Streptozotocin-induced diabetes and hormone sensitivity of adenylate cyclase in rat myocardial sarcolemma, aorta and liver

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.

Functional changes in adenylyl cyclases and associated decreases in relaxation responses in mesenteric arteries from diabetic rats

To assess the functional change in adenylyl cyclases (AC) associated with the diabetic state, we investigated AC-mediated relaxations and cAMP production in mesenteric arteries from rats with streptozotocin (STZ)-induced diabetes. The relaxations induced by the water-soluble forskolin (FSK) analog NKH477, which is a putative AC5 activator, but not by the beta-adrenoceptor agonist isoproterenol (Iso) and the AC activator FSK, were reduced in intact diabetic mesenteric artery. In diabetic rats, however, Iso-, FSK-, and NKH477-induced relaxations were attenuated in the presence of inhibitors of nitric oxide synthase and cyclooxygenase. To exclude the influence of phosphodiesterase (PDE), we also examined the relaxations induced by several AC activators in the presence of 3-isobutyl-1-methylxanthine (IBMX; a PDE inhibitor). Under these conditions, the relaxation induced by Iso was greatly impaired in STZ-diabetic rats. This Iso-induced relaxation was significantly attenuated by pretreatment with SQ-22536, an AC inhibitor, in mesenteric rings from age-matched controls but not in those from STZ-diabetic rats. Under the same conditions, the relaxations induced by FSK or NKH477 were impaired in STZ-diabetic rats. Neither FSK- nor A-23187 (a Ca2+ ionophore)-induced cAMP production was significantly different between diabetics and controls. However, cAMP production induced by Iso or NKH477 was significantly impaired in diabetic mesenteric arteries. Expression of mRNAs and proteins for AC5/6 was lower in diabetic mesenteric arteries than in controls. These results suggest that AC-mediated relaxation is impaired in the STZ-diabetic rat mesenteric artery, perhaps reflecting a reduction in AC5/6 activity.

Reversal of defective G-proteins and adenylyl cyclase/cAMP signal transduction in diabetic rats by vanadyl sulphate therapy

Vanadium salts exhibit a wide variety of insulinomimetic effects. In the present studies, we have examined the modulation of G-protein levels and adenylyl cyclase activity in the liver of streptozotocin-induced chronic diabetic rats (STZD) by vanadyl sulfate treatment and compared it with that of insulin. The basal enzyme activity, as well as the stimulatory effects of guanine nucleotides, glucagon, N-Ethylcarboxamideadenosine (NECA), isoproterenol, forskolin and sodium fluoride (NaF) on adenylyl cyclase were significantly increased in STZ-D rat liver as compared to control. In addition, the levels of stimulatory (Gs alpha) as well as inhibitory (Gi alpha-2 and Gi alpha-3) as determined by immunoblotting techniques were also significantly higher in the STZ-D rat liver, however, the inhibitory effects of oxotremorine and low concentrations of GTP gamma S on adenylyl cyclase were not different in the two groups. Vanadyl sulfate and insulin treatments restored the augmented basal enzyme activity, the stimulations exerted by stimulatory inputs on adenylyl cyclase and the G-protein levels to various degrees, however, vanadyl sulfate was more effective than insulin. In addition, unlike vanadyl sulfate, insulin was unable to improve the stimulation exerted by glucagon and isoproterenol on adenylyl cyclase activity in STZD rats. These results suggest that vanadyl sulfate mimics the effects of insulin to restore the defective levels of G-proteins and adenylyl cyclase activity. From these results it may be suggested that one of the mechanisms by which vanadyl sulfate improves the glucose homeostasis in STZ-D rats may be through its ability to modulate the levels of G-proteins and adenylyl cyclase signal transduction system.

Hormone-stimulated glucose production from glycogen in hepatocytes from streptozotocin diabetic rats

The contribution of hormone-stimulated glycogenolysis to hepatic glucose production was studied in hepatocytes from streptozotocin diabetic rats. To this end, the activation of glycogen phosphorylase by glucagon, vasopressin, and the alpha 1-adrenergic agonist phenylephrine was compared in hepatocytes from normal and diabetic rats and related to glycogen content, glucose production, and microsomal glucose-6-phosphatase activity. Streptozotocin-induced diabetes reduced the glycogen content and the amount of total (a + b) phosphorylase in hepatocytes proportionally to the severity of the disease. In cells from severely diabetic rats (group 1), the responsiveness of activation of phosphorylase to the hormones was reduced by about half, consistent with a 45% reduction in total phosphorylase. In addition, the sensitivity of phosphorylase activation to all hormones investigated was decreased by about 1 order of magnitude or more in cells of this group. In hepatocytes from rats with milder diabetes (group 2), maximal phosphorylase activation reached an intermediate value between that of the control group and of group 1. In response to all hormones investigated, group 2 diabetic rat hepatocytes produced less glucose than control rat liver cells, while in group 1 there was no increase in glucose production at all, presumably because glycogen concentration was too low. However, in group 2 diabetic rat hepatocytes, glucagon-stimulated glucose production, unlike phosphorylase activation, did not show decrease sensitivity, presumably because glucose-6-phosphatase activity is increased by diabetes. Our results thus indicate that hormone-stimulated liver glycogenolysis is unlikely to contribute to enhanced glucose production in insulin-deficient diabetes, despite increased glucose-6-phosphatase activity.

Defective response to cAMP-dependent protein kinase in non-insulin-dependent diabetic heart

Hearts isolated from 1-yr-old non-insulin-dependent diabetic rats exhibited reduced responsiveness to the beta-adrenergic agonist isoproterenol. Over a concentration range of 3 x 10(-9) to 10(-7) M, isoproterenol-mediated stimulation in the rate of left ventricular pressure decline, a measure of myocardial relaxation, and the rate of left ventricular pressure rise, a measure of myocardial contractility, were significantly depressed in the diabetic hearts. To clarify the basis for this defect, individual steps involved in the actions of the beta-adrenergic agonists were examined. Dihydroalprenolol binding assays revealed that neither beta-adrenergic receptor number nor binding affinity was affected by the diabetic condition. Also unaffected by diabetes was isoproterenol-mediated stimulation of adenylate cyclase activity, myocyte accumulation of adenosine 3',5'-cyclic monophosphate (cAMP), or the increase in cAMP-dependent protein kinase activity ratio. However, it was found that both in the presence and absence of cAMP-dependent protein kinase, activity of the sarcolemmal calcium transporter was significantly depressed in the diabetic heart. Also attenuated was protein kinase-induced enhancement of sarcoplasmic reticular calcium transport. The likelihood that these abnormalities contribute to alterations in calcium homeostasis and myocardial contractile function is discussed.