Effect of insulin on the decreased beta-adrenergic responses of duodenum and atrium isolated from streptozotocin diabetic rats

Expression and Signaling of β-Adrenoceptor Subtypes in the Diabetic Heart

Diabetes is a chronic, endocrine disorder that effects millions of people worldwide. Cardiovascular complications are the major cause of diabetes-related morbidity and mortality. Cardiac β₁- and β₂-adrenoceptor (AR) stimulation mediates positive inotropy and chronotropy, whereas β₃-AR mediates negative inotropic effect. Changes in β-AR responsiveness are thought to be an important factor that contributes to the diabetic cardiac dysfunction. Diabetes related changes in β-AR expression, signaling, and β-AR mediated cardiac function have been studied by several investigators for many years. In the present review, we have screened PubMed database to obtain relevant articles on this topic. Our search has ended up with wide range of different findings about the effect of diabetes on β-AR mediated changes both in molecular and functional level. Considering these inconsistent findings, the effect of diabetes on cardiac β-AR still remains to be clarified.

Glycoxidative stress and cardiovascular complications in experimentally-induced diabetes: effects of antioxidant treatment

Diabetes mellitus (DM) is a common metabolic disease, representing a serious risk factor for the development of cardiovascular complications, such as coronary heart disease, peripheral arterial disease and hypertension. Oxidative stress (OS), a feature of DM, is defined as an increase in the steady-state levels of reactive oxygen species (ROS) and may occur as a result of increased free radical generation and/or decreased anti-oxidant defense mechanisms. Increasing evidence indicates that hyperglycemia is the initiating cause of the tissue damage in DM, either through repeated acute changes in cellular glucose metabolism, or through long-term accumulation of glycated biomolecules and advanced glycation end products (AGEs). AGEs are formed by the Maillard process, a non-enzymatic reaction between ketone group of the glucose molecule or aldehydes and the amino groups of proteins that contributes to the aging of proteins and to the pathological complications of DM. In the presence of uncontrolled hyperglycemia, the increased formation of AGEs and lipid peroxidation products exacerbate intracellular OS and results in a loss of molecular integrity, disruption in cellular signaling and homeostasis, followed by inflammation and tissue injury such as endothelium dysfunction, arterial stiffening and microvascular complications. In addition to increased AGE production, there is also evidence of multiple pathways elevating ROS generation in DM, including; enhanced glucose auto-oxidation, increased mitochondrial superoxide production, protein kinase C-dependent activation of NADPH oxidase, uncoupled endothelial nitric oxide synthase (eNOS) activity, increased substrate flux through the polyol pathway and stimulation of eicosanoid metabolism. It is, therefore, not surprising that the correction of these variables can result in amelioration of diabetic cardiovascular abnormalities. A linking element between these phenomena is cellular redox imbalance due to glycoxidative stress (GOS). Thus, recent interest has focused on strategies to prevent, reverse or retard GOS in order to modify the natural history of diabetic cardiovascular abnormalities. This review will discuss the links between GOS and diabetes-induced cardiovascular disorders and the effect of antioxidant therapy on altering the development of cardiovascular complications in diabetic animal models.

Increased responses to adenosine in isolated left atria from streptozotocin-diabetic rats: evidence for the involvement of hypothyroidism

Direct and indirect (antiadrenergic) inotropic effects of adenosine in the isolated left atrial preparations from streptozotocin-induced diabetic rats were examined by comparing with those from propylthiouracil-induced hypothyroid rats. Experimental diabetes was induced by a single i.v. injection of streptozotocin (45 mg/kg). Subsets of diabetic rats were treated daily with either insulin (6-8 units/kg) or triiodothyronine (T3; 8-10 micrograms/kg). After 10 weeks, negative inotropic and antiadrenergic effects of adenosine were assessed in the atria from nondiabetic, diabetic, insulin- or T3-treated diabetic and hypothyroid rats. Diabetic rats exhibited a significant increase in negative inotropic and antiadrenergic effects of adenosine, as well as signs of hypothyroidism such as decreased thyroid hormone levels. The same changes in the adenosine effects also were noticed in the hypothyroid rats. In our study, the influence of T3 treatment on the hyperreactivity of diabetic rat atria to adenosine also was examined by comparing with that of insulin treatment. Both insulin and T3 treatments restored the hyperreactivity to adenosine, with the exception of adenosine receptor affinity (pD2 value) calculated for the negative inotropic effect of adenosine. These findings suggest that thyroid hormone deficiency is likely to be involved in the inotropic and antiadrenergic effects of adenosine on the left atria from rats with experimental diabetes for 10 weeks.

