Contrasting excitatory and inhibitory effects of adenosine in blood pressure regulation

Acute caffeine reverses the disruptive effects of chronic fluoxetine on the sexual behavior of female and male rats

RATIONALE

Sexual side effects of chronic treatment with selective serotonin reuptake inhibitors (SSRIs) in humans include anorgasmia and loss of sexual desire and/or arousal which interferes with treatment compliance. There are few options at present to reduce these effects. Because orgasm and desire are mediated in part by activation of sympathetic arousal, we asked whether the sympathomimetic effects of acute caffeine treatment could reverse these effects.

OBJECTIVE

The present study examined whether acute treatment with caffeine (CAF; 10 or 20 mg/kg, ip) versus vehicle could ameliorate the disruption of appetitive and consummatory measures of copulatory behavior produced by chronic fluoxetine (10 mg/kg, sc) in adult, sexually active female or male rats.

METHODS

Sexually experienced female or male rats received daily injections of FLU over a 24-day period and were tested for sexual behaviors five times at 4-day intervals during this period in bilevel pacing chambers. Females had been ovariectomized and given hormone replacement with estradiol benzoate and progesterone prior to each test. Males were left gonadally intact. Four days after the final FLU test, rats were randomly assigned to one of the three doses of CAF and received ip injections of CAF or the saline vehicle 60 min before testing.

RESULTS

Chronic FLU reduced solicitations and lordosis over time in females and reduced the number of ejaculations in males. Both doses of CAF restored solicitations and lordosis in females and ejaculations in males. On their own, both doses of CAF increased females' pacing behavior and the number of mounts and intromissions in the males.

CONCLUSIONS

Stimulation of sympathetic outflow by CAF may constitute a readily accessible on-demand treatment for the sexual side-effects of SSRIs.

Use of knockout mice to explore CNS effects of adenosine

The initial exploration using pharmacological tools of the role of adenosine receptors in the brain, concluded that adenosine released as such acted on A₁R to inhibit excitability and glutamate release from principal neurons throughout the brain and that adenosine A_2A receptors (A_2AR) were striatal-'specific' receptors controlling dopamine D₂R. This indicted A₁R as potential controllers of neurodegeneration and A_2AR of psychiatric conditions. Global knockout of these two receptors questioned the key role of A₁R and instead identified extra-striatal A_2AR as robust controllers of neurodegeneration. Furthermore, transgenic lines with altered metabolic sources of adenosine revealed a coupling of ATP-derived adenosine to activate A_2AR and a role of A₁R as a hurdle to initiate neurodegeneration. Additionally, cell-selective knockout of A_2AR unveiled the different roles of A_2AR in different cell types (neurons/astrocytes) in different portions of the striatal circuits (dorsal versus lateral) and in different brain areas (hippocampus/striatum). Finally, a new transgenic mouse line with deletion of all adenosine receptors seems to indicate a major allostatic rather than homeostatic role of adenosine and may allow isolating P2R-mediated responses to unravel their role in the brain, a goal close to heart of Geoffrey Burnstock, to whom we affectionately dedicate this review.

Comparison of the effects of ticagrelor and clopidogrel on heart rate variability and heart rate turbulence in patients with percutaneous coronary interventions

Introduction

Clopidogrel and ticagrelor are commonly used, antiplatelet agents. Ticagrelor has an effect of enhancing the plasma level of adenosine that may alter the autonomic function. The aim of this study is to compare the effects of ticagrelor and clopidogrel on heart rate variability (HRV) and heart rate turbulence (HRT).

Methods

Thirty subjects who performed percutaneous coronary intervention (PCI) were included in the randomized, crossover study. These patients were divided into two groups. Clopidogrel or ticagrelor was administered in two different testing sessions (1-month treatment for each session). In group 1, clopidogrel and ticagrelor treatment were used while in group 2, ticagrelor and clopidogrel treatment were used respectively. Three times rhythm Holter recording (baseline, 1st and 2nd month) was performed. The HRV (time domain and frequency domain) and HRT (turbulence onset (TO) and turbulence slope (TS)) parameters were analyzed from the Holter recordings.

Results

According to baseline Holter recording, pNN50 (5.82 ± 5.83 vs 10.56 ± 8.28; p = 0.03) and HF(nu) (6.85 ± 9.33 vs 9.53 ± 7.41; p = 0.04) parameters were higher in group 2 than in group 1, while TO(0.004 ± 0.02 vs −0.01 ± 0.02; p = 0.01) parameter was positive and higher in group 1 than in group 2. In the second month, the LF/HF ratio (4.47 ± 2.43 vs 3.18 ± 2.45; p = 0.04) was higher in group 1 than in group 2. However, when the evaluation was done within the groups themselves, there were no statistically significant differences in HRV and HRT parameters obtained before and after clopidogrel and ticagrelor administration in group 1 and group 2.

Conclusion

Ticagrelor and clopidogrel treatments did not have a significant effect on HRV and HRT parameters.

Evaluation of acetylcholine, seizure activity and neuropathology following high-dose nerve agent exposure and delayed neuroprotective treatment drugs in freely moving rats

Organophosphorus nerve agents such as soman (GD) inhibit acetylcholinesterase, producing an excess of acetylcholine (ACh), which results in respiratory distress, convulsions and status epilepticus that leads to neuropathology. Several drugs (topiramate, clobazam, pregnanolone, allopregnanolone, UBP 302, cyclopentyladenosine [CPA], ketamine, midazolam and scopolamine) have been identified as potential neuroprotectants that may terminate seizures and reduce brain damage. To systematically evaluate their efficacy, this study employed in vivo striatal microdialysis and liquid chromatography to respectively collect and analyze extracellular ACh in freely moving rats treated with these drugs 20 min after seizure onset induced by a high dose of GD. Along with microdialysis, EEG activity was recorded and neuropathology assessed at 24 h. GD induced a marked increase of ACh, which peaked at 30 min post-exposure to 800% of control levels and then steadily decreased toward baseline levels. Approximately 40 min after treatment, only midazolam (10 mg/kg) and CPA (60 mg/kg) caused a significant reduction of ACh levels, with CPA reducing ACh levels more rapidly than midazolam. Both drugs facilitated a return to baseline levels at least 55 min after treatment. At 24 h, only animals treated with CPA (67%), midazolam (18%) and scopolamine (27%) exhibited seizure termination. While all treatments except for topiramate reduced neuropathology, CPA, midazolam and scopolamine showed the greatest reduction in pathology. Our results suggest that delayed treatment with CPA, midazolam, or scopolamine is effective at reducing GD-induced seizure activity and neuropathology, with CPA and midazolam capable of facilitating a reduction in GD-induced ACh elevation.

