Ho-Shan Niu

Cardiac peroxisome proliferator-activated receptor δ (PPARδ) as a new target for increased contractility without altering heart rate.

Background and Aims Agents having a positive inotropic effect on the heart are widely used for the treatment of heart failure. However, these agents have the side effect of altering heart rate. It has been established that peroxisome proliferator-activated receptor δ (PPARδ) is mediated in cardiac contraction, however the effect on heart rate is unknown. Thus, we used an agonist of PPARδ, GW0742, to investigate this issue in the present study. Methods and Results We used isolated hearts in Langendorff apparatus and hemodynamic analysis in catheterized rats to measure the actions of GW0742 extra-vivo and in vivo. In diabetic rats with heart failure, GW0742 at a dose sufficient to activate PPARδ reversed cardiac contraction without changes in heart rate. In normal rats, PPARδ enhanced cardiac contractility and hemodynamic dP/dtmax significantly more than dobutamine. Both actions were diminished by GSK0660 at a dose enough to block PPARδ. However, GW0742 at the same dose failed to modify heart rate, although it did produce a mild increase in blood pressure. Detection of intracellular calcium level and Western blotting analysis showed that the intracellular calcium concentration and troponin I phosphorylation were both enhanced by GW0742. Conclusion Activation of PPARδ by GW0742 increases cardiac contractility but not heart rate. Thus, PPARδ may be a suitable target for the development of inotropic agents to treat heart failure without changing heart rate.

Canavanine induces insulin release via activation of imidazoline I3 receptors.

The aim of the present study was to identify the effect of canavanine on the imidazoline receptor because canavanine is a guanidinium derivative that has a similar structure to imidazoline receptor ligands. Transfected Chinese hamster ovary‐K1 cells expressing imidazoline receptors (nischarin (NISCH)‐CHO‐K1 cells) were used to elucidate the direct effects of canavanine on imidazoline receptors. In addition, the imidazoline I3 receptor has been implicated in stimulation of insulin secretion from pancreatic β‐cells. Wistar rats were used to investigate the effects of canavanine (0.1, 1 and 2.5 mg/kg, i.v.) on insulin secretion. In addition the a specific I3 receptor antagonist KU14R (4 or 8 mg/kg, i.v.) was used to block I3 receptors. Canavanine decreased blood glucose by increasing plasma insulin in rats. In addition, canavanine increased calcium influx into NISCH‐CHO‐K1 cells in a manner similar to agmatine, the endogenous ligand of imidazoline receptors. Moreover, KU12R dose‐dependently attenuated canavanine‐induced insulin secretion in HIT‐T15 pancreatic β‐cells and in the plasma of rats. The data suggest that canavanine is an agonist of I3 receptors both in vivo and in vitro. Thus, canavanine would be a useful tool in imidazoline receptor research.

Activation of receptors δ (PPARδ) by agonist (GW0742) may enhance lipid metabolism in heart both in vivo and in vitro.

It has been documented that cardiac agents may regulate the lipid metabolism through increased expression of PPARδ in cardiac cells. However, the effect on lipid metabolism by direct activation of PPARδ is still unknown. The present study applied specific PPARδ agonist (GW0742) to investigate this point in the heart of Wistar rats and in the primary cultured cardiomyocytes from neonatal rat. Expressions of PPARδ in the heart and cardiomyocytes after treatment with GW0742 were detected using Western blots. The fatty acid (FA) oxidation and the citric acid (TCA) cycle related genes in cardiomyocytes were also examined. In addition, PPARδ antagonist (GSK0660) and siRNA-PPARδ were employed to characterize the potential mechanisms. After a 7-day treatment with GW0742, expressions of PPARδ in the heart were markedly increased. Increased expressions of FA oxidation and TCA cycle related genes were also observed both in vivo and in vitro. This action of GW0742 was blocked by GSK0660 or by siRNA-PPARδ. The obtained results show that activation of PPARδ by GW0742 is responsible for the increase of FA oxidation and TCA cycle related genes in hearts. Role of PPARδ in the regulation of lipid metabolism in heart is then established.

Ginsenoside Rh2 Improves Cardiac Fibrosis via PPARδ–STAT3 Signaling in Type 1-Like Diabetic Rats

Ginsenoside Rh2 (Rh2) is an active principal ingredient contained in ginseng (Panax ginseng Meyer), a medicinal herb used to enhance health worldwide. The present study is designed to investigate the effect of Rh2 on myocardial fibrosis in diabetic rats. In a streptozotocin-induced model of type-1 diabetic rats (STZ-diabetic rats), the increased fasting blood glucose levels and heart weight/body weight (HW/BW) ratio were substantially alleviated by Rh2. Moreover, Rh2 improved cardiac performance in STZ-diabetic rats. Histological results from Masson staining showed that Rh2 attenuated cardiac fibrosis in STZ-diabetic rats. The effects of Rh2 were reversed by GSK0660 at a dose sufficient to inhibit peroxisome proliferator-activated receptor δ (PPARδ) in STZ-diabetic rats. The role of PPARδ was subsequently investigated in vitro. Rh2 restored the decreased PPARδ expression level in high glucose-cultured cardiomyocytes. Moreover, increased protein levels of fibrotic signals, including signal transducer and activator of transcription 3 (STAT3), connective tissue growth factor (CCN2) and fibronectin, were reduced by Rh2 in high glucose-cultured cardiomyocytes. These effects of Rh2 were reversed by GSK0660 or siRNA specific for PPARδ Taken together, PPARδ activation may inhibit STAT3 activation to reduce CCN2 and fibronectin expression in diabetic rats with cardiac fibrosis. Moreover, Rh2 improves cardiac function and fibrosis by increasing PPARδ signaling. Therefore, Rh2 is suitable to develop as an alternative remedy for cardiac fibrosis.

