The in-vitro effects of insulin and the effects of acute fasting on cardiac beta-adrenoceptor responses in the short-term streptozotocin-diabetic rat

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.

Cross-Talk Between Insulin Signaling and G Protein-Coupled Receptors

Diabetes is a major risk factor for the development of heart failure. One of the hallmarks of diabetes is insulin resistance associated with hyperinsulinemia. The literature shows that insulin and adrenergic signaling is intimately linked to each other; however, whether and how insulin may modulate cardiac adrenergic signaling and cardiac function remains unknown. Notably, recent studies have revealed that insulin receptor and β2 adrenergic receptor (β2AR) forms a membrane complex in animal hearts, bringing together the direct contact between 2 receptor signaling systems, and forming an integrated and dynamic network. Moreover, insulin can drive cardiac adrenergic desensitization via protein kinase A and G protein-receptor kinases phosphorylation of the β2AR, which compromises adrenergic regulation of cardiac contractile function. In this review, we will explore the current state of knowledge linking insulin and G protein-coupled receptor signaling, especially β-adrenergic receptor signaling in the heart, with emphasis on molecular insights regarding its role in diabetic cardiomyopathy.

Profiling of cardiac β-adrenoceptor subtypes in the cardiac left ventricle of rats with metabolic syndrome: Comparison with streptozotocin-induced diabetic rats

Little is known about metabolic syndrome (MetS)-associated cardiomyopathy, especially in relation to the role and contribution of beta-adrenoceptor (β-AR) subtypes. Therefore, we examined the roles of β-AR subtypes in the cardiac function of rats with MetS (MetS group) and compared it with that of rats with streptozotocin (STZ)-induced diabetes (STZ group). Compared with the normal control rats, the protein levels of cardiac β1- and β2-AR in the MetS group were significantly decreased and with no changes in their mRNA levels, whereas the protein levels of β3-AR were similar to those of the controls. However, as shown previously, the protein levels of cardiac β1- and β2-AR in the STZ group were decreased, whereas the β3-AR levels were significantly increased by comparison with the controls. Additionally, the mRNA levels of β2- and β3-AR were increased, but β1-AR mRNA was decreased in the STZ group. Furthermore, left ventricular developed pressure responses to β3-AR agonist BRL37344 were increased in the STZ group but not in the MetS group, whereas for both groups, the responses to noradrenaline were not different from those of the controls. However, the response to stimulation with high concentrations of fenoterol was depressed in the MetS group, compared with the controls, but not in the STZ group. Consequently, our data suggest that the contribution of the β-AR system to cardiac dysfunction in the rats with MetS is not the same as that in the STZ group, although they have similar cardiac dysfunction with similar ultrastructural changes to the myocardium.

Antioxidants but not doxycycline treatments restore depressed beta-adrenergic responses of the heart in diabetic rats

Reactive oxygen species (ROS) play important roles in the development of diabetic cardiomyopathy. Matrix metalloproteinases (MMPs) can get activated by ROS and contribute to loss of myocardial contractile function in oxidative stress injury. Previously we have shown that either a MMP-2 inhibitor doxycycline or an antioxidant selenium treatment in vivo prevented diabetes-induced cardiac dysfunction significantly. In addition, there is an evidence for impaired cardiac responsiveness to beta-adrenoceptor (beta AR) stimulation in experimental animals with diabetes. The exact nature of linkage between the functional depression in cardiac responses to catecholamines and the variations in uncoupling of beta AR in diabetes has not been clearly defined. Therefore, we aimed to evaluate the effect of in vivo administration of doxycycline on beta AR responses of isolated hearts from diabetic rats and compare these data with two well-known antioxidants; sodium selenate and (n-3) fatty acid-treated diabetic rats. We examined the changes in the basal cardiac function in response to the beta AR stimulation, adenylate cyclase activity, and beta AR affinity to its agonist, isoproterenol. These results showed that antioxidant treatment of diabetic rats could protect the hearts against diabetes-induced depression in beta AR responses, significantly while doxycycline did not have any significant beneficial action on these parameters. As a summary, present data, in part, demonstrate that antioxidants and MMP inhibitors could both regulate MMP function but may also utilize different mechanisms of action in cardiomyocytes, particularly related with beta AR signaling pathway.

