Attenuated cardiovascular response to adenosine in the brain stem nuclei of spontaneously hypertensive rats

Therapeutic potency of A1 adenosine receptor antagonists in the treatment of cardiovascular diseases, current status and perspectives

BACKGROUND

Cardiomyocytes form, transport, and metabolize the omnipresent metabolite adenosine. Depending upon the adenosine concentrations and the pharmacological properties of receptor subtypes, adenosine exerts (patho)physiological responses in the cardiovascular system. The objective of this review is to present different protective mechanisms of A1-adenosine receptor inhibitors in cardiovascular diseases.

METHODS AND RESULTS

Literature references were collected and sorted using relevant keywords and key phrases as search terms in scientific databases such as Web of Science, PubMed and Google Scholar. A1 adenosine receptor regulates free fatty acid metabolism, lipolysis, heart rate, blood pressure, and cardiovascular toxicity. The evidence clearly supporting the therapeutic potency of pharmacological A1 adenosine receptors agonists and antagonists in modulating cardiovascular risk factor parameters and treatment of cardiovascular diseases.

CONCLUSION

This review summarizes the protective role of pharmacological A1-adenosine receptor regulators in the pathogenesis of cardiovascular diseases for a better management of cardiovascular diseases.

Adenosine A2A receptors in the rostral ventrolateral medulla participate in blood pressure decrease with electroacupuncture in hypertensive rats

Acupuncture is increasingly used to manage high blood pressure (BP) as a complementary therapy. However, the mechanisms underlying its hypotensive effects remain unclear. Our previous studies have shown that electroacupuncture (EA) at the ST36-37 acupoints, overlying the deep peroneal nerve, attenuates pressor responses through adenosine A2A receptors (A2AR) in the rostral ventrolateral medulla (rVLM). However, it is uncertain whether rVLM A2AR contributes to EA's BP-lowering effect in sustained hypertension. We hypothesized that a course of EA treatment lowers BP, in part, through the activation of adenosine A2AR in the rVLM in hypertensive rats. To mimic essential hypertension in the clinic, we performed EA in conscious Dahl salt-sensitive hypertensive rats (DSHRs). EA (0.1-0.4 mA, 2 Hz) was applied at ST36-37 for 30 min twice weekly for four weeks, while sham-EA was conducted in a similar manner but without electrical input. In hypertensive rats, BP was reduced by EA (n = 14) but neither by sham-EA (n = 14) nor in the absence of needling (n = 8). Following four weeks of eight treatments and then under anesthesia, EA's modulatory effect on elevated BP was reversed by unilateral rVLM microinjection of SCH 58261 (1 mM in 50 nl; an A2AR antagonist; n = 7; P < 0.05) but not the vehicle (n = 5) in EA-treated DSHRs. Activation of rVLM A2AR in DSHRs treated with sham-EA by an A2AR agonist, CGS-21680 (0.4 mM in 50 nl; n = 8), decreased BP. Unilateral administration of SCH 58261 or CGS-21680 into the rVLM did not alter basal BP in Dahl salt-sensitive rats fed a regular diet with normal BP. The A2AR level in the rVLM after EA was increased compared to the sham-EA and untreated DSHRs (n = 5 in each group; all P < 0.05). These data suggest that a 4-week twice weekly EA treatment reduced BP in salt-sensitive hypertensive rats likely through adenosine-mediated A2AR in the rVLM.

Gabapentin Reduces Blood Pressure and Heart Rate through the Nucleus Tractus Solitarii

Background

Oral and intravenous gabapentin can markedly attenuate blood pressure (BP) in hypertensive rats. The nucleus tractus solitarii (NTS) is the primary integrative center for cardiovascular control and other autonomic functions in the central nervous system. However, the signaling mechanisms involved in gabapentin-mediated cardiovascular effects in the NTS remain unclear. We investigated whether the nitric oxide synthase (NOS) signaling pathway was involved in gabapentin-mediated BP regulation in the NTS of spontaneously hypertensive (SHR) rats.

Methods

SHR rats were anesthetized with urethane at age 10-12 weeks. Arterial pressure and heart rate (HR) were monitored through a femoral artery catheter. For stereotaxic intra-NTS microinjection, the dorsal surface of the medulla was exposed by limited craniotomy. We observed that unilateral microinjection of gabapentin into the NTS whether to change dose-related BP and HR. Then, unilateral microinjection of gabapentin into the NTS before and after N(ω)-nitro-L-arginine methyl ester (L-NAME) treatment whether to change blood pressure and heart rate.