Limitations and Pitfalls of Fractional Flow Reserve Measurements and Adenosine-Induced Hyperemia

Coronary hemodynamic measurements provide a critical tool to assess the ischemic potential of coronary stenoses. Fractional flow reserve (FFR) is a reliable method to relate translesional coronary pressures to hyperemic myocardial blood flow. Although a basic understanding in FFR can be quickly achieved, many of the nuances and potential pitfalls require special attention. The authors discuss the practical setup of coronary pressure measurement, the most common pitfalls in technique and ways to avoid them, and the limitations of available pharmacologic hyperemic methods.

Advances in coronary physiology

Pressure-wire technology, most typically fractional flow reserve (FFR), has provided interventional cardiologists with a means of determining the physiological importance of a stenosis during angiography. There has been renewed interest in coronary physiology in the light of guideline recognition, ongoing clinical research and new technologies changing the paradigm of how assessment is performed in the catheter laboratory. We reflect on FFR, with regards the potential effects of changing hemodynamics on FFR and the latest evidence with regards to outcomes. We also review the instantaneous wave-free ratio (iFR), a new pressure-only index, measured at rest, that is under active evaluation in several international randomized controlled trials. We review the accumulated evidence and discuss the important physiological concepts between pressure and flow that underlie the approach to using resting indices. Finally we investigate future developments, including physiological mapping with iFR-Pullback and the potential to predict the hemodynamic effect of stenting.

Hemodynamic response to intravenous adenosine and its effect on fractional flow reserve assessment: results of the Adenosine for the Functional Evaluation of Coronary Stenosis Severity (AFFECTS) study

BACKGROUND

We studied the hemodynamic response to intravenous adenosine on calculation of fractional flow reserve (FFR). Intravenous adenosine is widely used to achieve conditions of stable hyperemia for measurement of FFR. However, intravenous adenosine affects both systemic and coronary vascular beds differentially.

METHODS AND RESULTS

A total of 283 patients (310 coronary stenoses) underwent coronary angiography with FFR using intravenous adenosine 140 mcg/kg per minute via a central femoral vein. Offline analysis was performed to calculate aortic (Pa), distal intracoronary (Pd), and reservoir (Pr) pressure at baseline, peak, and stable hyperemia. Seven different hemodynamic patterns were observed according to Pa and Pd change at peak and stable hyperemia. The average time from baseline to stable hyperemia was 68.2±38.5 seconds, when both ΔPa and ΔPd were decreased (ΔPa, -10.2±10.5 mm Hg; ΔPd, -18.2±10.8 mm Hg; P<0.001 for both). The fall in Pa closely correlated with the reduction in peripheral Pr (ΔPr, -12.9±15.7 mm Hg; P<0.001; r=0.9; P<0.001). ΔPa and ΔPd were closely related under conditions of peak (r=0.75; P<0.001) and stable hyperemia (r=0.83; P<0.001). On average, 56% (10.2 mm Hg) of the reduction in Pd was because of fall in Pa. FFR lesion classification changed in 9% using an FFR threshold of ≤0.80 and 5.2% with FFR threshold <0.75 when comparing Pd/Pa at peak and stable hyperemia.

CONCLUSIONS

Intravenous adenosine results in variable changes in systemic blood pressure, which can lead to alterations in FFR lesion classification. Attention is required to ensure FFR is measured under conditions of stable hyperemia, although the FFR value at this point may be numerically higher.

Regulation of atherosclerosis and associated risk factors by adenosine and adenosine receptors

Adenosine is an endogenous metabolite that has an anti-inflammatory effect across the vasculature. Extracellular adenosine activates 4 G-protein coupled receptors (A1, A3, A2A, and A2B) whose expression varies in different cells and tissues, including the vasculature and blood cells. Higher levels of adenosine are generated during stress, inflammation, and upon tissue damage. Some of the adenosine receptors (AR), such as the A2BAR, are further up-regulated following such stresses. This review discusses the role of adenosine and adenosine receptors in the development of atherosclerosis and some of the risk factors associated with this pathology. These include adenosine receptor-regulated changes in atherosclerosis, blood pressure, thrombosis, and myocardial infarction. Potential therapeutic applications are reviewed, as well as reasons for phenotypic differences occasionally observed between receptor knockout and pharmacological inhibition via drug administration.

Acute adenosine increases cardiac vagal and reduces sympathetic efferent nerve activities in rats

Adenosine is the first drug of choice in the treatment of supraventricular arrhythmias. While the effects of adenosine on sympathetic nerve activity (SNA) have been investigated, no information is available on the effects on cardiac vagal nerve activity (VNA). We assessed in rats the responses of cardiac VNA, SNA and cardiovascular variables to intravenous bolus administration of adenosine. In 34 urethane-anaesthetized rats, cardiac VNA or cervical preganglionic sympathetic fibres were recorded together with ECG, arterial pressure and ventilation, before and after administration of three doses of adenosine (100, 500 and 1000 μg kg(-1)). The effects of adenosine were also assessed in isolated perfused hearts (n = 5). Adenosine induced marked bradycardia and hypotension, associated with a significant dose-dependent increase in VNA (+204 ± 56%, P < 0.01; +275 ± 120%, P < 0.01; and +372 ± 78%, P < 0.01, for the three doses, respectively; n = 7). Muscarinic blockade by atropine (5 mg kg(-1), i.v.) significantly blunted the adenosine-induced bradycardia (-56.0 ± 4.5%, P < 0.05; -86.2 ± 10.5%, P < 0.01; and -34.3 ± 9.7%, P < 0.01, respectively). Likewise, adenosine-induced bradycardia was markedly less in isolated heart preparations. Previous barodenervation did not modify the effects of adenosine on VNA. On the SNA side, adenosine administration was associated with a dose-dependent biphasic response, including overactivation in the first few seconds followed by a later profound SNA reduction. Earliest sympathetic activation was abolished by barodenervation, while subsequent sympathetic withdrawal was affected neither by baro- nor by chemodenervation. This is the first demonstration that acute adenosine is able to activate cardiac VNA, possibly through a central action. This increase in vagal outflow could make an important contribution to the antiarrhythmic action of this substance.

