Proposed metabolic dysfunctions in diabetic microthromboses and microangiopathy

Prostanoid production in umbilical vessels and its relation to glucose tolerance and umbilical artery flow resistance

OBJECTIVE

To study prostanoid synthesis in umbilical vessels relative to maternal glucose tolerance and umbilical artery blood flow resistance.

STUDY DESIGN

Umbilical artery pulsatility index was determined by Doppler velocimetry in 21 women with diabetes or impaired glucose tolerance and 10 healthy women. Segments from the umbilical artery and vein were incubated and prostacyclin (PGI2) and thromboxane (TxA2) metabolites determined. Statistical analyses with the Mann-Whitney U test, Kruskal-Wallis test, Wilcoxon signed-ranks matched-pairs test, contingency table analysis, Fisher's exact test, and simple linear regression analysis were used and a two-tailed P value of < 0.05 considered statistically significant.

RESULTS

No significant difference in PGI2 or TxA2 production was found in umbilical vessels between the women with diabetes/impaired glucose tolerance and controls, but the PGI2/TxA2 ratio in the vein was significantly lower in the diabetes/impaired glucose tolerance group. The umbilical artery pulsatility index was positively correlated to the PGI2/TxA2 ratio in cord vessel segments and to cord plasma TxA2 concentration. The cord plasma TxA2 concentration was significantly higher in cases with a high umbilical artery pulsatility index. The prostanoid production was not correlated to maternal HbA1c or cord plasma C-peptide concentrations.

CONCLUSIONS

In association with diabetes, an increased 'peroxide vascular tone' and an enhanced 'endoperoxide shift' between platelets and vascular endothelium may explain the unexpected positive correlation found between the umbilical artery pulsatility index and the vascular PGI2/TxA2 synthesis ratio.

Is hyperglycemia risky enough to justify the increased risk of hypoglycemia linked with tight diabetes control?

There is an ongoing debate about the possible disadvantages of human insulin use with respect to a possibly lower awareness of hypoglycemia than is associated with animal insulin usage. Participants in this debate have not, however, discussed a major contributory factor to this life-threatening acute complication of diabetes, the pressure on patients to achieve normal levels of blood glucose. This pressure stems from the view that hyperglycemia is the major causative factor in the long-term diabetic complications. However, the evidence that supranormal levels of tissue and plasma glucose contribute to the diabetic tissue damage is not as strong as the arguments on behalf of this position. Indeed, elevated glycemia may be no more than a crude index of other, unknown metabolic derangements which may be causative agents in diabetes-associated tissue damage. Intensive efforts to "normalize" glycemia lack experimental and clinical justification, distract attention from other possible mechanisms, and may impose an unnecessary risk on the insulin-dependent diabetic population since intensive "normalization" of glycemia lowers hypoglycemia awareness, and thus increases risk of hypoglycemia, irrespective of the type of insulin used.

Protein glycation and oxidative stress in diabetes mellitus and ageing

Hyperglycemia is increasingly regarded as the cause of the diabetic complications, in particular via the ability of glucose to glycate proteins and generate Maillard browning products which cross-link proteins and render them brown and fluorescent in vitro. Similar changes occur in vivo to long-lived proteins in diabetes mellitus as well as in ageing. The evidence supporting this route of glucose toxicity is discussed in the context of the ability of glucose to oxidize in vitro (catalyzed by trace amounts of transition metal) generating hydrogen peroxide, highly reactive oxidants, and protein-reactive ketoaldehyde compounds. It is suggested that protein browning in vivo may not result from the reactions of glucose with protein but from the transition metal-catalyzed reactions of other small autoxidisable substrates, such as ascorbate, with protein. Overall, studies of glycation and protein browning suggest a critical role for oxidative processes perhaps involving decompartmentalized transition metals and a variety of low molecular weight reducing agents in diabetes mellitus and ageing.

Is glucose the sole source of tissue browning in diabetes mellitus?

Reactions between glucose and protein are held to be responsible for the protein 'browning' reactions which occur in diabetes mellitus. In vitro, however, the formation of such novel protein fluorophores is dependent upon the metal-catalysed oxidation of the monosaccharide (glucose 'autoxidation'). Since other small oxidisable molecules are capable of 'browning' proteins via similar metal-catalysed oxidative mechanisms we suggest that protein modification in diabetes may not be restricted to reactions with monosaccharides but may also include other small autoxidisable molecules.