Results

Unilateral microinjection of gabapentin into the NTS produced prominent dose-related depressor and bradycardic effects in SHR rats. The cardiovascular effects of gabapentin were attenuated by the prior administration of the NOS inhibitor, L-NAME.

Conclusions

Gabapentin modulated central BP and HR control in the NTS of SHR rats in this study through NOS signaling.

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

Adenosine modulates alpha2-adrenergic receptors within specific subnuclei of the nucleus tractus solitarius in normotensive and spontaneously hypertensive rats

Adenosine is known to modulate neuronal activity within the nucleus tractus solitarius (NTS). The modulatory effect of adenosine A1 receptors (A1R) on alpha2-adrenoceptors (Adr2R) was evaluated using quantitative radioautography within NTS subnuclei and using neuronal culture of normotensive (WKY) and spontaneously hypertensive rats (SHR). Radioautography was used in a saturation experiment to measure Adr2R binding parameters (Bmax, Kd) in the presence of 3 different concentrations of N6-cyclopentyladenosine (CPA), an A1R agonist. Neuronal culture confirmed our radioautographic results. [3H]RX821002, an Adr2R antagonist, was used as a ligand for both approaches. The dorsomedial/dorsolateral subnucleus of WKY showed an increase in Bmax values (21%) induced by 10 nmol/L of CPA. However, the subpostremal subnucleus showed a decrease in Kd values (24%) induced by 10 nmol/L of CPA. SHR showed the same pattern of changes as WKY within the same subnuclei; however, the modulatory effect of CPA was induced by 1 nmol/L (increased Bmax, 17%; decreased Kd, 26%). Cell culture confirmed these results, because 10(-5) and 10(-7) mol/L of CPA promoted an increase in [3H]RX821002 binding of WKY (53%) and SHR cells (48%), respectively. DPCPX, an A1R antagonist, was used to block the modulatory effect promoted by CPA with respect to Adr2R binding. In conclusion, our study shows for the first time an interaction between A1R that increases the binding of Adr2R within specific subnuclei of the NTS. This may be important in understanding the complex autonomic response induced by adenosine within the NTS. In addition, changes in interactions between receptors might be relevant to understanding the development of hypertension.

Central A1-receptor activation associated with onset of torpor protects the heart against low temperature in the Syrian hamster

Body temperature drops dramatically during hibernation, but the heart retains the ability to contract and is resistant to induction of arrhythmia. Although adaptive changes in the heart prior to hibernation may be involved in the cold-resistant property, it remains unclear whether these changes are sufficient for maintaining cardiac pulsatility under an extreme hypothermic condition. We forcibly induced hypothermia in Syrian hamsters by pentobarbital anesthesia combined with cooling of the animals. This allows reproduction of a hypothermic condition in the absence of possible hibernation-specific reactions. Unlike hypothermia in natural hibernation, the forced induction of hypothermia caused atrioventricular block. Furthermore, J-waves, which are typically observed during hypothermia in nonhibernators, were recorded on an ECG. The origin of the J-wave seemed to be related to irreversible injury of the myocardium, because J-waves remained after recovery of body temperature. An abnormal ECG was also found when hypothermia was induced in hamsters that were well adapted to a cold and darkened environment or hamsters that had already experienced hibernation. These results suggest that acclimatization prior to hibernation does not have a crucial effect at least on acquisition of cardiac resistance to low temperature. In contrast, an abnormal ECG was not observed in the case of hypothermia induced by central administration of an adenosine A1-receptor agonist and subsequent cooling, confirming the importance of the adenosine system for inducing hibernation. Our results suggest that some specific mechanisms, which may be driven by a central adenosine system, operate for maintaining the proper cardiac pulsatility under extreme hypothermia.