Targeting adenosine receptors in the development of cardiovascular therapeutics

Adenosine receptor stimulation has negative inotropic and dromotropic actions, reduces cardiac ischemia-reperfusion injury and remodeling, and prevents cardiac arrhythmias. In the vasculature, adenosine modulates vascular tone, reduces infiltration of inflammatory cells and generation of foam cells, and may prevent the development of atherosclerosis as a result. Modulation of insulin sensitivity may further add to the anti-atherosclerotic properties of adenosine signaling. In the kidney, adenosine plays an important role in tubuloglomerular feedback and modulates tubular sodium reabsorption. The challenge is to take advantage of the beneficial actions of adenosine signaling while preventing its potential adverse effects, such as salt retention and sympathoexcitation. Drugs that interfere with adenosine formation and elimination or drugs that allosterically enhance specific adenosine receptors seem to be most promising to meet this challenge.

Modulation by salt intake of the vascular response mediated through adenosine A(2A) receptor: role of CYP epoxygenase and soluble epoxide hydrolase

High-salt intake can change the effect of adenosine on arterial tone in mice. The aim of this study was to clarify the mechanism by which this occurs. Using aortas from mice fed a 4% NaCl (HS) or 0.45% NaCl (NS) diet for 4-5 wks, concentration-response curves for ACh, 5'-N-ethylcarboxamidoadenosine (NECA; adenosine analog) and 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride hydrate [CGS-21680; A(2A) adenosine receptor (A(2A) AR) agonist] were obtained with N(omega)-nitro-L-arginine methyl ester (L-NAME; nitric oxide inhibitor, 10(-4) M), methylsulfonyl-propargyloxyphenylhexanamide [MS-PPOH; a CYP (cytochrome P-450) epoxygenase blocker, 10(-5) M including CYP2J2], 12-(3-adamantan-1-yl-ureido)dodecanoic acid [AUDA; soluble epoxide hydrolase (sEH) blocker, 10(-5) M], dibromo-dodecenyl-methylsulfimide [DDMS; CYP omega-hydroxylase (CYP4A blocker), 10(-5) M], glibenclamide (K(ATP) channel blocker; 10(-5) M) and 5-hydroxydecanoate (5-HD; mitochondrial-K(ATP) channel blocker, 10(-4) M). HS dose response to ACh (10(-7) - 10(-5) M) was not different from NS (P > 0.05). Relaxation to 10(-6) M NECA was greater in the HS group (28.4 +/- 3.9%) than in the NS group (4.1 +/- 2.3%). Relaxation to 10(-6) M CGS-21680 was also greater in HS (27.9 +/- 4.5%) than in NS (4.9 +/- 2.2%). L-NAME was able to block the dose response of ACh (10(-7) - 10(-5) M) equally in both HS and NS (P > 0.05), whereas L-NAME did not block CGS-21680-induced response in HS. In HS the CGS-21680 response was greatly reduced by MS-PPOH (to 4.7 +/- 2.0%) and 5-HD (to 8.9 +/- 2.2%), and also abolished by glibenclamide (-1.0 +/- 5.9%). In NS, the CGS-21680 response was increased by AUDA (to 26.3 +/- 3.4%) and DDMS (to 27.2 +/- 3.0%). Compared with NS, HS vessels showed increased CYP2J2 and A(2A) AR expression (46 and 74% higher, respectively) but decreased sEH, CYP4A, and A(1) AR expression (75, 30, and 55% lower, respectively). These data suggest that in mice fed NS-containing diet, upregulation of arterial A(1) receptor causes vasoconstriction via increased sEH and CYP4A proteins. However, in mice fed HS-containing diet, upregulation of A(2A) receptor protein triggers vascular relaxation through ATP-sensitive (K(+)) channels via upregulation of CYP2J2 enzyme.

Physiological roles of A1 and A2A adenosine receptors in regulating heart rate, body temperature, and locomotion as revealed using knockout mice and caffeine

Heart rate (HR), body temperature (Temp), locomotor activity (LA), and oxygen consumption (O(2)C) were studied in awake mice lacking one or both of the adenosine A(1) or A(2A) receptors (A(1)R or A(2A)R, respectively) using telemetry and respirometry, before and after caffeine administration. All parameters were lower during day than night and higher in females than males. When compared with wild-type (WT) littermates, HR was higher in male A(1)R knockout (A(1)RKO) mice but lower in A(2A)RKO mice and intermediate in A(1)-A(2A)R double KO mice. A single dose of an unselective beta-blocker (timolol; 1 mg/kg) abolished the HR differences between these genotypes. Deletion of A(1)Rs had little effect on Temp, whereas deletion of A(2A)Rs increased it in females and decreased it in males. A(1)-A(2A)RKO mice had lower Temp than WT mice. LA was unaltered in A(1)RKO mice and lower in A(2A)RKO and A(1)-A(2A)RKO mice than in WT mice. Caffeine injection increased LA but only in mice expressing A(2A)R. Caffeine ingestion also increased LA in an A(2A)R-dependent manner in male mice. Caffeine ingestion significantly increased O(2)C in WT mice, but less in the different KO mice. Injection of 30 mg/kg caffeine decreased Temp, especially in KO mice, and hence in a manner unrelated to A(1)R or A(2A)R blockade. Selective A(2B) antagonism had little or no effect. Thus A(1)R and A(2A)R influence HR, Temp, LA, and O(2)C in mice in a sex-dependent manner, indicating effects of endogenous adenosine. The A(2A)R plays an important role in the modulation of O(2)C and LA by acute and chronic caffeine administration. There is also evidence for effects of higher doses of caffeine being independent of both A(1)R and A(2A)R.