The role of gender differences in beta-adrenergic receptor responsiveness of diabetic rat heart

Since the mechanisms responsible for gender differences in cardiac contractile function have not been fully elucidated, we focused to determine the effect of gender difference on beta-adrenergic receptors (beta-ARs) signal transduction in ventricular cardiomyocytes from insulin-dependent diabetic (streptozotocin-induced) rats. Dose-response curves of left ventricular developed pressure (LVDP) to isoproterenol (ISO) in females showed that there was only a approximately 30% decrease in the maximum response without a significant shift in EC50 in diabetic females. On the other hand, diabetes induced a clear rightward shift in the potency (5-10 folds) without a significant change in the maximum response in the males. In order to further determine of the underlying mechanism for this difference, we measured cAMP production and obtained dose-response curves with ISO stimulation in isolated cardiomyocytes. In diabetic females, there was no obvious change in the cAMP dose-response curve. On the other hand, there was a significant decrease in the maximum response without any apparent change in the potency of diabetic males. Our findings indicate that male and female rats are affected differently by diabetes in terms of LVDP responses to beta-ARs stimulation. Also, the difference between their beta-ARs induced cAMP responses may underlie this disparity.

Altered vasoactivity in the early diabetic eye: measured in the isolated perfused rat eye

The effect of 4 weeks streptozotocin-induced diabetes on ocular vascular resistance responses to noradrenalin (NA), adrenalin (A), phenylephrine (PHE), isoproterenol (ISOP), prostaglandin F2 alpha (PGF2 alpha). 5-hydroxytryptamine (5-HT) and angiotensin II (ANG II), was determined using a newly-developed, isolated, arterially-perfused rat eye preparation, by comparing responses from control and diabetic eyes. After extensive preliminary experiments to establish optimum parameters, the ophthalmic artery of enucleated control and diabetic rat eyes was cannulated and the retinal and uveal vasculature perfused at a constant flow with Na(+)-Krebs solution after streptozotocin-induced diabetes had been established for 4 weeks. The eyes were maintained in an environment-controlled organ bath. Perfusion pressure was monitored as increasing log M concentrations of agonists were added to the perfusate. Total ocular resistance could be calculated from knowledge of flow and pressure. In control eyes, NA, A, PHE, PGF2 alpha, and 5-HT all produced dose-dependent increases in total vascular resistance, with the following order of potency: NA = A > 5-HT > PHE = PGF2 alpha at 10(-4) M. The ocular circulation was not sensitive to isoproterenol and angiotensin II. In diabetic eyes responses to NA, A, PGF2 alpha and 5-HT were altered. Diabetic responses to NA and A had lower thresholds with larger resistance increases at low concentrations. However, the rate of increase in resistance with concentration was more gradual in diabetic eyes so that at 10(-4) M control responses were larger. Diabetic resistance responses to PGF2 alpha had the same threshold as in control eyes, but were greater in magnitude with an earlier peak at 10(-4) M. In contrast diabetic resistance responses to 5-HT were reduced, peaked at a lower resistance at 10(-4) M, but had the same threshold as those in the control eye. Basal vascular resistances in control: 3.14 +/- 0.32 mmHg min microliter-1 (n = 28), and diabetic eyes: 3.44 +/- 0.19, mmHg min microliter-1 (n = 36), were not significantly different. Vasoactivity in the early diabetic eye is disturbed with the effective balance between different agonists altered in favour of catecholamines at physiological concentrations. This may be related to the early changes in blood flow and oxygen distribution already reported in the rat eye, as well as changes to autonomic function. The isolated perfused rat eye is a valuable technique for investigating such vascular reactivity in animal models of retinal disease.