Fumarate and its downstream signalling pathways in the cardiorenal system: Recent insights and novel expositions in the etiology of hypertension.

Hypertension, a risk factor for cardiorenal disease has a huge global health impact. Hence, there is a continuous search for new therapeutic targets and putative antihypertensive ligands. This search has transcended into the realm of mitochondrial metabolism which has been reported to underline the etiology of certain diseases, including hypertension. Recently, genetic alterations in the tricarboxylic acid (TCA) cycle enzyme, fumarase, which converts fumarate to malate, reportedly worsened salt-sensitive hypertension. These novel expositions shifted focus into the activity of TCA in the pathogenesis of hypertension. There is now evidence to show that a mechanistic link exists between blood pressure regulation and intermediaries in the TCA cycle involving fumarate metabolism. Fumarate has been reported to mediate the actions of endogenous ligands such as nitric oxide (NO), and hypoxia inducible factor (HIF)-1α. Similarly, there has been upregulation of protective genes such as nuclear erythroid factor 2 (Nrf2) and reduction in the expression of certain markers like kidney injury molecule 1 (KIM-1). There are reports of interactions with endogenous enzymes such as catalase (CAT) and renin via the activation of GPR91. Fumarate has also been shown to modulate the actions of renal ion channels and by extension, natriuresis. These actions of fumarate have conferred a reno- and cardio-protective effect in hypertension. This review evaluates the role of the TCA cycle, its mechanistic links, and significant contribution to blood pressure regulation with a view to understanding the possibility of a new pathological axis which may be involved in the pathogenesis of hypertension.

Malate reduced kidney injury molecule (KIM-1) expression and selectively upregulated the renal nitric oxide production in obstructive nephropathy.

BACKGROUND: Malate, the tricarboxylic acid (TCA) cycle intermediary, upregulates renal nitric oxide (NO) signaling, and NO is renoprotective in nephropathy. OBJECTIVES: This study explored the hypothesis that malate could increase renal NO and decrease renal injury and fibrotic markers in obstructive nephropathy. METHODS: Kidney injury was induced in rats via unilateral surgical ligation of the ureter, there after, rats were treated with malate (600 mg/kg, p.o.) for ten days. Urine was collected on days 0, 4, 7 and 10. Urinary sodium excretion was also determined. Western blot and biochemical analyses were carried on the nephropathic kidneys. RESULTS: Malate reduced kidney injury molecule (KIM-1) expression in the renal cortex and medulla of nephropathic rats (p < 0.05). NO production was selectively increased in the medulla of nephropathic rats treated with malate (58.3 ± 1.3 vs 77.8 ± 4.4 µM/ng, p < 0.05). Superoxide dismutase and catalase activity increased in the kidney of malate-treated nephropathic rats (p < 0.05). Transforming growth factor (TGF-ß), an index of fibrosis, increased in the cortex but not medulla of the malate-treated UUO group. There was a consistent increase in collagenase activity in the cortex, and a reduction in the medulla. CONCLUSION: Malate ameliorated the injury and inflammation but selectively reduced fibrosis in obstructive nephropathy (Fig. 6, Ref. 32). Text in PDF www.elis.sk Keywords: Malate, tricarboxylic acid cycle, nitric oxide, kidney injury molecule (KIM-1), obstructive nephropathy.

Malate reduced blood pressure and exerted differential effects on renal hemodynamics; role of the nitric oxide system and renal epithelial sodium channels (ENaC).

Malate regulates blood pressure via nitric oxide production in salt-sensitive rats, a genetic model of hypertension. This study investigated the possible contributions of malate to blood pressure regulation and renal haemodynamics in normotensive rats. Malate (0.1, 0.3 and 1 µg/kg, iv) was injected into rats or L-nitro-arginine methyl ester (L-NAME)-treated rats and mean arterial blood pressure (MABP), cortical blood flow (CBF), and medullary blood flow (MBF), was measured. The clearance study involved infusion of malate at 0.1 µg/kg/h into rats, and MABP, CBF, MBF, glomerular filtration rate (GFR), urine volume (UV) and sodium output (UNaV) were determined. Mechanistic studies to evaluate the role of renal sodium channels involved the treatment with malate (600 mg/kg, po), amiloride (2.5 mg/kg, po) or hydrochlorothiazide (HCTZ) (10 mg/kg, po), and UV and UNaV were determined. Malate elicited significant peak reductions in MABP (124 ± 6.5 vs 105 ± 3.1 mmHg) at 0.1 µg/kg), CBF (231 ± 18.5 vs 205 ± 10.9 PU). L-NAME did not reverse the effect of malate on MABP but tended to blunt the effect on CBF (40%) and MBF (87%) at 0.3 µg/kg. Infusion of malate reduced MABP, CBF, and MBF in a time-dependent manner (p<0.05). Malate exerted a three-fold decrease in GFR in a time-related fashion (p<0.05) as well as increased UV. UNaV increased by 86% in malate-treated-amiloride rats (p<0.05). These data indicate that malate modulates blood pressure and exerts vascular and tubular effects on renal function that may involve epithelial sodium channels (ENaC).

