Heme and hemoproteins in streptozotocin-diabetic female rats

Heme oxygenase-1 prevents superoxide anion-associated endothelial cell sloughing in diabetic rats

Heme oxygenase-1 (HO-1) represents a key defense mechanism against oxidative injury. Hyperglycemia has been linked to increased oxidative stress, leading to endothelial dysfunction, delayed cell replication, and enhanced apoptosis. The effect of streptozotocin (STZ)-induced diabetes on HO activity, HO-1 promoter activity, superoxide anion (O*-2, and the number of circulating endothelial cells was measured. The expression of HO-1/HO-2 protein was unchanged, but HO activity was decreased in aortas of diabetic rats compared with control (p < 0.05). High glucose decreased HO-1 promoter activity (p < 0.05). Hyperglycemia increased O*-2 and this increase was augmented with HO-1 inhibition and diminished with HO-1 upregulation (p < 0.05). Circulating endothelial cells were significantly higher in diabetic rats and were decreased or increased with administration of the HO-1 inducer (CoPP) or inhibitor (SnMP), respectively (p<0.05). In conclusion, HO-1 upregulation in diabetic rats brings about an increase in serum bilirubin, a reduction in O*-2 production, and a decrease in endothelial cell sloughing.

Insulin-dependent attenuation in alpha 2-adrenoreceptor-mediated nociception in experimental diabetes

Diabetes mellitus is associated with abnormalities in central noradrenergic dynamics, a system that appears to be involved in the regulation of nociception in both humans and experimental animals. To this end, we investigated the responsiveness of nociceptive threshold to the actions of clonidine (an alpha 2-adrenoreceptor agonist) and yohimbine (an alpha 2-adrenoreceptor antagonist) during diabetes. The induction of diabetes was achieved by the administration of streptozotocin (STZ) (55 mg/kg, intravenously). Nociceptive threshold, as indicated by the tail-flick latency of the tail immersion test, was progressively elevated as a function of the duration of diabetes. Systemic administration of clonidine and yohimbine respectively produced dose-dependent analgesic and hyperalgesic effects in control animals. Both of these phenomena were impaired in chronically diabetic animals. In contrast, insulin-treated diabetics displayed supersensitivity to clonidine's antinociceptive effect, especially at low doses. Acute hyperglycemia did not interfere with the alpha 2-agonist-mediated elevation in nociceptive threshold. Attenuation in clonidine antinociceptive effect was also observed following its intrathecal administration to diabetic animals. Overall, these data suggest that the impaired responsiveness of diabetic rats might be due to a central alpha 2-adrenoreceptor desensitization and/or biochemical defect in the postreceptor events.

Spontaneously diabetic biobreeding rats and impairment of bile acid-independent bile flow and increased biliary bilirubin, calcium and lipid secretion

Chemically induced diabetes has been reported to induce profound changes in bile formation, but possible toxic effects of the streptozotocin or alloxan used cannot be excluded totally. This study was undertaken to evaluate biliary function in spontaneously diabetic female biobreeding rats with a diabetes duration of 2 wk and compare them with nondiabetic littermates. Diabetic animals evidenced glycosuria, hyperglycemia and hypoinsulinemia. Biliary concentration and secretion of bile acids, cholesterol and phospholipids were significantly increased, with no enhancement in the lithogenic index of bile. Bile flow and the biliary secretion of sodium, potassium, chloride and bicarbonate were significantly reduced despite the increased bile acid secretion. The cholestatic condition was confirmed by an increased serum concentration of bile acids and a higher activity in serum of the alkaline phosphatase liver isoenzyme. Biliary calcium concentration increased without any change in its serum concentration. A linear relationship was observed between biliary calcium and bile acid secretion. Serum concentration of unconjugated and of conjugated bilirubin was increased 1.6-fold and 8-fold, respectively, with a 1.5-fold enhanced biliary secretion of bilirubins despite the cholestasis; this points to an enhanced bilirubin production. An increased proportion of conjugated bilirubin was found in serum together with an enhanced bilirubin diconjugate/monoconjugate ratio in bile. A higher UDP-glucuronyltransferase activity and a delayed transit of bilirubin could account for these effects. Administration of insulin to diabetic animals tended to reverse the above reported changes. The spontaneously diabetic biobreeding rat thus represents a model of bile acid-independent cholestasis with enhanced biliary bile acid and calcium secretion and with presumably an enhanced bilirubin production.