High-salt diet enhances mouse aortic relaxation through adenosine A2A receptor via CYP epoxygenases

We hypothesize that A(2A) adenosine receptors (A(2A) AR) promote aortic relaxation in mice through cytochrome P450 (CYP)-epoxygenases and help to avoid salt sensitivity. Aortas from male mice maintained on a high-salt (HS; 7% NaCl) or normal-salt (NS; 0.45% NaCl) diet for 4-5 wks were used. Concentration-response curves (10(-11)-10(-5) M) for 5'-N-ethylcarboxamidoadenosine (NECA; a nonselective adenosine analog) and CGS 21680 (A(2A) AR agonist) were obtained with different antagonists including ZM 241385 (A(2A) AR antagonist; 10(-6) M), SCH 58261 (A(2A) AR antagonist; 10(-6) M), N(omega)-nitro-l-arginine methyl ester (l-NAME; endothelial nitric oxide synthase inhibitor; 10(-4) M) and inhibitors including methylsulfonyl-propargyloxyphenylhexanamide (MS-PPOH; CYP epoxygenases inhibitor; 10(-5)M), 14,15-epoxyeicosa-5(z)-enoic acid (14,15-EEZE; EET antagonist; 10(-5)M), dibromo-dodecenyl-methylsulfimide (DDMS; CYP4A inhibitor; 10(-5)M), and HET0016 (20-HETE inhibitor; 10(-5)M). At 10(-7) M of NECA, significant relaxation in HS (+22.58 +/- 3.12%) was observed compared with contraction in NS (-10.62 +/- 6.27%, P < 0.05). ZM 241385 changed the NECA response to contraction (P < 0.05) in HS. At 10(-7) M of CGS 21680, significant relaxation in HS (+32.04 +/- 3.08%) was observed compared with NS (+10.45 +/- 1.34%, P < 0.05). SCH 58261, l-NAME, MS-PPOH, and 14,15-EEZE changed the CGS 21680-induced relaxation to contraction (P < 0.05) in HS. Interestingly, DDMS and HET0016 changed CGS 21680 response to relaxation (P < 0.05) in NS; however, there was no significant difference found between DDMS, HET0016-treated HS and NS vs. nontreated HS group (P > 0.05). CYP2C29 protein was 55% and 74% upregulated in HS vs. NS (P < 0.05) mice aorta and kidney, respectively. CYP4A protein was 30.30% and 35.70% upregulated in NS vs. HS (P < 0.05) mice aorta and kidneys, respectively. A(1) AR was downregulated, whereas A(2A) AR was upregulated in HS compared with NS. These data suggest that HS may activate CYP2C29 via A(2A) AR, causing relaxation, whereas NS may contribute to the upregulation of CYP4A causing contraction.

Snake venom dipeptidyl peptidase IV: taxonomic distribution and quantitative variation

The present study examined the taxonomic distribution of dipeptidyl peptidase IV (DPP IV) activity in venoms of 59 ophidian taxa, representing seven subfamilies of the Families Elapidae and Viperidae. DPP IV activity is extremely variable at all taxonomic levels. It ranged from essentially none in laticaudine, hydrophiine, and some bungarine and elapine venoms, to 10.72 mumol 4-methoxy-beta-naphthylamine liberated per min per 200 mug venom, for Ophiophagus hannah. Intra- and interpopulational variation were examined among eight populations of prairie rattlesnakes (Crotalus viridis viridis), Great Basin rattlesnakes (Crotalus viridis lutosus) and southern Pacific rattlesnakes (Crotalus viridis helleri). Among these populations, the mean weighted range of variation was 4.9-fold, and even among litter mates of C. v. lutosus, DPP IV activity varied as much as 5.6-fold. The two most salient findings, the near ubiquity of DPP IV in snake venoms and its great quantitative variability, even among full siblings, are paradoxical. The widespread distribution of the enzyme suggests an important role in envenomation, while the variable activity levels suggest that DPP IV and by extension, other individual enzymatic constituents, may not be under much individual selective pressure.

Blockade of adenosine A2A receptors reverses short-term social memory impairments in spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) exhibit impairment across several cognitive domains such as attention, short-term memory and spatial reference memory. These cognitive deficits have been variously attributed to disrupted dopaminergic, cholinergic and adenosinergic neurotransmitter function. However, social memory in SHR has not been investigated. In the present study, we therefore evaluated whether SHR exhibit altered short-term social memory abilities compared to normotensive Wistar rats (WIS) through two experimental paradigms (social recognition and habituation-dishabituation tests). We also compared the performance of SHR and WIS rats in the object recognition test. SHR exhibited significantly impaired performance in both models of social memory, but not in the object recognition test, demonstrating a selective deficit in the ability to recognize a juvenile rat after a short period of time. The administration of acute doses of the non-selective adenosine receptor antagonist caffeine (3.0 or 10.0 mg/kg, i.p.) and the adenosine A2A receptor antagonist 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo-[2,3-a][1,3,5]triazin-5-yl-amino]ethyl) phenol (ZM241385, 0.5 or 1.0 mg/kg, i.p.) but not the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1.0 or 3.0 mg/kg, i.p.) reversed this social memory impairment in SHR, but these treatments did not alter the hypertension state. These results demonstrate an impairment of short-term social memory in SHR and the involvement of the adenosine A2A receptors in this alteration.

Influence of L-nitro-arginine methyl ester, acetylcholine, and adenosine on mean blood pressure, pulse pressure, and pulse pressure amplification in rats

The blood pressure pattern in spontaneously hypertensive rats (SHRs) involves three main characteristics: increase in mean blood pressure (MBP); increase in thoracic aorta (proximal) and iliac (distal) pulse pressure (PP); disappearance of the normal PP amplification between the proximal and the distal arteries. Whether pharmacologic agents may reduce MBP with different or even opposite effects regarding PP and PP amplification has been poorly investigated. In SHRs and Wistar-Kyoto (WKY) anesthetized rats, the NO inhibitor l-nitro-arginine methyl ester (l-NAME) was infused at the dosage of 1 mg/kg for 30 min. Before and after infusion, 7 microg/kg/min acetylcholine (Ach) and 200 mg/kg adenosine (Ado) were perfused for 4 min. Proximal and distal intra-arterial BP was monitored throughout the procedure. In both WKYs and SHRs, l-NAME increased proximal and distal systolic (SBP), diastolic (DBP), and MBP but not PP. Before l-NAME, SBP, DBP, and MBP were significantly reduced by Ado and Ach. After l-NAME, such blood pressure reductions were abolished with Ach but not Ado. In both strains, the proximal and distal PP, when expressed in percent reduction of MBP, were significantly higher under Ado than under Ach. The Ado but not Ach changed PP amplification, causing a reduction in WKYs and an increase in SHRs independent of l-NAME. Vasodilating agents may reduce MBP with significantly different effects on PP. The Ado alters PP amplification, an effect not obtained with the nitric oxide endothelium-dependent vasorelaxing agent Ach. Tail SBP measurements cannot predict such dissociated changes.

Sympathetic nerve activity in end-stage renal disease

BACKGROUND

Uremia is proposed to increase sympathetic nerve activity (SNA) in hemodialysis patients. The aims of the present study were to determine whether reversal of uremia by successful kidney transplantation (RTX) eliminates the increased SNA and whether signals arising in the diseased kidneys contribute to the increased SNA in renal failure.

