Kynurenic Acid Selectively Reduces Heart Rate in Spontaneously Hypertensive Rats

Abstract

We found previously that intravenous kynurenic acid (KYNA), a native broad-spectrum glutamate antagonist, increases renal blood flow and induces natriuresis in anesthetized spontaneously hypertensive rats (SHR). Since such changes may affect systemic circulation and can potentially find therapeutic application, in this study we examined the long-term influence of orally administered KYNA on systemic and renal hemodynamics and renal excretion in conscious SHR. KYNA was administered in drinking water at a dose of 25 mg/kg/day for three weeks. Heart rate (HR), systolic blood pressure (SBP), and mean arterial pressure (MAP) were measured through telemetry. Records were taken at the beginning of the study (control, day 0) and then on day 7, 14, and 21 of treatment. Diuresis (V), total solute excretion (UosmV), and sodium excretion (UNaV) were determined on days 0, 7, and 14. KYNA consistently decreased HR from 319 ± 8 to 291 ± 5, 299 ± 9, and 284 ± 6 beats/min on day 7, 14, and 21 respectively (−9, −6, and −11%; p < 0.01–0.0001); HR was stable in the solvent group. SBP, MAP, V, and UNaV were not affected by KYNA, whereas UosmV increased modestly. Chronic oral administration of KYNA to conscious SHR decreased HR without affecting MAP. Since tachycardia is an independent risk factor for cardiovascular disorders, and most drugs used to decrease HR have strong negative inotropic or hypotensive effects, such selective action seems to have therapeutic potential. Moreover, food supplementation with KYNA can be considered in the prevention of heart diseases.

Introduction

Kynurenic acid (KYNA), a metabolite of tryptophan, is formed from L-kynurenine (KYN) by kynurenine aminotransferases (KATs). KATs were found in the heart and shown to selectively synthesize KYNA from low concentrations of KYN. Unlike KATs expressed in brain tissue where KAT I and KAT II dominate, high levels of KAT III were described in the heart. KYNA is an endogenous antagonist of glutamate (GLU) ionotropic receptors as well as an agonist of the G-protein-coupled receptor GPR35.

There is evidence that KYNA may modify heart function: it was reported to dose-dependently decrease respiratory parameters of heart mitochondria, potentially leading to cardiomyopathy symptoms. On the other hand, its cardioprotective effect has also been proposed. We recently showed that intravenous KYNA in anesthetized SHR induced natriuresis and diuresis. If such excretion changes are prolonged, they could alter body fluid balance and affect cardiovascular function, possibly impacting blood pressure. This prompted us to examine the long-term cardiovascular and renal effects of orally administered KYNA in conscious SHR.

Material and Methods

Animals

The experimental procedures were approved by the local ethical committee for animal experimentation in Warsaw. Experiments were performed on male SHR aged 10–12 weeks, weighing 270–300 g, with established hypertension. The rats were fed a standard diet with free access to food and water.

Protocols and Measurements

In chronic experiments with conscious SHR, we recorded MAP, SBP, HR, and renal excretion of water, sodium, and total solutes (V, UNaV, and UosmV) in response to KYNA administered in drinking water at 250 mg/l, alkalized to pH 7.8. Based on daily water intake, the actual mean dosage was 25 mg/kg/day. The rats were housed individually, and the volume of water consumed daily was measured to calculate the precise amount of KYNA ingested. This dose was selected based on earlier studies showing no adverse effects except for some morphologic changes and slowed body mass gain in newborn rats. The control group received solvent (tap water, pH adjusted to 7.8).

Since at the end of the experiments, baseline HR values were lower in the solvent group compared to the KYNA-treated group, an additional KYNA group of five rats was studied later, where baseline HR values were comparable to those of the solvent group.

Telemetry probes were surgically implanted five days before starting measurements, consisting of a radio transmitter and a catheter inserted into the abdominal aorta under anesthesia. The transmitter was placed in the peritoneal cavity. After surgery, analgesic and antibiotic drugs were administered. The rats were then kept in separate cages with access to food and water.