[3H]Adenosine uptake in brainstem membranes of CD-1 mice lacking the adenosine A2a receptor

Previous studies in our laboratory have demonstrated a decrease in [(3)H]nitrobenzylthioinosine binding sites in the brainstem of adenosine A(2a) receptor knockout mice, particularly in the brain nuclei involved in central control of cardiovascular function [Brain Research 877 (2000) 160]. The present study aimed to correlate this decrease, shown using autoradiography, with a functional change using a previously described method of [(3)H]adenosine uptake in a membrane preparation from the brainstem of wildtype CD - 1 and homozygous mutant mice lacking the adenosine A(2a) receptor. A statistically significant decrease was shown in the mean V(MAX) value obtained from homozygous mutant preparations (4.7 +/- 1.3 fmol/mg protein/20 s, P < 0.05, n = 4) compared to that obtained from wildtype controls (51.6 +/- 4.2 fmol/mg protein/20 s, n = 4). Competition studies using nucleoside uptake inhibitors showed a statistically significant increase in the log IC(50) values for dipyridamole (Wildtype: -4.3 +/- 0.2, Homozygous mutant: -8.3 +/- 0.4, n=5, P < 0.05) and dilazep (Wildtype: -3.9 +/- 0.8, Homozygous mutant: -8.3 +/- 0.8, n=5, P < 0.05) in the preparations using homozygous mutant tissue. The present study, in conjunction with the results of previous studies [Brain Research 877 (2000) 160], indicates that components of purinergic neurotransmission system have apparently adjusted in compensation for the lack of the A(2a) receptor.

Characterisation of central adenosine A(1) receptors and adenosine transporters in mice lacking the adenosine A(2a) receptor

The present study was designed to assess whether adenosine A(2a) receptor knockout mice exhibit altered purine utilisation in brain nuclei. Specifically, the properties of adenosine transporters and adenosine A(1) receptors were characterised in brain membranes and on slide-mounted sections. The B(MAX) for [(3)H]nitrobenzylthioinosine ([(3)H]NBTI) binding (adenosine transporter density) was significantly reduced in brainstem membranes of homozygotes (560+/-52 fmol/mg protein, n=5, P<0.05, Kruskal-Wallis ANOVA) compared to wildtype (1239+/-213 fmol/mg protein) and heterozygous mice (1300+/-558 fmol/mg protein). Quantitative autoradiography data indicated that [(3)H]NBTI binding in the medulla oblongata of heterozygous mice was seen to decrease significantly (P<0.05) in the subpostremal nucleus tractus solitarius (NTS), medial NTS, inferior olive and area postrema (AP). On the other hand, in the homozygous mice a decrease was seen in the medial NTS and AP. In the pons, [(3)H]1, 3-dipropyl-8-cyclopentylxanthine ([(3)H]DPCPX) (adenosine A(1) receptor density) binding increased significantly (P<0.05, Kruskal-Wallis ANOVA) in the lateral parabrachial nucleus, caudal pontine reticular nucleus and locus coeruleus of homozygotes compared to wildtype. In higher brain centres, [(3)H]NBTI binding was reduced in the paraventricular thalamic nucleus of both heterozygous and homozygous mice, whereas [(3)H]DPCPX binding was reduced in the hippocampus and lateral hypothalamus of heterozygotes. In homozygotes, [(3)H]DPCPX binding in the hippocampus increased compared to wildtype mice. The present study indicates that deletion of the A(2a) receptor may have contributed to region-specific compensatory changes in purine utilisation in brain nuclei associated with autonomic, neuroendocrine and behavioural regulation.

NTS A(2a) purinoceptor activation elicits hindlimb vasodilation primarily via a beta-adrenergic mechanism

