The nucleotide hydrolysis is altered in blood serum of streptozotocin-induced diabetic rats

Purinergic P2 receptors: Involvement and therapeutic implications in diabetes-related glomerular injury

The purinergic activation of P2 receptors initiates a powerful and rapid signaling cascade that contributes to the regulation of an array of physiological and pathophysiological processes in many organs, including the kidney. P2 receptors are broadly distributed in both epithelial and vascular renal cells. Disturbances of purinergic signaling can lead to impairments in renal function. A growing body of evidence indicates changes in P2 receptor expression and nucleotide metabolism in chronic renal injury and inflammatory diseases. Increasing attention has focused on purinergic P2X7 receptors, which are not normally expressed in healthy kidney tissue but are highly expressed at sites of tissue damage and inflammation. Under hyperglycemic conditions, several mechanisms that are linked to purinergic signaling and involve nucleotide release and degradation are disrupted, resulting in the accumulation of adenosine 5'-triphosphate in the bloodstream in diabetes. Dysfunction of the purinergic system might be associated with serious vascular complications in diabetes, including diabetic nephropathy. This review summarizes our current knowledge of the role of P2 receptors in diabetes-related glomerular injury and its implications for new therapeutics for diabetic nephropathy.

Purinergic signaling as a new mechanism underlying physical exercise benefits: a narrative review

In the last years, it has become evident that both acute and chronic physical exercise trigger responses/adaptations in the purinergic signaling and these adaptations can be considered one important mechanism related to the exercise benefits for health improvement. Purinergic system is composed of enzymes (ectonucleotidases), receptors (P1 and P2 families), and molecules (ATP, ADP, adenosine) that are able to activate these receptors. These components are widely distributed in almost all cell types, and they respond/act in a specific manner depending on the exercise types and/or intensities as well as the cell type (organ/tissue analyzed). For example, while acute intense exercise can be associated with tissue damage, inflammation, and platelet aggregation, chronic exercise exerts anti-inflammatory and anti-aggregant effects, promoting health and/or treating diseases. All of these effects are dependent on the purinergic signaling. Thus, this review was designed to cover the aspects related to the relationship between physical exercise and purinergic signaling, with emphasis on the modulation of ectonucleotidases and receptors. Here, we discuss the impact of different exercise protocols as well as the differences between acute and chronic effects of exercise on the extracellular signaling exerted by purinergic system components. We also reinforce the concept that purinergic signaling must be understood/considered as a mechanism by which exercise exerts its effects.

P2X7 receptor and klotho expressions in diabetic nephropathy progression

Diabetes mellitus is characterized by increased levels of reactive oxygen species (ROS), leading to high levels of adenosine triphosphate (ATP) and the activation of purinergic receptors (P2X₇), which results in cell death. Klotho was recently described as a modulator of oxidative stress and as having anti-apoptotic properties, among others. However, the roles of P2X₇ and klotho in the progression of diabetic nephropathy are still unclear. In this context, the aim of the present study was to characterize P2X₇ and klotho in several stages of diabetes in rats. Diabetes was induced in Wistar rats by streptozotocin, while the control group rats received the drug vehicle. From the 1st to 8th weeks after the diabetes induction, the animals were placed in metabolic cages on the 1st day of each week for 24 h to analyze metabolic parameters and for the urine collection. Then, blood samples and the kidneys were collected for biochemical analysis, including Western blotting and qPCR for P2X₇ and klotho. Diabetic rats presented a progressive loss of renal function, with reduced nitric oxide and increased lipid peroxidation. The P2X₇ and klotho expressions were similar up to the 4th week; then, P2X₇ expression increased in diabetes mellitus (DM), but klotho expression presented an opposite behavior, until the 8th week. Our data show an inverse correlation between P2X₇ and klotho expressions through the development of DM, which suggests that the management of these molecules could be useful for controlling the progression of this disease and diabetic nephropathy.

Exercise partially reverses the inhibitory effect of caffeine on liver gluconeogenesis in type 1 diabetic rats with hypoglycemia

The purpose was to determine the possible effects of exercise and/or caffeine on hypoglycemia and liver gluconeogenesis in diabetic rats. These were divided into four subgroups: (a) intraperitoneal insulin only, (b) exercise bout before insulin, (c) caffeine after insulin, and (d) exercise bout before and caffeine after insulin. The marked glycemic drop 45 min after insulin (0 min = 229.00, 45 min = 75.75) was considerably reduced (p < 0.05) by caffeine or exercise (45 min: exercise = 127.00, caffeine = 104.78). However, this systemic effect was lost (p > 0.05) when they were combined (45 min: exercise + caffeine = 65.44) (Mean, in mg·dL^-1). Caffeine alone strongly inhibited liver glucose production from 2 mM lactate 45 min after insulin (without caffeine = 3.05, with caffeine = 0.27; p < 0.05), while exercise + caffeine partially re-established the liver gluconeogenic capacity (exercise + caffeine = 1.61; p < 0.05 relative to the other groups) (Mean, in μmol·g^-1). The improved hypoglycemia with caffeine or exercise cannot be explained by their actions on liver gluconeogenesis. As their beneficial effect disappeared when they were combined, such association in diabetic patients should be avoided during the period of hyperinsulinemia due to the risk of severe hypoglycemia.