METHODS AND RESULTS

We compared muscle sympathetic nerve activity (MSNA) in 13 hemodialysis patients wait-listed for RTX and in renal transplantation patients with excellent graft function treated with cyclosporine (RTX-CSA, n=13), tacrolimus (RTX-FK, n=13), or without calcineurin inhibitors (RTX-Phi, n=6), as well as in healthy volunteers (CON, n=15). In addition to the above patients with present diseased native kidneys, we studied 16 RTX patients who had undergone bilateral nephrectomy (RTX-NE). Data are mean+/-SEM. MSNA was significantly elevated in hemodialysis patients (43+/-4 bursts/min), RTX-CSA (44+/-5 bursts/min), RTX-FK (34+/-3 bursts/min), and RTX-Phi (44+/-5 bursts/min) as compared with CON (21+/-3 bursts/min), despite excellent graft function after RTX. RTX-NE had significantly reduced MSNA (20+/-3 bursts/min) when compared with RTX patients. MSNA did not change significantly with RTX in 4 hemodialysis patients studied before and after RTX (44+/-6 versus 43+/-5 bursts/min, P=NS). In contrast, nephrectomy resulted in reduced MSNA in all 6 RTX patients studied before and after removal of the second native kidney.

CONCLUSIONS

Despite correction of uremia, increased SNA is observed in renal transplant recipients with diseased native kidneys at a level not significantly different from chronic hemodialysis patients. The increased SNA seems to be mediated by signals arising in the native kidneys that are independent of circulating uremia related toxins.

Role of endothelial adenosine receptor-mediated vasorelaxation in ethanol-induced hypotension in hypertensive rats

Our previous findings showed that chronic ethanol feeding lowers blood pressure in spontaneously hypertensive rats. The present study investigated the role of the adenosine receptor-endothelial nitric oxide (NO) pathway in the hypotensive response to ethanol. Changes in blood pressure were evaluated in radiotelemetered pair-fed rats receiving liquid diet with or without ethanol (2.5% or 5%, w/v) for 12 weeks. The vasorelaxant activity of the adenosine analogue 5'-N-ethylcarboxamidoadenosine (NECA) in isolated aortic rings obtained from ethanol and control rats were evaluated. Ethanol (2.5% and 5%) lowered blood pressure in a dose-dependent manner. The hypotension started at week 1, reached its maximum at week 4 and remained so thereafter. In aortas with intact endothelium, NECA (10(-10) to 10(-4) M) produced a concentration-dependent relaxation of the phenylephrine-precontracted aortas. Compared with control rats, ethanol (2.5% and 5%) caused significant and concentration-related increases in NECA responses. This effect of ethanol was attenuated by the adenosine receptor antagonist 8-sulfophenyltheophylline and the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA). Further, endothelium denudation abolished the ethanol-evoked enhancement of NECA responses. The vasorelaxant responses to acetylcholine or sodium nitroprusside in aortic rings were not influenced by ethanol. In conclusion, the present findings suggest that chronic ethanol enhances the NO-dependent vasorelaxant responses to adenosine receptor activation and this may explain, at least partly, the mechanism of the hypotensive effect of ethanol in spontaneously hypertensive rats.

Ophidian envenomation strategies and the role of purines

Snake envenomation employs three well integrated strategies: prey immobilization via hypotension, prey immobilization via paralysis, and prey digestion. Purines (adenosine, guanosine and inosine) evidently play a central role in the envenomation strategies of most advanced snakes. Purines constitute the perfect multifunctional toxins, participating simultaneously in all three envenomation strategies. Because they are endogenous regulatory compounds in all vertebrates, it is impossible for any prey organism to develop resistance to them. Purine generation from endogenous precursors in the prey explains the presence of many hitherto unexplained enzyme activities in snake venoms: 5'-nucleotidase, endonucleases (including ribonuclease), phosphodiesterase, ATPase, ADPase, phosphomonoesterase, and NADase. Phospholipases A(2), cytotoxins, myotoxins, and heparinase also participate in purine liberation, in addition to their better known functions. Adenosine contributes to prey immobilization by activation of neuronal adenosine A(1) receptors, suppressing acetylcholine release from motor neurons and excitatory neurotransmitters from central sites. It also exacerbates venom-induced hypotension by activating A(2) receptors in the vasculature. Adenosine and inosine both activate mast cell A(3) receptors, liberating vasoactive substances and increasing vascular permeability. Guanosine probably contributes to hypotension, by augmenting vascular endothelial cGMP levels via an unknown mechanism. Novel functions are suggested for toxins that act upon blood coagulation factors, including nitric oxide production, using the prey's carboxypeptidases. Leucine aminopeptidase may link venom hemorrhagic metalloproteases and endogenous chymotrypsin-like proteases with venom L-amino acid oxidase (LAO), accelerating the latter. The primary function of LAO is probably to promote prey hypotension by activating soluble guanylate cyclase in the presence of superoxide dismutase. LAO's apoptotic activity, too slow to be relevant to prey capture, is undoubtedly secondary and probably serves principally a digestive function. It is concluded that the principal function of L-type Ca(2+) channel antagonists and muscarinic toxins, in Dendroaspis venoms, and acetylcholinesterase in other elapid venoms, is to promote hypotension. Venom dipeptidyl peptidase IV-like enzymes probably also contribute to hypotension by destroying vasoconstrictive peptides such as Peptide YY, neuropeptide Y and substance P. Purines apparently bind to other toxins which then serve as molecular chaperones to deposit the bound purines at specific subsets of purine receptors. The assignment of pharmacological activities such as transient neurotransmitter suppression, histamine release and antinociception, to a variety of proteinaceous toxins, is probably erroneous. Such effects are probably due instead to purines bound to these toxins, and/or to free venom purines.

