Izabela Zakrocka

Angiotensin-converting enzyme inhibitors modulate kynurenic acid production in rat brain cortex in vitro

It is well established that the renin-angiotensin system (RAS) is present in the brain and that glutamate activates the brain centers responsible for blood pressure control. An antagonist of glutamate, kynurenic acid (KYNA) was shown to decrease blood pressure after intracerebral administration. KYNA is an endogenous metabolite of tryptophan produced from the breakdown of kynurenine by kynurenine aminotransferases (KAT), mainly within astrocytes. The purpose of this study was to evaluate the influence of three angiotensin-converting enzyme inhibitors (lisinopril, perindopril and ramipril) on KYNA production and KAT activity in the rat brain cortex in vitro. The effect of the angiotensin-converting enzyme inhibitors on KYNA production was examined on rat brain cortical slices incubated for 2h in the presence of l-kynurenine and the angiotensin-converting enzyme inhibitors. To analyze KAT I and KAT II activity, brain cortical homogenates were incubated for 2h with L-kynurenine and the tested drugs. KYNA was separated by HPLC and quantified fluorometrically. Among the examined angiotensin-converting enzyme inhibitors, lisinopril increased KYNA production, perindopril was ineffective, and ramipril decreased KYNA synthesis in rat brain cortical slices. Lisinopril increased KAT I activity and perindopril did not affect it. However, ramipril lowered KAT I activity in rat brain cortex in vitro. Neither lisinopril nor perindopril affected KAT II activity, but ramipril decreased KAT II activity in the rat brain cortex in vitro. Our study reveals that angiotensin-converting enzyme inhibitors show various influences on KYNA production in rat brain cortical slices and activity of KATs.

Angiotensin II Type 1 Receptor Blockers Inhibit KAT II Activity in the Brain—Its Possible Clinical Applications

Angiotensin II receptor blockers (ARBs) are one of the most frequently recommended antihypertensive drugs. Apart from their activity towards the circulatory system, ARBs also penetrate the blood-brain barrier and display neuroprotective effects. Kynurenic acid (KYNA) is an endogenous metabolite of tryptophan produced by kynurenine aminotransferase II (KAT II) in the brain. Antagonism towards all ionotropic glutamate (GLU) receptors is the main mechanism of KYNA action. An elevated brain level of KYNA is linked with memory impairment and psychotic symptoms. The aim of this study was to examine the influence of three ARBs: irbesartan, losartan, and telmisartan on KYNA production and KAT II activity in rat brain. The effect of ARBs on KYNA production was analyzed in rat brain cortical slices and on isolated KAT II enzyme. Irbesartan, losartan, and telmisartan decreased KYNA production and KAT II activity in a dose-dependent manner in rat brain cortex in vitro. Molecular docking suggested that the examined ARBs could bind to an active site of KAT II. In conclusion, ARBs decrease KYNA production in rat brain by direct inhibition of KAT II enzymatic activity. This novel mechanism of ARBs action may be advantageous in the treatment of cognitive impairment or the management of schizophrenia.

Effects of systemic administration of kynurenic acid and glycine on renal haemodynamics and excretion in normotensive and spontaneously hypertensive rats

Both NMDA receptor and kynurenic acid (KYNA), a glycine-site NMDA receptor antagonist, are present in the kidney yet their functional role remains unclear. Our aim was to examine effects of intravenous KYNA and glycine on arterial blood pressure (MAP) and renal haemodynamics and excretion in anaesthetized normotensive Sprague-Dawley (S-D) and in spontaneously hypertensive (SHR) rats. Renal blood flow (RBF, renal artery probe) and renal cortical (CBF) and outer- and inner medullary perfusion (laser-Doppler) were measured, along with diuresis (V) and sodium excretion (UNaV). KYNA given alone (150mgkg(-1) iv) or during infusion of glycine at 1gkg(-1)h(-1) iv (G+K) increased or decreased RBF, respectively, in both S-D and SHR. Neither treatment altered MAP. In both strains glycine alone increased RBF and CBF 50-60% and was clearly diuretic and natriuretic, less so in SHR. KYNA increased UNaV by 4.1±1.7μmolmin(-1)and V by 11.1±4.3μlmin(-1) in S-D (P<0.05 for both); the respective increases in SHR were by 1.7±0.6μmolmin(-1) and 4.7±1.7μlmin(-1) (P<0.02 for both). G+K treatment increased UNaV by 5.2±1.4μmolmin(-1) (P<0.01) and V by 29.6±4.6μmolmin(-1) (P<0.001) in S-D, and by 2.7±0.7μmolmin(-1) (P<0.05) and 19.3±3.5μlmin(-1) (P<0.0006) in SHR. In conclusion, KYNA increased renal excretion, apparently by inhibiting tubular reabsorption, whereas glycine substantially increased renal haemodynamics by an ill-defined mechanism, with a secondary increase in the excretion. Combined G+K treatment could be utilised to combat body fluid retention and possibly alleviate hypertension, without endangering renal perfusion and function.