Previously, we have shown that activation of adenosine A(2a) receptors in the subpostremal nucleus tractus solitarii (NTS) via microinjection of the selective A(2a) receptor agonist CGS-21680 elicits potent, dose-dependent decreases in mean arterial pressure and preferential, marked hindlimb vasodilation. Although A(2a) receptor activation does not change lumbar sympathetic nerve activity, it does markedly enhance the preganglionic adrenal sympathetic nerve activity, which will increase epinephrine release and could subsequently elicit hindlimb vasodilation via activation of beta(2)-adrenergic receptors. Therefore we investigated whether this hindlimb vasodilation was due to neural or humoral mechanisms. In chloralose-urethan-anesthetized male Sprague-Dawley rats, we monitored cardiovascular responses to stimulation of NTS adenosine A(2a) receptors (CGS-21680, 20 pmol/50 nl) in the intact control animals; after pretreatment with propranolol (2 mg/kg iv), a beta-adrenergic antagonist; after bilateral lumbar sympathectomy; after bilateral adrenalectomy; and after combined bilateral lumbar sympathectomy and adrenalectomy. After beta-adrenergic blockade, stimulation of NTS adenosine A(2a) receptors produced a pressor response and a hindlimb vasoconstriction. Lumbar sympathectomy reduced the vasodilation seen in the intact animals by approximately 40%, and adrenalectomy reduced it by approximately 80%. The combined sympathectomy and adrenalectomy virtually abolished the hindlimb vasodilation evoked by NTS A(2a) receptor activation. We conclude that the preferential, marked hindlimb vasodilation produced by stimulation of NTS adenosine A(2a) receptors is mediated by both the efferent sympathetic nerves directed to the hindlimb and the adrenal glands via primarily a beta-adrenergic mechanism.

Autoradiographic visualisation of axonal transport of adenosine A1 receptors along the rat vagus nerve and characterisation of adenosine A1 receptor binding in the dorsal vagal complex of hypertensive and normotensive rats

The present study had employed in vitro receptor autoradiography with [3H]DPCPX to visualise the presence of adenosine A1 receptors on the rat nodose ganglion, which contains the perikarya of vagal afferent neurons projecting the the nucleus tractus solitarius (NTS). In addition, unilateral vagal ligation resulted in an accumulation of [3H]DPCPX binding adjacent to the ligatures, indication that adenosine A1 receptors are subject to axoplasmic flow along the rat vagus nerve. Radioligand binding assays were utilised to characterise the properties of adenosine A1 receptors in the dorsal vagal complex (NTS, area postrema and dorsal motor nucleus of the vagus) of pup and adult normotensive (Wistar Kyoto, WKY) and hypertensive (spontaneously hypertensive, SHR) rats. Saturation binding indicated that the affinity (KD) of [3H]DPCPX, and the binding site density (Bmax) were not different between the adult WKY and SHR, although the pup SHR had a lower KD value than the pup WKY rat. Competition binding assays revealed complex differences between the two rat strains; however, with respect to hypertension, the affinity of the selective adenosine A1 agonist, cyclohexyladenosine (CHA), was markedly reduced in the membranes from SHR (Ki approximately 93 nM) compared to WKY (approximately 6 nM). Such an observation is consistent with the attenuated responses of SHRs to intra-NTS injections of adenosine.

Reduced responsiveness of [Ca2+]i to adenosine A1- and A2-receptor stimulation in the isoproterenol-stimulated ventricular myocytes of spontaneously hypertensive rats

To determine the modulatory action of adenosine-receptor stimulation on [Ca2+]i responses to beta-adrenoceptor stimulation in the heart of the spontaneously hypertensive rat (SHR), the electrically induced [Ca2+]i transient in response to isoproterenol (ISO) in single ventricular myocytes pretreated with adenosine agonists in SHRs and its normotensive control Wistar-Kyoto (WKY) rats was measured with a spectrofluorometric method by using fura-2/AM as the calcium indicator. In both types of rat, ISO at 0.001-1 microM augmented the electrically induced [Ca2+]i transient, and the effect was blocked by a beta-adrenoceptor blocker, propranolol. In SHRs that did not exhibit cardiac hypertrophy, the resting level of [Ca2+]i and the amplitude of the electrically induced [Ca2+]i transient were the same as those in WKY rats, whereas the augmentation of the electrically induced [Ca2+]i transient in response to ISO was significantly lower than that in WKY rats. In WKY rats, the effects of ISO on the electrically induced [Ca2+]i transient were inhibited by the adenosine A1-receptor agonist, R(-)-N6-(2-phenylisopropyl)adenosine (R-PIA) at 0.01-10 microM. In contrast, the effects of ISO were further enhanced by the adenosine A2-receptor agonist, N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl)]adenosine (DPMA) at 1-10 microM. In SHRs, the inhibitory effect of R-PIA was significantly reduced, whereas the excitatory effect of DPMA was absent. The effects of both adenosine-receptor agonists in both types of rat were abolished by the respective adenosine-receptor antagonists, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and 3,7-dimethyl-1-propargylxanthine (DMPX). The results indicate that the modulatory actions of adenosine-receptor stimulation on [Ca2+]i response to beta-adrenoceptor stimulation in the hearts of SHRs are reduced, which is independent of cardiac hypertrophy.

