanged. Significantly, the lens content of glutathione was decreased to 60% of the normal value in the diabetic group, but was sustained at normal levels with Sorbinil treatment. The ATP content of the lens was not altered by diabetes or Sorbinil treatment at this time interval. Sorbinil has no significant effect on the above metabolites in the normal rat lens. The effect of Sorbinil in restoring normal levels of glutathione and glycerol 3-phosphate may be a potentially important facet of the action of this drug. The interlocking of metabolic pathways by the redox state of NAD+ /NADH and NADP+/NADPH, their derangement in diabetes, and the wider effects of Sorbinil on the network of reactions in the lens are discussed.

Effect of Aldose Reductase Inhibitor (Sorbinil) on Integration of Polyol Pathway, Pentose Phosphate Pathway, and Glycolytic Route in Diabetic Rat Lens

This study examines the effect of an aldose reductase inhibitor (sorbinil) on the flux of specifically labeled glucose through alternative pathways of metabolism in the lens of normal and diabetic rats 1 wk after the induction of diabetes with alloxan. In the diabetic rat lens, there was an apparent increase in the flux of glucose through the pentose phosphate pathway (PPP), as measured by the difference in the yields of 14CO2 from]1-14C[glucose and [6-14C]glucose [C1–C6], this value was 0.087 ± 0.005 and 0.263 ± 0.034 μmol · g lens−1 · h (mean + SE of 6 values) for control and diabetic rats, respectively; sorbinil treatment decreased the values to 0.065 ± 0.008 and 0.171 ± 0.028, respectively. With glucose tritiated on carbon 2 or 3, it has been shown that the flux of glucose through the polyol route is increased, whereas the flux through the glycolytic pathway is decreased in the diabetic rat lens; both are restored toward normal in the sorbinil-treated diabetic group. These results suggest that the dual effects of diabetes in increasing 1) the lens content of glucose and glucose 6-phosphate and 2) the flux of glucose in the polyol pathway will result in an increased utilization of NADPH and production of NADH, factors favoring the flow of glucose through the PPP and restricting the glycolytic route in the diabetic rat lens. The inhibition of aldose reductase by sorbinil tends to normalize the redox state of the nicotinamide nucleotides, reimposing the NADPH limitation on the PPP and increasing the availability of NAD+ for the glycolytic route.

Renal Hypertrophy in Experimental Diabetes: Changes in Pentose Phosphate Pathway Activity

An examination was made of the effect of different periods of experimental diabetes on the activity of the pentose phosphate pathway in rat kidney. A rapid increase in kidney weight, expressed both in absolute terms and in terms of body weight, occurred shortly after the induction of diabetes. The activity of the enzymes of the oxidative segment of the pentose phosphate pathway and the flux of glucose through the pathway were both increased during the first 7 days after induction of diabetes. Thereafter, enzyme activity returned toward control levels, but the increased functional activity of the pathway, as measured using specifically labeled glucose, persisted. In contrast, transketolase was significantly depressed at the time of most rapid kidney growth. A positive correlation was found between the rate of kidney growth and the change in activity of glucose-6-phosphate dehydrogenase and a negative correlation with changes in transketolase activity. The possible roles of the oxidative and nonoxidative segments of the pentose phosphate pathway in the kidney in early diabetes-induced renal hypertrophy are discussed.

Regulation of pathways of glucose metabolism in kidney. The effect of experimental diabetes on the activity of the pentose phosphate pathway and the glucuronate-xylulose pathway

Abstract Measurements have been made of the activities of enzymes of the pentose phosphate pathway, the glucuronate-xylulose pathway, hexokinase and phosphofructokinase in kidney of diabetic and normal rats. The activities of these enzymes keep pace with kidney growth, remaining constant per gram tissue but showing a marked increase on the basis of total activity per 100 g body wt. The formation of 14 CO 2 from [1- 14 C]glucose and [6- 14 C]glucose by kidney slices from diabetic rats was decreased to approximately half the control value; evidence was obtained for an equivalent dilution of the glucose 6-phosphate pool. Correction of the 14 CO 2 yields for the change in specific activity of glucose 6-phosphate yielded values consistent with the enzyme profile. Calculations from specific yields of 14 CO 2 provided evidence for an increased flux of glucose via the pentose phosphate pathway in the kidney in diabetes. The results are discussed in relation to kidney hypertrophy in diabetes and the requirement for ribose 5-phosphate and NADPH for biosynthetic reactions and in relation to the thickening of the basement membrane in diabetes. These results are in accord with the concept of glucose overutilization by non-insulin-requiring tissues.

Regulation of pathways of glucose metabolism in kidney. Specific linking of pentose phosphate pathway activity with kidney growth in experimental diabetes and unilateral nephrectomy.