Duration-dependent changes in calcium responsiveness in the alloxan-diabetic rat intestine

Disturbances of the gastrointestinal tract that are common in diabetes mellitus seem to be related to intestinal motility. In experimental models of diabetes, decreased calcium sensitivity has been demonstrated in various smooth muscles including those in the gastrointestinal tract. The main purpose of the present study was to examine further the calcium sensitivity in diabetic rat intestine and to understand if changes in the calcium sensitivity occur at an earlier stage of the disease. For this purpose, the effects of potassium and calcium were evaluated on nondepolarized and depolarized duodenum from rats with alloxan diabetes for 1 and 8 weeks and their age-matched controls. To evaluate the calcium sensitivity in rat duodenum, apparent affinity constants (pD2 values) and intrinsic activities (alpha E values) were calculated for every experimental conditions examined in this study. Both values (pD2 and alpha E) for the effects of potassium and calcium on the nondepolarized and depolarized duodenum, respectively, were not changed in 1-week diabetic rats. In contrast, intrinsic activities for the effects of potassium and calcium were found to be significantly decreased (p < 0.001) in the nondepolarized and depolarized duodenum from rats with alloxan diabetes for 8 weeks, whereas apparent affinity constants were not altered in this case. Taking into consideration all these experimental findings, the decreased calcium sensitivity in gastrointestinal tract seems to be closely related to decreased calmodulin levels and may occur at a later stage of diabetes as a linkage to long-term gastrointestinal complications.

The effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastrointestinal tract in rats with non-insulin-dependent diabetes

1. Decreased beta-adrenergic responses have been reported in gastro-intestinal tract of rats with diabetes mellitus. Effects of glyburide and insulin on the decreased beta-adrenergic responsiveness of the gastro-intestinal tract due to non-insulin-dependent diabetes were investigated using duodenum, jejunum and ileum from rats which were injected with alloxan in their neonatal periods. 2. Insulin treatment of non-insulin-dependent diabetic rats for 10 days corrected the decreased beta-adrenergic responses of the isolated duodenum, jejunum and ileum confirming the previous results obtained from insulin-dependent diabetic rats. 3. Glyburide treatment alone for 3 weeks also reversed the changes in the gastro-intestinal beta-adrenergic responses of non-insulin-dependent diabetic rats. Combination of glyburide with insulin, however, did not cause an additive or supra-additive interaction in terms of beta-adrenergic sensitivities of the diabetic tissues. 4. The results obtained in the present study strongly suggested that non-insulin-dependent diabetes may cause a decrease in the number of gastro-intestinal beta-adrenoceptors, while glyburide and insulin treatments correct the changes related to beta-adrenoceptors. The effect of insulin on the beta-adrenergic sensitivity of diabetic rat duodenum, jejunum and ileum may occur via a direct mechanism, whereas glyburide seems to be effective on the beta-adrenergic responses through the increases in the insulin secretion and/or in the number of gastro-intestinal insulin receptors.