On the fifth day post-surgery (day 0), pre-treatment measurements of MAP, SBP, HR, and locomotor activity were recorded telemetrically for four hours in the morning. The data were averaged in short sampling intervals. Afterwards, the rats were kept for 24 hours in metabolic cages to measure food and water intake, feces weight, urine flow, osmolality, and sodium concentration.

After these control measurements, the experimental group received KYNA solution for 21 days, and the control group received solvent. Telemetric measurements were repeated at one-week intervals, and measurements of renal excretion were repeated on days 0, 7, and 14.

At the end of the chronic treatment, final acute experiments were performed under anesthesia. The left kidney was exposed and immobilized for measurement of renal blood flow (RBF) using a Transonic flowmeter. After observation, the right kidney and heart were collected for morphometric examination.

Analytical Procedures

Since HR could be modulated by stress, HR variability (HRV) analysis was performed to estimate sympathetic cardiac input using the low-frequency band (LF). Telemetry recordings were checked for accuracy. HRV was analyzed using specialized software, calculating power in predefined spectral bands: very low frequency (below 0.2 Hz), low frequency (0.2–0.74 Hz), and high frequency (0.74–2.50 Hz). Urine volume was measured gravimetrically, osmolality by freezing point depression, and sodium concentration by flame photometry.

Statistics

Differences within and between groups were evaluated by repeated measures ANOVA followed by post-hoc tests, with correction for multiple comparisons. Data are expressed as mean ± standard error, and p ≤ 0.05 was considered significant.

Results

Throughout the observation period, both KYNA and solvent groups showed comparable increases in body weight, food and water intake, and feces weight. Plasma sodium and osmolality remained unchanged. No abnormal behavior was observed in KYNA-treated rats. Daytime locomotor activity was not significantly different between groups.

KYNA administration consistently decreased HR, with significant reductions already evident on day 7 and persisting to day 21. In contrast, solvent treatment did not change HR. A separate group of rats with similar baseline HR values to the solvent group also showed gradual HR reduction after KYNA.

Analysis of HRV showed no significant differences in LF values between KYNA- and solvent-treated rats during the experiment.

MAP and SBP were not significantly affected by KYNA. Diuresis did not significantly change, and while sodium excretion tended to increase, this was not specific to KYNA. Total solute excretion increased modestly under KYNA.

Final morphometric data showed no adverse effects of KYNA on organ weights or renal blood flow.

Discussion

This study’s main finding is that oral administration of KYNA for three weeks decreased HR in conscious SHR without affecting MAP or SBP. Unlike previously reported central administration of KYNA, which decreased blood pressure, oral KYNA selectively reduced HR, possibly through peripheral receptors.

KYNA did not significantly change sodium and water excretion during the chronic study, suggesting HR reduction was independent of body fluid volume. Chronic KYNA treatment did not produce adverse effects on body or organ weights or RBF.

The HR decrease without blood pressure reduction is notable since most HR-lowering agents also reduce BP. The study’s limitation is that measurements were only done during the day. Since KYNA poorly crosses the blood-brain barrier, the HR reduction likely involved peripheral NMDA receptors, which have been found in cardiomyocytes and other tissues. Previous studies have shown that NMDA receptor activation can increase HR, supporting the idea that KYNA, as an NMDA antagonist, reduced HR.

Other possible mechanisms include KYNA’s action through GPR35 receptors, which have been implicated in cardiovascular regulation. The analysis of HRV did not show decreased sympathetic activity, suggesting a direct effect on the heart.

Tachycardia is an independent cardiovascular risk factor associated with higher risks of heart disease, stroke, and metabolic disorders. Current HR-lowering drugs often have side effects like hypotension and negative inotropic action, limiting their use. The selective HR-lowering effect of KYNA without lowering BP suggests potential clinical applications, though further studies on toxicity and mechanism are needed. Since KYNA is present in some foods, dietary supplementation might also be explored.

Conclusions

Chronic oral KYNA administration to conscious SHR decreased HR without affecting MAP. While the blood-brain barrier in SHR may allow some central effects, the primary action is likely peripheral. Further studies are needed to clarify the molecular mechanism and dose dependency. Given that tachycardia is a risk factor for cardiovascular disease, KYNA’s selective HR reduction may have therapeutic potential.