Oxidative stress enhances the production and actions of adenosine in the kidney

The purpose of this study was to determine whether superoxide anions (O.) activate 5'-nucleotidase (5'-ND), thereby increasing the production of renal adenosine and regulating renal function. Using HPLC analysis, we found that incubation of renal tissue homogenate with the O. donor KO(2) doubled adenosine production and increased the maximal reaction velocity of 5'-ND from 141 to 192 nmol. min(-1). mg protein(-1). The O.-generating system, xanthine/xanthine oxidase increased the maximal reaction velocity of 5'-ND from 122 to 204 nmol. min(-1). mg protein(-1). Superoxide dismutase (SOD) with catalase produced a concentration-dependent reduction of 5'-ND activity in renal tissue homogenate, while the SOD inhibitor diethyldithiocarbamic acid significantly increased 5'-ND activity. Inhibition of disulfide bond formation by thioredoxin or thioredoxin reductase significantly decreased xanthine/xanthine oxidase-induced activation of renal 5'-ND. In in vivo experiments, inhibition of SOD by diethyldithiocarbamic acid (0.5 mg. kg(-1). min(-1) iv) enhanced renal vasoconstriction induced by endogenously produced adenosine and increased renal tissue adenosine concentrations under control condition and in ischemia and reperfusion. We conclude that oxidative stress activates 5'-ND and increases adenosine production in the kidney and that this redox regulatory mechanism of adenosine production is important in the control of renal vascular tone and glomerular perfusion.

Evidence for the presence of A(1) adenosine receptors in the aorta of spontaneously hypertensive rats

1. Isolated aortic rings (endothelium-intact and -denuded) from spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats were used in this study to examine the vasoactive effects of various adenosine analogues. 2. In phenylephrine contracted aortic rings, concentration-response curves were constructed by cumulative additions (10(-11) - 10(-5) M) of (2S)-N(6)-[2-endo-Norbornyl] adenosine (ENBA), N(6)-cyclopentyladenosine (CPA), R-N(6)-(2-phenylisopropyl) adenosine (R-PIA), 2-p-(-2-carboxyethyl) phenethylamino-5'-N-thylcarboxamido adenosine (CGS-21680). 3. A non-specific adenosine receptor agonist 2-chloroadenosine (CAD) resulted in biphasic response with a small contraction at lower concentrations (10(-9) - 10(-8) M) followed by a significant relaxation at higher concentration in endothelium-intact SHR tissues, suggesting presence of both A(1) and A(2) adenosine receptors in SHR aorta. However, only relaxation was observed in WKY. 4. Contractile response in SHR had the following rank order of potency: ENBA>CPA>R-PIA>CAD. The relaxation response in SHR and WKY had the following rank order of potency: CGS 21680>CAD>R-PIA>CPA>ENBA. 5. Removal of endothelium abolished the adenosine analogue induced contractions in SHR aorta and attenuated the vasorelaxation responses in the WKY and SHR. 6. The contractile response in SHR was abolished by A(1) adenosine receptor antagonist N(6)-endonorbornan-2-yl-9-methyladenine (N-0861). A(2) adenosine receptor antagonist, 3,7-dimethyl-1-proparglyxanthine (DMPX) did not affect the contraction response of adenosine analogues. 7. Endothelium-dependent contractions elicited by A(1) receptor agonists were blocked by indomethacin and by free radical scavengers. 8. These data suggest that the contractile response to adenosine analogues in SHR aorta is probably mediated by free radicals which are generated through the increased cyclo-oxygenase activity occurring in the vascular endothelium of SHR but not the WKY rats.

Role of endothelium in adenosine receptor-mediated vasorelaxation in hypertensive rats

The present study was designed to investigate the role of endothelium derived relaxing factor nitric oxide (NO) in adenosine A2 receptor mediated vasorelaxation in normotensive (WKY) and hypertensive (SHR) rat aortic ring preparations. Adenosine analogues, 2-chloroadenosine (CAD) and 5-ethylcarboxamidoadenosine (NECA) produced concentration-dependent (10(-9)-10(-4) M) relaxation in phenylephrine (1 x 10(-6) M) precontracted vascular rings, which was significantly shifted to the right in SHR compared to WKY rats. Endothelium removal attenuated CAD and NECA relaxation responses in both SHR and WKY and abolished the difference in relaxation between SHR and WKY vascular tissues. The relaxation response to CAD was antagonised by adenosine A2 receptor antagonist, 8-sulfophenyltheophylline (8-SPT, 50 x 10(-6) M). The antagonism by 8-SPT was lower in SHR as compared to WKY tissues. L-monomethylarginine (L-LMMA) (30 x 10(-6) M) significantly shifted the CAD relaxation to the right, which was reversed by the addition of L-arginine (100 x 10(-6) M) in both SHR and WKY rats. However, the rightward shift by L-NMMA was smaller in SHR compared to WKY vascular tissues. Vasorelaxation response to acetylcholine (1 x 10(-6) M) was significantly inhibited (50%) in SHR rings compared to WKY. The relaxation produced by sodium nitroprusside (10(-9)-10(-5) M) in endothelium-intact and -denuded aortic rings showed no difference between SHR and WKY. Isoproterenol produced concentration-dependent (10-9-10-5 M) relaxation, which was shifted to the right in SHR compared to WKY rings with an intact endothelium, while the removal of endothelium abolished the difference in the response between SHR and WKY. The results suggest: (i) adenosine A2 receptors mediate vasorelaxation in part through endothelium possibly by releasing nitric oxide (NO); (ii) the impairment of endothelium may be one of the factors for the attenuation of adenosine receptor and receptor-mediated responses in SHR.

Adenosine, a metabolic trigger of the exercise pressor reflex in humans

There is substantial evidence that adenosine activates muscle afferent nerve fibers leading to sympathetic stimulation, but the issue remains controversial. To further test this hypothesis, we used local injections of adenosine into the brachial artery while monitoring systemic muscle sympathetic nerve activity (MSNA) with peroneal microneurography. The increase in MSNA induced by 3 mg intrabrachial adenosine (106+/-32%) was abolished if forearm afferent traffic was interrupted by axillary ganglionic blockade (21+/-19%, n=5, P:<0.05). Furthermore, the increase in MSNA induced by intravenous adenosine was 3.7-fold lower and later (onset latency 20.9+/-4.8 seconds versus 8.5+/-1 seconds) than intrabrachial adenosine. Finally, we used forearm exercise (dynamic handgrip at 50% and 15% maximal voluntary contraction, MVC), with or without superimposed ischemia, to modulate interstitial levels of adenosine (estimated with microdialysis) while monitoring MSNA. Fifteen minutes of intense (50% MVC) and moderate (15% MVC) exercise increased adenosine dialysate concentrations from 0.31+/-0.1 to 1.24+/-0.4 micromol/L (528+/-292%) and from 0.1+/-0.02 to 0.419+/-0.16 micromol/L (303+/-99%), respectively (n=7, P:<0.01). MSNA increased 88+/-25% and 38+/-28%, respectively. Five minutes of moderate exercise increased adenosine from 0.095+/-0.02 to 0.25+/-0.12 micromol/L, and from 0.095+/-0.02 to 0.48+/-0.19 micromol/L when ischemia was superimposed on exercise (n=7, P:=0.01). The percent increase in MSNA induced by the various interventions correlated with the percent increase in dialysate adenosine levels (r=0.96). We conclude that adenosine activates muscle afferent nerves, triggering reflex sympathetic activation.