In vivo evidence that insulin does not inhibit hepatic tryptophan pyrrolase activity in rats

Previous reports have indicated that insulin administration triggers an early increase in plasma tryptophan (TRP) levels in fasted rats. Then, the present study was undertaken to investigate the putative role of liver tryptophan pyrrolase (TPO) in this short-term effect of insulin. In 24 hr fasted rats, doses of insulin that triggered an increase in plasma TRP levels (i.e., 2-3 I.U./kg, 1 hr) did not alter either holoenzyme or total enzyme activity. In another series of experiments, the administration of insulin (2 I.U./kg) to 24 hr fasted rats promoted biphasic time effects on plasma TRP levels and liver TPO activity. Thus, insulin initially triggered a rise in plasma TRP (without any change in liver TPO activity) and then increased liver TPO activity whilst plasma TRP returned toward control levels. In addition, hypercorticosteronemia was evidenced throughout the first phasis. Lastly, the influence of insulin administration (2 I.U./kg) on fasting-induced TPO induction was analysed. Whereas fasting increased liver TPO activity in a time-dependent manner, insulin administration (2 I.U./kg, 30 min) did not modify either plasma TRP or liver TPO activity. The data reported herein bring evidence that the effect of insulin administration on circulating TRP is not mediated by an inhibition of hepatic TPO.

Studies on induction of delta-aminolevulinic acid synthase, ferrochelatase, cytochrome P-450 and cyclic AMP by phenformin. Chlorpropamide, allylisopropylacetamide and lead in hepatocytes from normal and experimental diabetic rats

The present work demonstrates that phenformin exerted an inducing effect on delta-aminolevulinic acid synthase (ALA-S) and ferrochelatase activities and on cytochrome P-450 content in isolated hepatocytes from rats with experimental diabetes. Similar results were obtained with respect to ALA-S activity and cytochrome P-450 content when chlorpropamide was used. The inducing effect exerted by allylisopropylacetamide (AIA) on ALA-S and ferrochelatase activities in diabetic hepatic cells was markedly greater than that observed in normal hepatocytes. This stimulatory response was not enhanced by adding dibutyryl cyclic AMP (cAMP). When phenformin was added to isolated rat hepatocytes of normal rats, induction of ALA-S and ferrochelatase activities and cytochrome P-450 content was observed only in the presence of added dibutyryl cAMP. Addition of chlorpropamide to this in vitro system did not exert an inducing effect on the same enzymes even in the presence of dibutyryl cAMP. The present results add more experimental evidence about the lability of the heme pathway of diabetic hepatocytes.

Free radicals and diabetes

The role of active oxygen species in diabetes is discussed in this review. Type I diabetes is caused by destruction of the pancreatic beta cells responsible for producing insulin. In humans, the diabetogenic process appears to be caused by immune destruction of the beta cells; part of this process is apparently mediated by white cell production of active oxygen species. Diabetes can be produced in animals by the drugs alloxan and streptozotocin; the mechanism of action of these two drugs is different, but both result in the production of active oxygen species. Scavengers of oxygen radicals are effective in preventing diabetes in these animal models. Not only are oxygen radicals involved in the cause of diabetes, they also appear to play a role in some of the complications seen in long-term treatment of diabetes. Changes in antioxidants in the diabetic state and their consequences are discussed.

Diabetes-induced changes in monoamine concentrations of rat hypothalamic nuclei

Streptozotocin-induced diabetes produced marked alterations of monoamine concentrations in several hypothalamic nuclei of male and female rats. Norepinephrine (NE) concentrations were significantly elevated in the median eminence (ME), supraoptic nucleus (SON) and periventricular nucleus (PEVN) in both sexes of diabetic rats. NE concentrations in the suprachiasmatic nucleus (SCN) and ventromedial nucleus (VMN) of male and female diabetic animals remained unaltered. Serotonin (5-HT) concentrations were increased in PEVN of male and female diabetic rats. No significant changes in hypothalamic dopamine (DA) concentrations were observed. Insulin treatment reversed the diabetes-related changes in monoamine concentrations in most of the nuclei. The significance of these biochemical changes relative to the endocrine and behavioral abnormalities in diabetes is discussed.