Fumarate exerted an antihypertensive effect and reduced kidney injury molecule (KIM)-1 expression in deoxycorticosterone acetate-salt hypertension.

Background: The tricarboxylic (TCA) acid cycle provides the energy needed for regulatory functions in the cardio-renal system. Recently, a genetic defect in the TCA cycle enzyme, fumarase hydratase, altered L-arginine metabolism and exacerbated hypertension in salt-sensitive rats. This study evaluated the effect of fumarate and its possible link to L-arginine metabolism in deoxycorticosterone (DOCA)-salt hypertension, a non-genetic model of hypertension.Method: Hypertension was induced with DOCA (25 mg/kg s.c, twice weekly) + 1% NaCL in uninephrectomised rats placed on fumarate (1 g/L, ad libitum). Blood pressure was measured in conscious rats via carotid cannulation. Biochemical and western blot analyses were carried out on kidney fractions.Results: Fumarate reduced mean blood pressure (198 ± 5 vs 167 ± 7 mmHg, p < .01), increased nitric oxide levels in the renal cortex (36.1 ± 2 vs 61.3 ± 4 nM/µg) and medulla (27.4 ± 1 vs 54.1 ± 2 nM/µg) of DOCA-salt rats (p < .01). Consistent with this, arginase activity was reduced (threefold) in the renal medulla but not cortex of DOCA-salt rats. Fumarate increased superoxide dismutase activity in the medulla (p < .001) of DOCA-hypertensive rats. However, catalase activity was exacerbated by fumarate in both renal cortex (4.5 ± 1 vs 11.2 ± 1) and medulla (3.7 ± 1 vs 16.3 ± 1 units/mg) of DOCA-salt rats (p < .001). Proteinuria (64.6%), kidney injury molecule-1 expression and kidney weight were reduced in DOCA-hypertensive rats treated with fumarate (p< .05). However, there was a paradoxical increase in TGF-ß expression in fumarate-treated DOCA-salt rats. Conclusion: These data show that fumarate attenuated hypertension, renal injury and improved the redox state of the kidney in DOCA/salt hypertension by mechanisms involving selective reduction of L-arginine metabolism.

Antinociceptive and antioxidant activities of Hunteria umbellata stem bark: possible role of the serotonergic, opioidergic and dopaminergic pathways.

Background Hunteria umbellata (HU) (K. Schum) is used in ethnomedicine for the management of pain, diabetes mellitus and dysmenorrhoea. This study evaluated the analgesic and antioxidant activities of aqueous extract of HU stem bark and the possible mechanism(s) of action. Methods The antinociceptive effect of HU was evaluated using acetic acid mouse writhing, tail flick, hot plate and formalin-induced paw licking models. To establish the possible mechanism(s) of action of HU, separate group of animals were pretreated with naloxone (1 mg/kg, i.p.), atropine (1 mg/kg, i.p.), haloperidol (0.1 mg/kg, i.p.), ondansetron (1 mg/kg, i.p.) and phenoxybenzamine (0.1 mg/kg, i.p.), 15 min before HU. The in vivo and in vitro antioxidant potential was evaluated using established methods. Results The extract at 150 and 300 mg/kg, significantly (p<0.05) reduced the number of writhes and paw licking times and increased pain threshold in writhing assay, paw licking and hotplate tests respectively. Pretreatment of animals with ondansetron, naloxone and haloperidol, significantly (p<0.05 and p<0.01) attenuated the analgesic activity of HU. The extract demonstrated significant (p<0.05) radical scavenging activity (IC50 0.39 µg/mL), with high phenol content and reducing property. The total phenol content was 124.19 per gram of gallic acid. In vivo antioxidant assay showed significant (p<0.05) increase in catalase and superoxide levels. Conclusions Results obtained in this study suggest the involvement of serotonergic, opioidergic and dopaminergic pathways in the analgesic effect of HU stem bark, in addition to its potent antioxidant potential.