Effects of purified fish oil on platelet lipids and function in diabetic women

Purified fish oil (MaxEpa, 10 g/day) treatment for six weeks increased consistently the content of eicosapentaenoic (20:5, n-3), docosapentaenoic (22:5, n-3) and docosahexaenoic acid (22:6, n-3) and decreased that of arachidonic acid (20:4, n-6) and other n-6 polyunsaturated C-20 fatty acids (PUFA) of platelets both in insulin dependent diabetic (IDDM) (n = 13) and healthy women (n = 7), but it had no effect on the number and aggregation of platelets or on plasma beta-thromboglobulin. Serum TxB2, produced by diabetic platelets was reduced, but the urine 6-keto PGF1 alpha excretion, believed to reflect prostacyclin (PGI2) production was normal in the diabetics. During the MaxEpa treatment the response of thromboxane B2 (TxB2) release to ADP was decreased in platelet-rich plasma in the healthy subjects. However, in diabetics the fish oil treatment resulted in increased TxB2 formation from exogenous arachidonic acid. The results demonstrate the dependence of platelets fatty acid composition on dietary sources and suggest that at least in the diabetic platelets the diminished arachidonic acid content could be compensated by an activation of enzymes of thromboxane B2 pathway.

Platelet aggregation and coagulation factors in insulin dependent diabetics with and without microangiopathy

Abnormalities of platelet aggregation and coagulation have been reported in insulin dependent diabetes mellitus (IDDM), although there is controversy concerning their relationship to microangiopathy. We have studied platelet function and haemostasis in 55 patients with IDDM, 23 without, 14 with mild (background retinopathy) and 18 with severe (proliferative retinopathy, or background retinopathy plus proteinuria) complications. Studies were done on 2 occasions 8 weeks apart and the results compared with 28 control subjects. There was evidence of increased in vivo platelet aggregation in the diabetic group v controls shown by raised values of beta-thromboglobulin (61 +/- 42, mean +/- SD, v 18 +/- 14 micrograms/ml, p less than 0.001), platelet factor 4 (62 +/- 76 v 14 +/- 11 micrograms/ml, p less than 0.01), and platelet micro-aggregates (20 +/- 16 v 12 +/- 11%, p less than 0.01). There was no significant difference in fibrinogen and fibrinopeptide A levels, nor in 'in vitro' tests of platelet aggregation between the groups. Dilute whole blood clot lysis time was increased in the diabetic group v controls (6.4 +/- 2.6 v 4.8 +/- 0.5 hours, respectively, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Improvement of diabetic control by continuous subcutaneous insulin infusion therapy changes fatty acid composition of serum lipids and erythrocytes in type 1 (insulin-dependent) diabetes

The influence of improved diabetic control on the fatty composition of serum lipids, erythrocytes and platelets was investigated in 24 patients with Type 1 (insulin-dependent) diabetes treated for 6 months with either continuous subcutaneous insulin infusion (n = 14) or conventional insulin therapy (n = 10). The groups were matched for age, sex, body mass index, serum lipids, duration of diabetes, glycosylated haemoglobin and insulin dose. Glycaemic control improved, and the contents of dihomogammalinolenic acid and arachidonic acid but not linoleic acid rose significantly (p less than 0.05), in serum lipids of patients treated with continuous infusion. No changes were observed in the group treated with insulin injections. Both in serum and erythrocytes the n-6 polyunsaturated fatty acid ratios rose consistently in the patients, with improvement of control regardless of the mode of treatment. Furthermore, the change of HbA1 was negatively correlated with that of arachidonic acid in erythrocytes. No changes were found in the platelet fatty acid compositions. The findings suggest that improved diabetic control enhances the conversion of linoleic acid to arachidonic acid, probably by activating enzymes needed for chain elongation and desaturation.