Allantoin activates imidazoline I-3 receptors to enhance insulin secretion in pancreatic β-cells

BackgroundImidazoline I3 receptors (I-3R) can regulate insulin secretion in pancreatic β-cells. It has been indicated that allantoin ameliorates hyperglycemia by activating imidazoline I2 receptors (I-2R). Thus, the effect of allantoin on I-3R is identified in the present study.MethodsWe used male Wistar rats to screen allantoin’s ability for lowering of blood glucose and stimulation of insulin secretion. Chinese hamster ovary-K1 cells transfected with imidazoline receptors (NISCH-CHO-K1 cells) were also applied to characterize the direct effect of allantoin on this receptor. Additionally, KU14R as specific antagonist was treated to block I-3R in rats and in the cultured pancreatic β-cells named Min 6 cells.ResultsIn rats, allantoin decreased blood sugar with an increase in plasma insulin. Also, allantoin enhanced calcium influx into NISCH-CHO-K1 cells in a way similar to agmatine, an I-R agonist. Moreover, KU14R dose-dependently blocked allantoin-induced insulin secretion both in Min 6 cells and in Wistar rats.ConclusionAllantoin can activate I-3R to enhance insulin secretion for lowering of blood sugar in Wistar rats. Thus, allantoin may provide beneficial effects as a supplement for diabetic patients after clinical trials.

Canavanine activates imidazoline I-2 receptors to reduce hyperglycemia in type 1-like diabetic rats.

Canavanine is a guanidinium derivative that has the basic structure of a ligand for the imidazoline receptor (I-R). Furthermore, canavanine is found in an herb that has been shown to improve diabetic disorders. Thus, the present study was designed to investigate the anti-hyperglycemic action of canavanine in rats with streptozotocin (STZ)-induced type 1-like diabetes. Canavanine decreased hyperglycemia in the STZ-induced diabetic rats, and this action was blocked by the antagonist specific to imidazoline I-2 receptors (I-2R), BU224, in a dose-dependent manner. Additionally, canavanine increased the plasma β-endorphin level, as measured using enzyme-linked immunosorbent assay (ELISA), and this increase was also blocked by BU224 in the same manner. Moreover, amiloride at a dose sufficient to block I-2AR attenuated the actions of canavanine, including the increased β-endorphin level and the antihyperglycemic effect. Otherwise, canavanine increased the radioactive glucose uptake into skeletal muscles isolated from the diabetic rats. Furthermore, canavanine increased the phosphorylation of AMPK measured using Western blot analysis in these isolated skeletal muscles in a dose-dependent manner. Additionally, the insulin sensitivity of the diabetic rats was markedly increased by canavanine, and this action was also blocked by BU224. Overall, canavanine is capable of activating imidazoline I-2R; I-2AR is linked to an increase in the plasma level of β-endorphin, and I-2BR is related to effects on the glucose uptake by skeletal muscle that reduces hyperglycemia in type 1-like diabetic rats. Therefore, canavanine can be developed as effective agent to treat the diabetic disorders in the future.

Canavanine increases glucose uptake in C2C12 cells through the activation of imidazoline I-2B receptors

Canavanine is a guanidinium derivative that contains the basic structure of the ligand(s) of imidazoline receptor (I‐R). Canavanine has been reported to activate the imidazoline I‐3 receptor (I‐3R) both in vivo and in vitro. Additionally, the activation of the imidazoline I‐2B receptor (I‐2BR) by guanidinium derivatives may increase glucose uptake. Therefore, the effect of canavanine on the I‐2BR was investigated in the present study. Glucose uptake into cultured C2C12 cells was determined using the radio‐ligated tracer 2‐[14C]‐deoxy‐glucose. The changes in 5′ AMP‐activated protein kinase (AMPK) expression were also identified using Western blotting analysis. The canavanine‐induced glucose uptake was inhibited in a dose‐dependent manner by BU224 (0.01–1 μmol/L), which is a specific I‐2BR antagonist, in the C2C12 cells. Additionally, the canavanine‐stimulated AMPK phosphorylation and glucose transporter (GLUT4) expression were also sensitive to BU224 inhibition in the C2C12 cells. Moreover, both canavanine‐stimulated glucose uptake and AMPK phosphorylation were attenuated by high concentrations of amiloride (1–2 μmol/L), which is another established I‐2BR inhibitor, in a dose‐dependent manner in C2C12 cells. Additionally, compound C abolished the canavanine‐induced glucose uptake and AMPK phosphorylation at a concentration (0.1 μmol/L) sufficient to inhibit AMPK. In conclusion, these data demonstrated that canavanine has an ability to activate I‐2BR through the AMPK pathway to increase glucose uptake, which indicates I‐2BR as a new target for diabetic therapy.