The effect of three angiotensin-converting enzyme inhibitors on kynurenic acid production in rat kidney in vitro

BACKGROUND The renin-angiotensin system (RAS) is commonly known to regulate blood pressure, water and electrolyte homeostasis, however it also exerts paracrine and autocrine actions on the kidney. Angiotensin-converting enzyme inhibitors (ACE-Is), alongside their hypotensive properties, have been shown to decrease kidney function decline in animal models of nephropathy. Glutamate (GLU) is the main stimulatory neurotransmitter in the central nervous system, however its importance in the periphery should also be considered. Activation of renal GLU receptors has been linked to normal kidney function and also renal injury. The wide spectrum GLU receptor antagonist kynurenic acid (KYNA) possesses neuroprotective and central hypotensive effects, however its actions outside the brain are less well recognized. KYNA is a tryptophan metabolite synthesized from kynurenine by kynurenine aminotransferases (KATs). The purpose of this study was to examine the influence of three ACE-Is: lisinopril, perindopril and ramipril on KYNA production and KATs activity in rat kidney in vitro. METHODS The effect of ACE-Is on KYNA production and KATs activity was examined in rat kidney homogenates. KYNA was detected by high-performance liquid chromatography (HPLC) and quantified fluorometrically. RESULTS All examined ACE-Is: lisinopril, perindopril and ramipril decreased KYNA production in rat kidney in vitro. KAT I activity was decreased by lisinopril and ramipril whereas the activity of KAT II was lowered by ramipril. CONCLUSION Our study shows that ACE-Is can decrease KYNA production in rat kidney in vitro. Further studies are required to determine the clinical importance of the inhibitory action of ACE-Is on KYNA synthesis in the kidney.BACKGROUND The renin-angiotensin system (RAS) is commonly known to regulate blood pressure, water and electrolyte homeostasis, however it also exerts paracrine and autocrine actions on the kidney. Angiotensin-converting enzyme inhibitors (ACE-Is), alongside their hypotensive properties, have been shown to decrease kidney function decline in animal models of nephropathy. Glutamate (GLU) is the main stimulatory neurotransmitter in the central nervous system, however its importance in the periphery should also be considered. Activation of renal GLU receptors has been linked to normal kidney function and also renal injury. The wide spectrum GLU receptor antagonist kynurenic acid (KYNA) possesses neuroprotective and central hypotensive effects, however its actions outside the brain are less well recognized. KYNA is a tryptophan metabolite synthesized from kynurenine by kynurenine aminotransferases (KATs). The purpose of this study was to examine the influence of three ACE-Is: lisinopril, perindopril and ramipril on KYNA production and KATs activity in rat kidney in vitro. METHODS The effect of ACE-Is on KYNA production and KATs activity was examined in rat kidney homogenates. KYNA was detected by high-performance liquid chromatography (HPLC) and quantified fluorometrically. RESULTS All examined ACE-Is: lisinopril, perindopril and ramipril decreased KYNA production in rat kidney in vitro. KAT I activity was decreased by lisinopril and ramipril whereas the activity of KAT II was lowered by ramipril. CONCLUSION Our study shows that ACE-Is can decrease KYNA production in rat kidney in vitro. Further studies are required to determine the clinical importance of the inhibitory action of ACE-Is on KYNA synthesis in the kidney.

Influence of Cyclooxygenase-2 Inhibitors on Kynurenic Acid Production in Rat Brain in Vitro

Significant body of evidence suggests that abnormal kynurenic acid (KYNA) level is involved in the pathophysiology of central nervous system disorders. In the brain, KYNA is synthesized from kynurenine (KYN) by kynurenine aminotransferases (KATs), predominantly by KAT II isoenzyme. Blockage of ionotropic glutamate (GLU) receptors is a main cellular effect of KYNA. High KYNA levels have been linked with psychotic symptoms and cognitive dysfunction in animals and humans. As immunological imbalance and impaired glutamatergic neurotransmission are one of the crucial processes in neurological pathologies, we aimed to analyze the effect of anti-inflammatory agents, inhibitors of cyclooxygenase-2 (COX-2): celecoxib, niflumic acid, and parecoxib, on KYNA synthesis and KAT II activity in rat brain in vitro. The influence of COX-2 inhibitors was examined in rat brain cortical slices and on isolated KAT II enzyme. Niflumic acid and parecoxib decreased in a dose-dependent manner KYNA production and KAT II activity in rat brain cortex in vitro, whereas celecoxib was ineffective. Molecular docking results suggested that niflumic acid and parecoxib interact with an active site of KAT II. In conclusion, niflumic acid and parecoxib are dual COX-2 and KAT II inhibitors.

Clonidine decreases kynurenic acid production in rat brain cortex in vitro – a novel antihypertensive mechanism of action?

Introduction and objective. Clonidine, an antihypertensive agent, is known to activate presynaptic α2-adrenoreceptors and imidazoline receptors in the central nervous system. Clonidine may also have influence on glutamatergic neurotransmission. Kynurenic acid (KYNA) is an endogenous antagonist of excitatory amino acid receptors. Cerebral KYNA synthesis from its bioprecursor L-kynurenine is regulated by aminotransferases localized preferentially within astrocytes. KYNA was shown to display potent neuroprotective properties. Moreover, it was reported that KYNA could reduce blood pressure levels in animal model of hypertension. The aim of the study was to discover whether the central antihypertensive agent clonidine has an influence on kynurenic acid (KYNA) production in rat brain cortex in vitro. Materials and method. Cortical slices and brain cortical homogenates were incubated for 2 hours in the presence of KYNA precursor – L-kynurenine and clonidine. KYNA was separated chromatographically and detected fluorometrically. Results. Clonidine at the concentration of 0.001 mM – 0.1 mM did not affect KYNA production in rat brain cortex in vitro. It decreased KYNA production in rat brain cortical slices at concentrations 0.5–5 mM. Clonidine at all tested concentrations influenced neither KAT I nor KAT II activity. Conclusion. The study revealed that clonidine decreases KYNA production in rat brain cortex in vitro. The obtained results suggest that augmentation of glutamatergic transmission may play an important role in the antihypertensive action of