Autonomic neural control of cardiac function: modulation by adenosine and adenosine 5'-triphosphate

Adenosine and adenosine 5'-triphosphate (ATP) are found in every cell of the human body. These molecules are released from cells into the extracellular fluid under physiologic and pathophysiologic conditions. Outside of cells, adenosine and ATP act as physiologic regulators of cells, tissues, and organs. In the heart, extracellular adenosine and ATP exert pronounced inotropic, lusitropic, electrophysiologic, and metabolic effects, which are mediated by specific cell surface receptors. In addition, both compounds can modulate sympathetic and parasympathetic input to the heart by interacting with neural elements within and without the heart, thereby modulating autonomic neural control of cardiac functions. This article briefly reviews these indirect, neurally-mediated actions of adenosine and ATP.

Complex interactions between nitric oxide and adenosine receptors in the rat isolated nodose ganglion

The present study has employed in vitro electrophysiology, utilising the isolated rat nodose ganglion preparation, to determine whether nitric oxide (NO) and adenosine interact with each other in vagal afferent neurons. The nucleophile NO donor, diethylamine-NO, caused reproducible, concentration-related depolarisations of the isolated rat nodose ganglia. Pre-incubation of the isolated rat nodose ganglia with the adenosine A2A receptor agonists CGS 21680 (2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride) and DPMA (N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)-ethyl]adenosine) (both 1 microM) resulted in a functional antagonism of the ability of diethylamine-NO to depolarise the preparation. A similar effect was observed with adenosine (10 microM) only in the presence of the adenosine A1 receptor antagonist PACPX (1,3-dipropyl-8-(2-amino-4-chlorophenyl)-xanthine, 100 nM). Conversely, the adenosine A1 receptor agonists ENBA (N6-[2-endo-norbomyl]adenosine, 1 microM) and cyclohexyladenosine (100 nM) potentiated the effect of diethylamine-NO on isolated rat nodose ganglia. Inclusion of either adenosine A3 agonists or ATP had no effect on the diethylamine-NO concentration-response curve. These data suggest an ability of NO to interact, in opposing manner, with adenosine A2A and A1 receptors in rat vagal afferent neurons. On the other hand, neither A3 receptors nor ATP appear capable of interacting with NO.

Purinergic modulation of neural control of cardiac function

1. The purine nucleotide adenosine 5'-triphosphate (ATP) and its related nucleoside, adenosine (Ado), exert pronounced electrophysiologic, inotropic, lusitropic and metabolic effects in the mammalian heart. 2. These effects are the result of direct actions of these compounds on cardiac myocytes and endothelial cells, mediated by cell surface receptors. 3. In addition, ATP and Ado can stimulate neural elements inside and outside the heart and thereby modulate neural control of cardiac function. These latter actions of ATP and Ado are briefly reviewed and their hypothetical physiological role is outlined.

Neurochemical modulation of cardiovascular control in the nucleus tractus solitarius

The central control of cardiovascular function has been keenly studied for a number of decades. Of particular interest are the homeostatic control mechanisms, such as the baroreceptor heart-rate reflex, the chemoreceptor reflex, the Bezold-Jarisch reflex and the Breuer-Hering reflex. These neurally-mediated reflexes share a common termination point for their respective centrally-projecting sensory afferents, namely the nucleus tractus solitarius (NTS). Thus, the NTS clearly plays a critical role in the integration of peripherally initiated sensory information regarding the status of blood pressure, heart rate and respiratory function. Many endogenous neurochemicals, from simple amino acids through biogenic amines to complex peptides have the ability to modulate blood pressure and heart rate at the level of the NTS. This review will attempt to collate the current knowledge regarding the roles of neuromodulators in the NTS, the receptor types involved in mediating observed responses and the degree of importance of such neurochemicals in the tonic regulation of the cardiovascular system. The neural pathway that controls the baroreceptor heart-rate reflex will be the main focus of attention, including discussion of the identity of the neurotransmitter(s) thought to act at baroafferent terminals within the NTS. In addition, this review will provide a timely update on the use of recently developed molecular biological techniques that have been employed in the study of the NTS, complementing more classical research.