Altered extracellular ATP, ADP, and AMP hydrolysis in blood serum of sedentary individuals after an acute, aerobic, moderate exercise session

Nucleotidases participate in the regulation of physiological and pathological events, such as inflammation and coagulation. Exercise promotes distinct adaptations, and can influence purinergic signaling. In the present study, we investigated soluble nucleotidase activities in the blood serum of sedentary young male adults at pre- and post-acute moderate aerobic exercise. In addition, we evaluated how this kind of exercise could influence adenine nucleotide concentrations in the blood serum. Sedentary individuals were submitted to moderate aerobic exercise on a treadmill; blood samples were collected pre- and post-exercise, and serum was separated for analysis. Results showed increases in ATP, ADP, and AMP hydrolysis post-exercise, compared to pre-exercise values. The ecto-nucleotide pyrophosphatase/phosphodiesterase was also evaluated, showing an increased activity post-exercise, compared to pre-exercise. Purine levels were analyzed by HPLC in the blood serum, pre- and post-exercise. Decreased levels of ATP and ADP were found post-exercise, in contrast with pre-exercise values. Conversely, post-exercise levels of adenosine and inosine increased compared to pre-exercise levels. Our results indicate an influence of acute exercise on ATP metabolism, modifying enzymatic behavior to promote a protective biological environment.

Upregulation of inducible NO synthase by exogenous adenosine in vascular smooth muscle cells activated by inflammatory stimuli in experimental diabetes

Background

Adenosine has been shown to induce nitric oxide (NO) production via inducible NO synthase (iNOS) activation in vascular smooth muscle cells (VSMCs). Although this is interpreted as a beneficial vasodilating pathway in vaso-occlusive disorders, iNOS is also involved in diabetic vascular dysfunction. Because the turnover of and the potential to modulate iNOS by adenosine in experimental diabetes have not been explored, we hypothesized that both the adenosine system and control of iNOS function are impaired in VSMCs from streptozotocin-diabetic rats.

Methods

Male Sprague–Dawley rats were injected with streptozotocin once to induce diabetes. Aortic VSMCs from diabetic and nondiabetic rats were isolated, cultured and exposed to lipopolysaccharide (LPS) plus a cytokine mix for 24 h in the presence or absence of (1) exogenous adenosine and related compounds, and/or (2) pharmacological agents affecting adenosine turnover. iNOS functional expression was determined by immunoblotting and NO metabolite assays. Concentrations of adenosine, related compounds and metabolites thereof were assayed by HPLC. Vasomotor responses to adenosine were determined in endothelium-deprived aortic rings.

Results

Treatment with adenosine-degrading enzymes or receptor antagonists increased iNOS formation in activated VSMCs from nondiabetic and diabetic rats. Following treatment with the adenosine transport inhibitor NBTI, iNOS levels increased in nondiabetic but decreased in diabetic VSMCs. The amount of secreted NO metabolites was uncoupled from iNOS levels in diabetic VSMCs. Addition of high concentrations of adenosine and its precursors or analogues enhanced iNOS formation solely in diabetic VSMCs. Exogenous adenosine and AMP were completely removed from the culture medium and converted into metabolites. A tendency towards elevated inosine generation was observed in diabetic VSMCs, which were also less sensitive to CD73 inhibition, but inosine supplementation did not affect iNOS levels. Pharmacological inhibition of NOS abolished adenosine-induced vasorelaxation in aortic tissues from diabetic but not nondiabetic animals.

Conclusions

Endogenous adenosine prevented cytokine- and LPS-induced iNOS activation in VSMCs. By contrast, supplementation with adenosine and its precursors or analogues enhanced iNOS levels in diabetic VSMCs. This effect was associated with alterations in exogenous adenosine turnover. Thus, overactivation of the adenosine system may foster iNOS-mediated diabetic vascular dysfunction.

Renal vasculature reactivity to agonist of P2X7 receptor is increased in streptozotocin-induced diabetes

BACKGROUND

Diabetic nephropathy is characterized by the dysfunction of renal microvasculature. The involvement of the P2X7 receptor, being a part of the purinergic system, is presumable in this process. The aim of our study was to investigate the P2X7 receptor-mediated renal microvasculature response and renal metabolism of extracellular adenine nucleotides in diabetic rats.

METHODS

Study was performed on streptozotocin-induced diabetic Wistar rats. The vascular response to BzATP, an agonist of the P2X7 receptor, was monitored based on the changes of cortical blood flow (CBF), glomerular filtration rate (GFR) and glomerular inulin space (GIS). The renal interstitial fluid (RIF) was probed by microdialysis technique and concentrations of ATP and adenosine were measured. Activity on NTDPase and 5'-nucleotidases was measured on renal membranes.

RESULTS

Diabetic kidneys were characterized by decreased ATP RIF and increased adenosine RIF concentrations with accompanied enhancement of NTDPase and 5'-nucleotidase activities. BzATP induced a more pronounced increase of CBF and decrease of GFR and GIS in diabetes rats. These effects were abolished by A438079, an antagonist of the P2X7 receptor.

