Streptozotocin-Induced diabetes mellitus is associated with increased pancreatic tissue levels of noradrenaline and adrenaline in the rat

Serotonin transporter imaging agent as a probe for β-cells of pancreas

OBJECTIVE

Diabetes mellitus (DM) is one of the major diseases in the world. Nuclear medicine imaging may be able to detect functional status of pancreatic β cells in vivo, which might elucidate the pathological mechanisms of diabetes and develop individualized treatment plans. In this study, we evaluated the ability of [125I]ADAM, a serotonin transporter (SERT) imaging agent, as a probe for detecting pancreatic β-cell mass (BCM).

METHODS

In vitro cell studies were evaluated in INS-1 cells (rat islet β cell line). Biodistribution studies were performed in male normal Sprague-Dawley rats and alloxan-induced type 1 diabetes mellitus (T1DM) rats. Distribution and expression of SERT protein in pancreas of rats were also measured by immunofluorescence staining and Western blot.

RESULTS

In vitro cell studies showed that the concentration of [125I]ADAM associated with the INS-1 cells was increased gradually with incubation time, and the SERT specific inhibitor, escitalopram, exhibited the inhibitory effect on this interaction. Biodistribution studies also showed that the uptake of [125I]ADAM in the pancreas of normal rats was decreased in the presence of escitalopram. However, in the T1DM rat model with a significant β cells reduction, the uptake of pancreas was increased when compared with the control. Through immunofluorescence staining and Western blot, it was found that both the endocrine and exocrine cells of the normal pancreas expressed SERT protein, and the level of SERT protein in the exocrine cells was higher than islets. In the diabetic state, the expression of SERT in the exocrine cells was further increased.

CONCLUSIONS

The SERT imaging agent, [125I]ADAM, at the present form will not be suitable for imaging β cells, specifically because there were extraordinarily high non-specific signals contributing from the exocrine cells of pancreas. In addition, we noticed that the level of SERT expression was abnormally elevated in the diabetic state, which might provide an unexpected target for studying the pathological mechanisms of diabetes.

Knockdown the P2X3 receptor in the stellate ganglia of rats relieved the diabetic cardiac autonomic neuropathy

Diabetic cardiac autonomic neuropathy (DCAN) is a common and serious complication of diabetes mellitus (DM), is manifested by nerve fiber injury in the sympathetic and parasympathetic nerve of the autonomic nervous system, and causes hypertension, cardiac arrhythmias, silent myocardial infarction, and sudden death. Our previous study observed that P2X3 receptor in superior cervical ganglia in rat was associated with sympathetic neuropathy caused by myocardial ischemia. However, whether the P2X3 receptor is involved in the diabetic cardiac autonomic neuropathy and the underlying mechanisms remain unclear. The aim of this research was explored the effect of P2X3 short hairpin RNA (shRNA) on information transmission of sympathetic nerve induced by DCAN. Sprague-Dawley (SD) male rats were randomly divided into four groups: Control, DM, DM treated with P2X3 shRNA and DM treated with scramble shRNA. Blood pressure, heart rate and heart rate variability were measured in each group. The expression of P2X3 in stellate ganglion (SG) was detected by immunohistochemistry, western blotting and QPCR. Results showed that P2X3 shRNA alleviated blood pressure and heart rate, improved heart rate variability, decreased the up-regulated expression levels of P2X3, interleukin-1beta and tumor necrosis factor alpha in stellate ganglion (SG) of diabetic rats. P2X3 shRNA also reduced the incremental concentration of serum epinephrine and the phosphorylation level of extracellular regulated protein kinases1/2 in diabetic rats. These results indicated that P2X3 shRNA could decrease sympathetic activity via inhibiting P2X3 receptor in the SG to alleviate DCAN.

Differential activations of PKC/PKA related to microvasculopathy in diabetic GK rats

Microvasculopathy is the most serious and predictable threat to the health of diabetic patients, which often results in end-stage renal disease, blindness, and limb amputations. Up to the present, the underlying mechanisms have remained elusive. Here, it was found that the differential activations of PKC/PKA were involved in diabetic microvasculopathy in diabetic GK rats. By real-time PCR, Western blot, immunohistochemistry, and enzyme activity assay, upregulation of PKC was prominent in kidney but was not significant in liver and brain. The expression and activity of PKA were lowered in kidney but comparable in brain and liver during diabetic nephropathy. Furthermore, the generation of reactive oxygen species, production of nitric oxide, and expression of inducible nitric oxide synthase induced by advanced glycation end products were inhibited by PKCβ inhibitor LY-333531 or a PKA agonist in rat glomerular microvascular endothelial cells. Finally, albuminuria was significantly lowered by a PKA agonist and boosted by a PKA antagonist. It suggested that the differential activations of PKC/PKA related to microvasculopathy in diabetes and that activation of PKA may protect the diabetic microvasculature.

Is elevated noradrenaline an aetiological factor in a number of diseases?

1 Here I put forth the hypothesis that noradrenaline (NA), which is a signalling molecule in the brain and sympathetic nervous system (SNS), is an aetiological factor in a number of diseases. 2 In a previous paper (Fitzgerald, Int. J. Cancer, 124, 2009, 257), I examined evidence that elevated NA is a factor in various types of cancer. Here I extend the argument to several other diseases, including diabetes mellitus, open-angle glaucoma, osteoarthritis and rheumatoid arthritis and asthma. 3 The principal hypothesis is that, largely as a result of genetics, elevated noradrenergic tone in the SNS predisposes a large number of individuals to a broad range of diseases. 4 For each of the above five diseases, I briefly examine the following four lines of evidence to assess the hypothesis: i) whether pharmacological studies in rodents that manipulate NA levels or receptors affect these diseases; ii) whether pharmacological manipulation of NA in humans affects these diseases; iii) whether bipolar disorder, excessive body weight, and hypertension, which may all three involve elevated NA, tend to be comorbid with these diseases and iv) whether psychological stressors tend to cause or exacerbate these conditions, since psychological stress is associated with increased release of NA. 5 The four lines of evidence tend to support the hypothesis.

