Effects of long-acting somatostatin analogues on redox systems in rat lens in experimental diabetes

Abstract

The effects of long-acting somatostatin analogues, angiopeptin (AGP) and Sandostatin (SMS), on the early decline in the lens content of glutathione (GSH), ATP and NADPH and increase in sorbitol were studied in STZ diabetic rats, and comparison was made with the effect of insulin. Three factors prompted this study: (i) the known increase in IGF-1 in ocular tissue in diabetes and antagonistic effect of somatostatins, (ii) the known effect of IGF-1 in increasing lens aldose reductase and (iii) the lack of effect of somatostatins on diabetic hyperglycaemia, the latter enabling a differentiation to be made between effects of hyperglycaemia per se and site(s) of IGF-1/somatostatins. All four metabolites studied showed a significant restoration towards the normal control level after 7 days of treatment with AGP and SMS, and AGP was more effective on levels of GSH and ATP. A significant correlation was found between GSH and ATP across all groups at 7 days treatment. The redox state changes in diabetes include both NADP+/NADPH and NAD+/NADH in the conversion of glucose to sorbitol and via sorbitol dehydrogenase to fructose with a linked decrease in ATP formation via NAD+/NADH regulation of the glycolytic pathway. The interlinked network of change includes the requirement for ATP in the synthesis of GSH. The present study points to possible loci of action of somatostatins in improving metabolic parameters in the diabetic rat lens via effects on aldose reductase and/or glucose transport at GLUT 3.

Keywords: ATP; angiopeptin; experimental diabetes (STZ); glutathione; insulin; rat lens; redox systems; sandostatin.

Figure 1

Effect of long-acting somatostatin analogues (Sandostatin, SMS and angiopeptin, AGP) and insulin on the sorbitol (a), glutathione (b), ATP (c) and NADPH (d) content of diabetic rat lens. The columns represent the mean values and the vertical bars the SEM of not <7 separate values for each of the following treatments: D, diabetic injected with normal saline s.c. twice daily; D+SMS, diabetic treated with 100 μg Sandostatin s.c. twice daily; D+AGP, diabetic treated with 100 μg angiopeptin s.c. twice daily; D+INS, diabetic treated with 1–3 U of long-acting Ultralente insulin s.c.. Total period of all treatments was 7 days; for further details of treatment, see Materials and methods section. The significance of differences between control and diabetic groups with or without treatment with SMS, AGP or INS is shown at the foot of each column. (a) Sorbitol: (i) CvD, P < 0.001; CvD+SMS, P < 0.001; CvD+AGP, P < 0.001; CvD+INS, P < 0.001. (ii) DvD+SMS, P < 0.01; DvD+AGP, P < 0.001; DvD+INS, P < 0.001. (iii) D+SMSv D+AGP, P-NS. (b) Glutathione: (i) CvD, P < 0.001; CvD+SMS, P < 0.01; CvD+AGP, P < 0.05; CvD+INS, P-NS. (ii) DvD+SMS, P < 0.01; DvD+AGP, P < 0.001; DvD+INS, P < 0.001. (iii) D+SMSv D+AGP, P < 0.001. (c) ATP: (i) CvD, P < 0.01; CvD+SMS, P < 0.01; CvD+AGP, P-NS; CvD+INS, P-NS. (ii) DvD+SMS, P-NS; Dv D+AGP, P < 0.001; DvD+INS, P < 0.01. (iii) D+SMSv D+AGP, P < 0.001. (d) NADPH: (i) CvD, P < 0.02; CvD+SMS, P-NS; CvD+AGP, P-NS; CvD+INS, P-NS. (ii) DvD+SMS, P < 0.02; Dv D+AGP, P-NS; DvD+INS, P < 0.02. (iii) D+SMSv D+AGP, P-NS.

Figure 2

Changes in metabolites and interlocking pathways in STZ diabetic rat lens. Pathways indicated by heavy lines are increased in STZ diabetes; those with broken lines are decreased in STZ diabetes. Metabolites increased in uncontrolled diabetes are highlighted in red, and the linked disturbances in lens glutathione, ATP, NAD and NADPH are highlighted in yellow. Potential sites of action of long-acting somatostatin analogues on aldose reductase and GLUT 3 are shown in green. Key enzymes and their kinetic parameters are as follows: (A) Glutathione reductase (E.C.1.6.4.2) Km NADPH 8–21 μM, Km GSSG 26–65 μM; Carlberg & Mankervik ; Tandogan & Ulusu . (B) Aldose reductase (E.C. 1.1.1.21) NADPH 4–9 μM, Km glucose 90–120 mM; von Wartburg & Wermuth ; Halder & Crabbe . (C) Sorbitol dehydrogenase (E.C.1.1.1.4) Km NAD 27 μM, sorbitol 10 μM; Jedziniak et al. ; Lessing & McGuiness . (D) Glyceraldehyde dehydrogenase (EC 1.2.1.59) Km NAD 40–58 μM; Jedziniak et al. . (E) Glutamate cysteine ligase (EC 2.3.2.15) Km ATP 400 μM, Ki GSH 2.3 mM; Raftos et al. . (F) Glutathione loss from lens in diabetes; Lu et al. ; Milton et al. . (G) Glucose transport in lens in diabetes – GLUT3 increased; Merriman-Smith et al. . (H) Potential sites of action of SMS & AGP; Civil et al. ; Merriman-Smith et al. ; Lu .