CONCLUSIONS

It is possible that increased P2X7 receptor reactivity may be involved in the pathogenesis of diabetic nephropathy.

Mechanisms underlying reduced P2Y(1) -receptor-mediated relaxation in superior mesenteric arteries from long-term streptozotocin-induced diabetic rats

AIM

Extracellular nucleotides activate cell-surface purinergic (P2) receptors, contribute to the local regulation of vascular tone and play important roles in pathophysiological states. However, little is known about the vasodilator effects of P2Y(1) -receptor activation in diabetic states. We hypothesized that in a model of established type 1 diabetes, long-term streptozotocin (STZ)-induced diabetic rats, the arterial relaxation elicited by a P2Y(1) -receptor agonist would be impaired.

METHODS

Relaxations to adenosine 5'-diphosphate sodium salt (ADP), 2-MeSADP (selective P2Y(1) -receptor agonist) and adenosine 5'-triphosphate disodium salt (ATP) were examined in superior mesenteric artery rings from long-term STZ-induced diabetic rats (at 50-57 weeks after STZ injection). ADP-stimulated nitric oxide (NO) production in the superior mesenteric artery was assessed by measuring the levels of NO metabolites. Mesenteric artery expressions of P2Y(1) receptor, and ADP-stimulated levels of phosphorylated endothelial NO synthase (eNOS) (at Ser(1177) and at Thr(495) ) and eNOS were detected by Western blotting.

RESULTS

Arteries from diabetic rats exhibited (vs. those from age-matched control rats): (i) reduced ADP-induced relaxation, which was partly or completely inhibited by endothelial denudation, by NOS inhibitor treatment and by a selective P2Y(1) -receptor antagonist, (ii) reduced 2-MeSADP-induced relaxation, (iii) reduced ADP-stimulated release of NO metabolites and (iv) impaired ADP-induced stimulation of eNOS activity (as evidenced by reduced the fold increase in eNOS phosphorylation at Ser(1177) with no difference in fold increase in eNOS phosphorylation at Thr(495) ). The protein expression of P2Y(1) receptor did not differ between diabetic and control arteries.

CONCLUSIONS

These results suggest that P2Y(1) -receptor-mediated vasodilatation is impaired in superior mesenteric arteries from long-term type 1 diabetic rats. This impairment is because of reduced P2Y(1) -receptor-mediated NO signalling, rather than to reduced P2Y(1) -receptor expression.

Physical training normalizes nucleotide hydrolysis and biochemical parameters in blood serum from streptozotocin-diabetic rats

Ectonucleotidases and the nucleotide metabolism have been implicated as important regulators in diabetes disease. We evaluated the ectonucleotidase activities and biochemical parameters in blood serum of streptozotocin (STZ)-induced diabetic rats submitted a physical training protocol. We observed a raise in ATP, ADP, AMP and p-Nph-5'-TMP hydrolysis rate and in the levels of cholesterol and triglycerides in rat blood serum, after 30 days of diabetes induction. However, in serum of rats submitted a physical training protocol by forced swimming, both the nucleotide hydrolysis rate and the lipids levels returned to the control values. Considering that diabetes leads to multiple pathophysiological alterations, the modulations observed in ectonucleotidase activities may be part of the events involved in these alterations. Then the physical training is a very important way to control the vascular alterations developed in diabetes.

High glucose concentration impairs ATP outflow and immunoglobulin production by human peripheral B lymphocytes: involvement of P2X7 receptor

AIMS/HYPOTHESIS

Patients with diabetes are more prone to bacterial infections mostly due to hyperglycemia-induced suppression of immune cells function. B lymphocytes by secreting antibodies inhibit microbial replication, but the impact of high glucose concentration on humoral immune response is not fully resolved. The aim of this work was to investigate the effect of high glucose concentration on B cells response to stimulation with a bacterial antigen and autocrine regulation.

METHODS

Purified human peripheral blood B cells were cultured at different glucose concentrations and stimulated in vitro with Staphylococcus aureus Cowan I (SAC) plus IL-2. B cells proliferation, differentiation and IgM expression were analyzed by flow cytometry. B cell ATP release and involvement of P2 purinergic receptors in regulation of IgM secretion was assessed.

RESULTS

B cells cultured at 25 mM glucose in response to SAC stimulation released significantly less (≈ 55%) IgM comparing to cells maintained in 5mM glucose. Under resting and stimulatory conditions B cells released significant quantities of ATP to the culture media, but ATP level decreased when B cells were maintain in high glucose. SAC-induced B cell IgM release was totally blocked by highly selective antagonist (Az11645373) of P2X7 receptor. IgM secretion increased in the presence of potent P2X7 receptor agonist (BzATP), but this effect was abolished by high glucose concentration.

CONCLUSIONS/INTERPRETATION

High glucose concentration impairs B cell function by suppression of P2X7 receptor-dependent IgM release in response to in vitro bacterial antigen stimulation. This alteration may greatly contribute to the impaired humoral immune response in diabetics.