Aberration of heme and hemoprotein in aged female rats

The magnitude and duration of drug action is determined partially by the activity of the drug metabolizing enzyme systems in the liver. The pharmacological effectiveness of many drugs is altered during the aging process. In this study, the regulation of heme metabolism and hemoprotein content was examined in livers of aged female rats. The activities of hexobarbital hydroxylase and aniline hydroxylase, indicators of mono-oxygenase function, were decreased in aged rats by 31% and 24%, respectively, as compared to values in young rats. This was accompanied by a proportional decrease in the level of cytochrome P-450 (26%). Additionally, the activity of delta-aminolevulinic acid synthetase (ALA-S), the rate-limiting enzyme in heme synthesis, and the microsomal concentration of heme were also decreased by 33% and 26%, respectively, in these animals. In contrast, the basal activity of microsomal heme oxygenase (MHO), the rate-limiting enzyme in heme degradation, and the percent heme saturation of tryptophan pyrrolase (TPO), a sensitive indicator of changes in the availability of heme in the "regulatory" heme pool, were increased by (87%) and (31%), respectively, in the aged rats. The serum concentration of bilirubin, an indicator of erythrocyte breakdown and/or liver function was likewise increased in these animals. In view of these findings, we suggest that the high activity of MHO and the low level of ALA-S may be a significant causative factor for the decreased microsomal concentration of heme, cytochrome-P-450 and its dependent monooxygenase activities in senescent female rats.

Studies on regulatory mechanisms of heme biosynthesis in hepatocytes from experimental-diabetic rats

Isolated hepatocytes from rats with experimental diabetes exhibit increased content of cytochrome P-450 and cyclic AMP and normal activities of the regulatory enzymes delta-aminolevulinic acid synthase and ferrochelatase. The inducing effect exerted by phenobarbital on cytochrome P-450, delta-aminolevulinic acid synthase and ferrochelatase biosynthesis and cyclic AMP content in diabetic hepatic cells is markedly greater than that observed in normal hepatocytes. This stimulatory response is neither enhanced by added dibutyryl cyclic AMP nor repressed by glucose. The present results suggest that the heme pathway of diabetic hepatocytes is more susceptible to porphyrinogenic factors.

Maintenance and induction of cytochrome P-450 in cultured rat hepatocytes

Maintenance of microsomal cytochrome P-450 content by cultured rat hepatocytes has proven an elusive goal. It is reported here that exogenous heme maintains cytochrome P-450 content of cultured rat hepatocytes at high levels during the first 72 h of incubation. The maintenance studies have been expanded to demonstrate the in vitro induction of cytochrome P-450 by phenobarbital treatment. The induction of P-450 in vitro by phenobarbital required the trace element, selenium, in the presence of exogenous heme. The present findings suggest that selenium, and other trace elements, may have an essential role in the formation of holocytochrome P-450 in vitro.

Activation of liver tryptophan oxygenase by hydrocortisone, hematin and tryptophan in streptozotocin-diabetic rats

This study compared changes in liver tryptophan oxygenase (TPO) activity in response to hydrocortisone, hematin and tryptophan administration to non-diabetic and diabetic (streptozotocin) rats. Hydrocortisone caused similar increases in apoenzyme (inactive), holoenzyme (heme-saturated) and total (holoenzyme + apoenzyme) TPO activities in non-diabetic and diabetic rats. The ability of hematin to increase total TPO activity was significantly less in diabetic rats. The largest differences between diabetic and non-diabetic rats were found with tryptophan which increased total TPO and holoenzyme activities 300% and 650% respectively in non-diabetic rats. However, tryptophan increased both apoenzyme (unchanged in non-diabetic rats) and holoenzyme activities by 300% in diabetic rats. These results indicate that in the diabetic state, the TPO-heme conjugation process is impaired, especially substrate mediated TPO-heme saturation.

Modification of age-induced changes in heme and hemoproteins by testosterone in male rats

Age-related changes in heme and hemoproteins, as well as the effect of testosterone treatment on these modifications were examined in male Sprague-Dawley rats. The activity of delta-aminolevulinic acid synthase (ALA-S) and the microsomal concentration of heme in aged rats were decreased by 37% and 33%, respectively, as compared to young values. In contrast, a marked increase in the activity of microsomal heme oxygenase (MHO) was seen in these animals. In aged rats, the level of cytochrome P-450 was decreased by 37%, as compared to young values. Furthermore, the activities of benzo[a]pyrene hydroxylase and aniline hydroxylase were decreased in proportion to the microsomal content of cytocyrome P-450. Steroid delta 4-hydrogenase, an index of endogenous substrate metabolism, exhibited no changes in activity during the aging process. The level of various hemoproteins such as cytochrome b5 and tryptophan pyrrolase in aged animals remained unaltered despite the decreased hepatic concentration of heme. It is worth noting that testosterone treatment of aged castrated rats restored the level of heme and cytochrome P-450 and the altered enzymatic activities of ALA-S and MHO to the "young" condition. In view of these findings, it is concluded that the events which lead to the low level of heme and cytochrome P-450 and its dependent mixed function oxidase activity during the senescent period could be due to increased rates of MHO and diminished ALA-S activities in these animals.