Relaxation of rat aorta by adenosine in diabetes with and without hypertension: role of endothelium

Effects of diabetes on the responses of aortic rings of normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rat to adenosine analogues were examined. Streptozotocin-induced diabetes caused an increase in blood glucose and plasma levels of cholesterol and triglycerides in normotensive (diabetic-WKY) as well as hypertensive (diabetic-SHR) rats. In diabetic-SHR group, the body weight was significantly low (50%) as compared to SHR (non-diabetic). Diabetic-SHR group showed the largest heart weight-to-body weight ratio indicating cardiac enlargement. The relaxation responses to adenosine analogues were obtained in endothelium-intact and -denuded aortic rings precontracted with phenylephrine. The IC(50) values of adenosine analogues were lower in endothelium-intact aortic rings of WKY as compared to diabetic-WKY and -SHR. Aortic rings from diabetic-SHR showed the greatest attenuation in adenosine analogue-mediated relaxation. Removal of endothelium from the aortic rings inhibited the relaxant response of adenosine analogues and abolished the differences among the groups. Nitric oxide (NO) synthase inhibitor L-monomethylarginine (L-NMMA) caused a significant rightward shift in the concentration-response curves in WKY and diabetic-WKY groups, only a small shift in SHR and no change in diabetic-SHR group indicating that it is primarily the inhibition of NO release which is responsible for attenuation of adenosine receptor responses in SHR and diabetic-WKY and there was absence of NO release in diabetic-SHR. Forskolin and sodium nitroprusside equally relaxed the aortic rings in all the groups. This suggested that there was no abnormality in the relaxant property of vascular smooth muscle due to hypertension and/or diabetes. Therefore, it is concluded that streptozotocin-induced diabetes in SHR aggravates the severity of vascular endothelial dysfunction which led to impairment in adenosine receptor-mediated vascular responses.

Effects of adenosine receptor agonists on efferent renal nerve activity in anesthetized rats

The aim of this study was to investigate the effects of A1 and A2 adenosine-receptor activation on the sympathetic nervous system. The effects on efferent renal nerve activity of selective A1 (CCPA; 2-chloro-N-6-cyclopentyladenosine) and A2 (2HE-NECA; 2-hexynyl-5'-N-ethylcarboxamidoadenosine) adenosine-receptor agonists were studied in anesthetized rats either with intact baroreflexes (intact rats) or with bilateral sinoaortic denervation and vagotomy (denervated rats). After a control period of 5 min, A1 or A2 agonist or vehicle were intravenously infused for 8 min in separate groups of intact or denervated rats, in which arterial pressure and heart rate were continuously recorded. CCPA (5.0 microg/kg/min) and 2HE-NECA (0.7 microg/kg/min) were selected to obtain comparable blood pressure changes over the period of observation. Arterial pressure significantly and equally decreased during the A1 (-41 +/- 8%), and A2 (-35 +/- 5%) agonist administration. Heart rate significantly decreased during A1 agonist infusion, but it did not change during A2 agonist administration. Bilateral sinoaortic denervation and vagotomy did not modify the hemodynamic responses to both drugs. The A1 and A2 administration caused a large and significant increase in efferent renal nerve activity (+66 +/- 22% and +76 +/- 15%, respectively), and this effect was entirely abolished in denervated rats. A linear relation with a significant negative slope between changes in arterial pressure and changes in neural discharge was observed for each treatment. The comparison of the regression slopes showed that the reflex increase of efferent sympathetic activity caused by the administration of both agonists was significantly smaller than the increment induced by equipotent hypotensive dose of sodium nitroprusside (10 microg/kg). These data show that the selective activation of A1 and A2 receptors elicits a reflex increase in efferent renal nerve activity. This neural activation is smaller as compared with the effect of equihypotensive doses of sodium nitroprusside, thus indicating a blunting effect of both adenosine agonists on baroreceptor sensitivity.

Effect of chronic salt loading on adenosine metabolism and receptor expression in renal cortex and medulla in rats

Previous studies have shown that chronic salt loading increased renal interstitial adenosine concentrations and desensitized renal effects of adenosine, a phenomenon that could facilitate sodium excretion. However, the mechanisms responsible for the increased adenosine production and decreased adenosine response are poorly understood. This study examined the effects of the dietary high salt intake on adenosine metabolism and receptor expression in the renal cortex and medulla in Sprague Dawley rats. Fluorescent high-performance liquid chromatography analyses were performed to determine adenosine levels in snap-frozen kidney tissues. Comparing rats fed a normal (1% NaCl) versus high salt (4% NaCl) diet, renal adenosine concentrations in rats fed a high salt diet were significantly higher (cortex: 43+/-3 versus 85+/-4, P<0.05; medulla: 183+/-4 versus 302+/-8 nmol/g wet tissue, P<0.05). Increased adenosine concentrations were not associated with changes in the 5'-nucleotidase or adenosine deaminase activity, as determined by quantitative isoelectric focusing and gel electrophoresis. Western blot analyses showed that a high salt diet (4% NaCl for 3 weeks) downregulated A1 receptors (antinatriuretic type), did not alter A2A and A2B receptors (natriuretic type), and upregulated A3 receptors (function unknown) in both renal cortex and medulla. The data show that stimulation of adenosine production and downregulation of A1 receptors with salt loading may play an important role in adaptation in the kidney to promote sodium excretion.