Plasma eicosanoid levels in rats with nonketotic diabetes mellitus: effect of severity

Nonketotic diabetes mellitus (DM) is associated with increased platelet production of thromboxane (TX) A2 and decreased endothelial production of prostacyclin (prostaglandin [PG]I2), but measurements of stable derivatives of these substances in the circulation have yielded discordant results. We studied the relationship between the severity of nonketotic DM and the plasma levels of 13,14-dihydro-15-keto-PGE2, 6-keto-PGF1 alpha, and TXB2 (stable derivatives of PGE2, PGI2, and TXA2, respectively) in rats, using three commonly employed doses of streptozotocin (40, 50, and 60 mg/kg body weight) to induce nonketotic DM of varying severity. Small differences in the severity of DM were associated with considerable differences in the plasma levels of 13,14-dihydro-15-keto-PGE2 and 6-keto-PGF1 alpha but not TXB2. Each eicosanoid responded differently to variations in the severity of DM. The plasma 13,14-dihydro-15-keto-PGE2 level was significantly lower than normal in the rats given 40 mg/kg streptozotocin, was unchanged from normal in the rats given 50 mg/kg and was significantly higher than normal in the rats given 60 mg/kg. The plasma 6-keto-PGF1 alpha level was significantly increased in rats given 40 mg/kg and 60 mg/kg, but was unchanged in those given 50 mg/kg. The plasma TXB2 level was not significantly different from normal in any one of the three groups of rats with nonketotic DM. The effect of severity on the plasma levels of the PGE2 and PGI2 derivatives is unexplained, but may reflect the origin of these derivatives from diverse organs and tissues, and the differing effects of abnormal metabolic factors (eg, fatty acids, glucose, insulin, pH) on the synthesis of these derivatives.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific thromboxane synthetase inhibition and albumin excretion rate in insulin-dependent diabetes

Albumin excretion rates (AER) were measured in 30 insulin-dependent diabetics during a 16-week double-blind, randomised, placebo-controlled study of the specific thromboxane synthetase inhibitor UK-38,485.6 of 15 subjects in the active group had microalbuminuria (defined as mean pretreatment AER 20-150 micrograms/min); in these patients AER fell from 32 +/- 3 micrograms/min to 11 +/- 1 micrograms/min at 8 weeks and 9 +/- 1 micrograms/min at 16 weeks. The AER rose again (to 29 +/- 8 micrograms/min) within 12 weeks of stopping the drug. There was no significant change in the 10 patients with microalbuminuria who received placebo. There was a strong correlation between change from baseline values and the baseline values themselves in the active, but not in the placebo group, and the change from baseline differed significantly between the two groups. There was no change in glycosylated haemoglobin or mean blood glucose levels during the study. In a separate study UK-38,485 caused significant suppression of thromboxane B2 synthesis in diabetic and non-diabetic subjects.

Insulin secretory capacity, beta-thromboglobulin and blood viscosity in long standing, non-insulin dependent diabetics with and without microangiopathy

A comparison was made of insulin secretion in response to i.v. glucose between noninsulin dependent diabetics with and without severe microangiopathy. During i.v. glucose tolerance testing those with complications had significantly lower serum insulin concentration (at 40 and 60 min 15.2 +/- 2.9 and 11.7 +/- 2.5 mU-1 respectively) than those without (26.1 +/- 4.7 and 24.3 +/- 3.6 mU-1, p less than 0.01). The differences were also significant when insulin increment above basal was determined. All subjects had raised plasma beta-thromboglobulin concentration (indicating increased tendency to platelet aggregation), but with no significant difference between the two groups. There was also no difference in fasting glucose, kg values, HbAl or blood viscosity. We conclude that non-insulin dependent diabetics with severe microangiopathy have significantly reduced insulin secretory capacity in response to i.v. glucose when compared with those without.

Blockade of chlorpropamide-alcohol flushing by indomethacin suggests an association between prostaglandins and diabetic vascular complications

Chlorpropamide/alcohol flushing (CPAF), found in many patients with non-insulin-dependent diabetes (NIDD), can be blocked by indomethacin in most patients who are free of vascular complications but not in those with such complications. Since indomethacin is a prostaglandin inhibitor this finding suggests that prostaglandins may be involved in the aetiology of vascular diseases in NIDD. All 6 pairs of identical twins with CPAF, of whom 2 pairs were disocrdant for diabetes, were concordant for indomethacin blocking, which suggests that the block has a genetic basis. The difference in the response of CPAF to indomethacin in diabetic patients with and without vascular complications is probably the first indication of a metabolic difference between these two types of patient.