Altered alpha-adrenergic responses of vas deferens to noradrenaline and tyramine from rats with short- and long-term alloxan diabetes

1. Functional and morphological abnormalities in vas deferens have been reported by both experimental and clinical studies as a cause of genital function abnormalities in diabetic males. 2. In the present study, contractile effects of noradrenaline and tyramine in isolated vas deferens from rats with short- and long-term alloxan diabetes were investigated by comparing with those from control rats. For this purpose, intrinsic activities (alpha E value) and apparent affinity constants (pD2 value) for contractile effects of noradrenaline and tyramine in the isolated rat vas deferens were calculated in normal rats and rats with short- and long-term alloxan diabetes. 3. Apparent affinity constants for contractile effects of noradrenaline in the isolated rat vas deferens were increased depending on both short- and long-term alloxan diabetes. By contrast, apparent affinity constants for contractile effects of tyramine in the isolated rat vas deferens were attenuated due to both short- and long-term alloxan diabetes. Intrinsic activities for both noradrenaline- and tyramine-induced contractions of rat vas deferens, however, were increased due to short-term diabetes and decreased due to long-term diabetes. 4. Experimental findings obtained in this study indicate that vas deferens preparations from rats with short- and long-term alloxan-induced diabetes exhibit altered alpha-adrenergic responsiveness depending on time elapsed. While short-term alloxan diabetes causes enhanced alpha-adrenergic responses in the rat vas deferens, the long-term diabetes decreases the responses in this tissue.

Vanadate treatment reverses gastrointestinal complications in the streptozotocin-diabetic rats

1. Insulin-like effects of vanadium compounds have been reported in various experimental conditions. Effects of vanadate on the decreased beta-adrenergic responsiveness of the rat duodenum due to streptozotocin diabetes were investigated to determine its influence on diabetic gastro-intestinal complications as well as its effects on the carbohydrate metabolism. 2. Administration of sodium orthovanadate to streptozoticin-diabetic rats in drinking water (0.7 mg/ml) for 4 weeks resulted in an improvement of carbohydrate metabolism noticed by increased serum levels of insulin and decreased blood levels of glucose as reported in previous studies. 3. Vanadate treatment of streptozotocin-diabetic rats also corrected the diabetic changes in the beta-adrenergic responsiveness of the rat duodenum to salbutamol suggesting a beneficial effect on the diabetic complications of rat gastro-intestinal tract. The same treatment with vanadate did not cause any alteration in the beta-adrenergic responsiveness of isolated duodenum from non-diabetic rats. 4. From the findings obtained, it is concluded that vanadate possesses an insulin-like effect on the beta-adrenergic responsiveness of the rat gastro-intestinal tract. Since vanadate treatment did not alter the beta-adrenergic responses of isolated duodenum from non-diabetic rats it seems likely that the insulin-like effect of vanadate is dependent on increased responsiveness of the gastrointestinal tract to circulating insulin.

Increased alpha 1-adrenergic responsiveness of alloxan diabetic rat atria: effects of insulin therapy and thyroidectomy

1. Experimental models of diabetes are increasingly used for the investigation of cardiovascular complications as well as other complications due to diabetes mellitus. However, animal models have been used in relatively few studies on the myocardial alpha 1-adrenergic responses. Hence, this report describes the effect of alloxan-induced diabetes and insulin-treatment on the alpha 1-adrenergic responses of the isolated rat atria. 2. Alloxan-induced diabetes caused an increase in the alpha 1-adrenergic responses assessed in isolated spontaneously beating rat atria. Both pD2 and alpha E values for phenylephrine, an alpha 1-adrenergic agonist were increased in atria from rats with alloxan diabetes. 3. Insulin treatment (4 IU/kg/day, s.c.) for 10 days normalized the changes in diabetic rat atria. pD2 and alpha E values for phenylephrine were slightly lower than those from control rats. Serum levels of thyroid hormones returned to normal following insulin treatment as well. 4. The effect of insulin on the increased alpha 1-adrenergic responses of rat atria due to alloxan diabetes seems to be related to thyroid hormone metabolism, since thyroidectomy also changed the atrial parameters measured. 5. The finding obtained in this study suggest that experimentally-induced diabetes increases alpha 1-adrenergic sensitivity of the rat atria possibly due to an increased receptor affinity, but these changes can be reversed with insulin treatment.