Diabetes induces changes of catecholamines in primary mesangial cells

Diabetes mellitus is a frequent cause of kidney function damage with diabetic nephropathy being predominantly related to glomerular dysfunction. Diabetes is capable of interfering with distinct hormonal systems, as well as catecholamine metabolism. Since mesangial cells, the major constituent of renal glomerulus, constitute a potential site for catecholamine production, the present study was carried out to investigate alterations in catecholamine metabolism in cultured mesangial cells from the nonobese diabetic mouse, a well-established model for type I diabetes. We evaluated mesangial cells from normoglycemic and hyperglycemic nonobese diabetic mice, as well as cells from normoglycemic Swiss mice as control. Mesangial cells from normoglycemic mice presented similar profiles concerning all determinations. However, cells isolated from hyperglycemic animals presented increased dopamine and norepinephrine production/secretion. Among the studied mechanisms, we observed an upregulation of tyrosine hydroxylase expression accompanied by increased tetrahydrobiopterin consumption, the tyrosine hydroxylase enzymatic cofactor. However, this increase in synthetic pathways was followed by decreased monoamine oxidase activity, which corresponds to the major metabolic pathway of catecholamines in mesangial cells. In addition, whole kidney homogenates from diabetic animals also presented increased dopamine and norepinephrine levels when compared to normoglycemic animals. Thus, our results suggest that diabetes alters catecholamine production by interfering with both synthesizing and degrading enzymes, suggesting a possible role of catecholamine in the pathogenesis of acute and chronic renal complications of diabetes mellitus.

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.

Early changes in insulin secretion and action induced by high-fat diet are related to a decreased sympathetic tone

To evaluate the relationship between the development of obesity, nervous system activity, and insulin secretion and action, we tested the effect of a 2-mo high-fat diet in rats (HF rats) on glucose tolerance, glucose-induced insulin secretion (GIIS), and glucose turnover rate compared with chow-fed rats (C rats). Moreover, we measured pancreatic and hepatic norepinephrine (NE) turnover, as assessment of sympathetic tone, and performed hypothalamic microdialysis to quantify extracellular NE turnover. Baseline plasma triglyceride, free fatty acid, insulin, and glucose concentrations were similar in both groups. After 2 days of diet, GIIS was elevated more in HF than in C rats, whereas plasma glucose time course was similar. There was a significant increase in basal pancreatic NE level of HF rats, and a twofold decrease in the fractional turnover constant was observed, indicating a change in sympathetic tone. In ventromedian hypothalamus of HF rats, the decrease in NE extracellular concentration after a glucose challenge was lower compared with C rats, suggesting changes in overall activity. After 7 days, insulin hypersecretion persisted, and glucose intolerance appeared. Later (2 mo), there was no longer insulin hypersecretion, whereas glucose intolerance worsened. At all times, HF rats also displayed hepatic insulin resistance. On day 2 of HF diet, GIIS returned to normal after treatment with oxymetazoline, an alpha(2A)-adrenoreceptor agonist, thus suggesting the involvement of a low sympathetic tone in insulin hypersecretion in response to glucose in HF rats. In conclusion, the HF diet rapidly results in an increased GIIS, at least in part related to a decreased sympathetic tone, which can be the first step of a cascade of events leading to impaired glucose homeostasis.

Diabetes-related changes in cAMP-dependent protein kinase activity and decrease in relaxation response in rat mesenteric artery

Using superior mesenteric artery rings isolated from age-matched controls and streptozotocin (STZ)-induced diabetic rats, we recently demonstrated that EDHF-type relaxation is impaired in STZ-induced diabetic rats, possibly due to a reduced action of cAMP via increased phosphodiesterase (PDE) activity (Matsumoto T, Kobayashi T, and Kamata K. Am J Physiol Heart Circ Physiol 285: H283-H291, 2003). Here, we investigated the activity and expression of cAMP-dependent protein kinase (PKA), an enzyme that is produced by a pleiotropic and plays key roles in the transduction of many external signals through the cAMP second messenger pathway and in cAMP-mediated vasorelaxation. The relaxation induced by cilostamide, a selective PDE3 inhibitor, was significantly weaker in superior mesenteric artery rings from STZ-induced diabetic rats than in those from age-matched controls. The relaxation responses to 8-bromo-cAMP (8Br-cAMP) and N6,O2-dibutyryl-adenosine-cAMP (db-cAMP), a cell-permeant cAMP analog, were also impaired in the STZ diabetic group. PKA activity in the db-cAMP-treated mesenteric artery was significantly lower in the STZ diabetic group. The expression levels of the mRNA and protein for PKA catalytic subunit Cat-alpha were significantly decreased in the STZ diabetic group, but those for PKA regulatory subunit isoform RII-beta were increased. We conclude that the abnormal vascular relaxation responsiveness seen in STZ-induced diabetic rats may be attributable not only to increased PDE activity but also to decreased PKA activity. Possibly, the decreased PKA activity may result from an imbalance between PKA catalytic and regulatory subunit expressions.