The pentose phosphate pathway operates at an elevated level in rat kidney following induction of diabetes and in the compensatory hypertrophy following unilateral nephrectomy in control and alloxan‐diabetic rats, as shown by the yields of 14Co2 from [1‐14C]glucose, [6‐14C]glucose and 3H2O yields from [2‐3H]glucose. The elevated flux through the pentose phosphate pathway is correlated with the increased RNA content and weight of the kidney. The direct utilization of NADPH for reductive synthetic reactions and the potential for indirect utilization via the sorbitol route and the linked transhydrogenase reactions of the glucuronate‐xylulose pathway, for NADH and ATP generation, are also discussed.

Glucose overutilization in diabetes: Evidence from studies on the changes in hexokinase, the pentose phosphate pathway and glucuronate-xylulose pathway in rat kidney cortex in diabetes

Abstract Kidney cortex from alloxan-diabetic rats has an elevated activity of hexokinase, enzymes of the pentose phosphate pathway and of certain enzymes of the glucuronate-xylulose route relative to age-matched control rats. These changes are highly significant when expressed as total kidney units/100 g body weight, a parameter relating biochemical activity to the functional requirement of the whole animal. These changes are discussed in relation to the hypothesis of ‘glucose over-utilization’ in diabetes in tissues not requiring insulin for glucose uptake [R.G. Spiro (1976) Diabetologia 12 , 1–14].

Effect of experimental diabetes on glycolytic intermediates and regulation of phosphofructokinase in rat lens

Abstract The pattern of glycolytic intermediates in the lens of alloxan-diabetic rats was indicative of regulation at phosphofructokinase. The changes in metabolites influencing phosphofructokinase activity in the diabetic, relative to the normal, rat lens were: glucose 6-phosphate, 182%; fructose 6-phosphate, 107%; fructose diphosphate, 57%. There was also a marked decrease in phosphoenolpyruvate, pyruvate, lactate and ATP but no significant change in other triose phosphates or cyclic AMP. The resuts are considered in relation to the early changes in [Ca2+] known to occur in lens in diabetes and to the coordinating effect of fructose diphosphate on flux through the glycolytic route.

The effect of a somatostatin analogue (SMS 201-995, Sandostatin) on the concentration of phosphoribosyl pyrophosphate and the activity of the pentose phosphate pathway in the early renal hypertrophy of experimental diabetes in the rat.

The effect of a long-acting somatostatin analogue on the acute renal hypertrophy following induction of experimental diabetes in the rat has been studied. The kidney weight increase occurring at 2 and 7 days after alloxan injection was significantly lower in the diabetic group receiving somatostatin. Similarly, the previously reported increase in glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and 6-phosphogluconate dehydrogenase (EC 1.1.1.44) found in the kidney at 2 and 7 days of diabetes was less marked in the group receiving SMS 201-995. The fall in renal phosphoribosyl pyrophosphate associated with early diabetic renal hypertrophy (7) was also lessened by administration of SMS 201-995. No effects of the drug were found in the normal rat on the same regimen of treatment. These observations indicate involvement of glucagon and/or growth hormone in the initiation of kidney growth in diabetes.

Effect of experimental diabetes on the activity of hexokinase in rat lens: An example of glucose overutilization in diabetes

Abstract Hexokinase activity of lens increases two-fold in alloxan-diabetic rats four weeks after induction of diabetes; both type I and type II isoenzymes of hexokinase are increased. This increase in lens hexokinase is considered in relation to alternative routes of glucose metabolism.

Regulation of Glucose Metabolism in Livers and Kidneys of NOD Mice

Measurements were made of the levels of metabolic intermediates and activities of enzymes of the glycolytic route, pentose phosphate pathway, and polyol pathway in livers and kidneys of NOD mice. A 34% decrease in UDP-glucose, a 40% decrease in glucose-6-phosphate (G6P) and fructose-6-phosphate, and a 75% decrease in fructose-2,6-bisphosphate (F2,6P) were found in the livers of NOD mice. The fall in the level of F2,6P (the important regulator of glycolysis) is accompanied by a 20% reduction in the activity of phosphofructokinase. These changes are in agreement with previously reported liver depletion of glycogen and reduced synthesis of proteins and nucleic acids in the diabetic state. In the kidney, the increase in hexokinase activity is consistent with increased levels of G6P and glycogen content of kidney in diabetes. The decreased level of phosphoribosyl pyrophosphate was reported to be a regulator of kidney growth in the initial period of diabetes but can still be found in NOD mice 6 wk after development of hyperglycemia. The reported changes are similar to those seen in alloxan- or streptozocin-induced diabetic animals, but certain changes are more marked in NOD mice, especially those directed to increase nucleic acid and protein synthesis in the diabetic kidney.