Microassay of 5'-nucleotidase and adenosine deaminase activity in microdissected nephron segments

The present study describes a new method for microassay of the activity of 5'-nucleotidase (5'-ND) and adenosine deaminase (ADA) in the microdissected nephron segments. The nephron segments including glomeruli, proximal convoluted and straight tubules (PCT and PST), cortical and medullary thick ascending limbs, and cortical and medullary collecting ducts were microdissected. 5'-ND and ADA in the nondenatured lysate of 20-mm microdissected tubules and 20 glomeruli were separated by agarose gel electrophoresis and by isoelectric focusing, respectively. The gels were incubated with specific substrates and staining dyes to exhibit the dephosphorylation by 5'-ND or deamination by ADA. The enzyme activity was estimated by measuring the intensity of the reaction bands on the gels. The 5'-ND activity was detected in all microdissected tubular segments and glomeruli. Among these nephron segments, PCT and PST exhibited the greatest enzyme activity, averaging 1142 and 939 mU/mg tissue protein, respectively. The activity of ADA was also detected in all tubular segments and glomeruli. However, the greatest activity of this enzyme was found in the glomeruli (649.8 mU/mg protein). Using reverse transcriptase-polymerase chain reaction technique, we verified the presence of mRNA of 5'-ND and ADA in all microdissected tubular segments and glomeruli. Based on these results, we conclude that 5'-ND and ADA are present in all nephron segments studied, but the activity of these enzymes is nonuniformly expressed along the nephron. This microassay is a highly specific, sensitive, and reliable method for the segmental analysis of adenosine metabolism in the kidney.

The role of A3 adenosine receptors in central regulation of arterial blood pressure

1. Pharmacological studies have suggested that A3 receptors are present on central neurons. Recently this adenosine receptor subtype has been identified in the rat and its presence in the central nervous system has been confirmed. 2. In this study we investigated the effects of acute intracerebroventricular (i.c.v.) injections of N6-2-(4-aminophenyl)-ethyladenosine (APNEA), a non-selective A3 adenosine receptor agonist, on arterial blood pressure (ABP) and heart rate (HR), after treatment with 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective antagonist of A1 adenosine receptors. 3. Anaesthetized rats, after DPCPX (12 microg(-1) kg i.c.v.), were treated with APNEA (0.4-4 microg kg(-1) i.c.v.) resulting in a transitory and dose-dependent decrease in arterial blood pressure without a change in heart rate. APNEA also induced hypotensive responses after i.c.v. pretreatment with aminophylline, at a dose of 20 microg kg(-1). In contrast, pretreatment 48 h before, with 4 microg kg(-1) i.c.v. of pertussis toxin reduced the hypotensive effect induced by APNEA. Administration of APNEA at a higher dose (20 microg kg(-1) i.c.v.), after DPCPX, induced a decrease in ABP of -66+/-5.4 mmHg and after 3 min a decrease in heart rate of -62+/-6.0 beats min(-1). Transection of the spinal cord abolished this significant fall in ABP, but not the decrease of HR. 4. These results suggest that a population of A3-receptors is present in the CNS, whose activation induces a decrease in blood pressure with no change of heart rate.

Angiotensin AT1 receptor blockade abolishes the reflex sympatho-excitatory response to adenosine

We tested the hypothesis that endogenous angiotensin II participates in the direct and reflex effects of adenosine on the sympathetic nervous system. Nine healthy men were studied after 1 wk of the angiotensin II type I receptor antagonist losartan (100 mg daily) or placebo, according to a double-blind randomized crossover design. Bilateral forearm blood flows, NE appearance rates, and total body NE spillover were determined before and during graded brachial arterial infusion of adenosine (0.5, 1.5, 5, and 15 microg/100 ml forearm tissue) and nitroprusside. Adenosine increased total body NE spillover (P < 0.05) whereas nitroprusside did not. Losartan lowered BP (P < 0.05), had no effect on total body NE spillover at rest, or forearm vasodilation during either infusion, but reduced the systemic noradrenergic response to adenosine from 1.0+/-0.4 nmol/min on the placebo day to 0.2+/-0.3 nmol/min (P < 0.01), and forearm NE appearance rate in response to adenosine was lower in the infused, as compared with the contralateral arm (P = 0.04). The sympatho-excitatory reflex elicited by adenosine is mediated through pathways involving the angiotensin II type I receptor. Interactions between adenosine and angiotensin II may assume importance during ischemia or congestive heart failure and could contribute to the benefit of converting enzyme inhibition in these conditions.

Localization and functional effects of adenosine A1 receptors on cardiac vagal afferents in adult rats

There is evidence to suggest that during ischemia adenosine acts on cardiac vagal afferent neurons to activate systemic reflexes and to modulate cardiac nociception. The purpose of this study was to determine whether adenosine receptors are present and have direct cellular electrophysiological actions on cardiac vagal afferent neurons. In radioreceptor assays of nodose ganglion tissue from rats, binding was detectable for A1 (39.6 +/- 1.2 fmol/mg protein) but not A2a adenosine receptors. These findings were confirmed using the complementary approach of receptor-labeling autoradiography. Using in situ hybridization, we saw specific labeling over approximately 50% of neurons in the nodose ganglia, but not over nonneuronal cells. In colabeling studies, cardiac vagal afferent neurons were identified by retroneuronal labeling with fluororuby. Of cardiac vagal afferents approximately one-half were strongly positive for A1 adenosine receptors (immunocytochemistry). In patch-clamping experiments, adenosine inhibited peak inward calcium current in 7 of 11 cells by 48 +/- 13%. In conclusion, adenosine A1 receptors reside on a subset of vagal afferent neurons, including cardiac vagal afferents, and have electrophysiological effects that modulate neuroexcitability in cultured nodose ganglion neurons.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

Differential effects of diadenosine phosphates on purinoceptors in the rat isolated perfused kidney

1. The activation of various purinoceptors in rat renal vasculature by P1,P2-diadenosine pyrophosphate (Ap2A), P1,P3-diadenosine triphosphate (Ap3A), P1,P4-diadenosine tetraphosphate (Ap4A), P1,P5-diadenosine pentaphosphate (Ap5A), P1,P6-diadenosine hexaphosphate (Ap6A) was studied by measuring their effects of perfusion pressure of a rat isolated perfused kidney. 2. The vasoconstrictive response to Ap5A was completely due to P2x purinoceptor activation, that to Ap4A and Ap6 was P2x purinoceptor mediated to a large extent, as evidenced by the inhibitory effects of suramin and pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid tetrasodium (PPADS). 3. The vasoconstrictive effects of Ap2A and Ap3A were mostly due to stimulation of A1-receptors, as shown by the inhibitory effect of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). 4. The vasoconstrictive response to Ap6A was partially insensitive to A1 and P2x purinoceptor blockers. 5. In raised tone preparations Ap2A and Ap3A evoked vasodilatation, which was blocked by the A2 receptor blocker, 3,7-dimethyl-1-propargylxanthine (DMPX). 6. In raised tone preparations Ap4A evoked vasodilatation when the P2-purinoceptors were blocked by suramin. 7. The activation of different purinoceptor subtypes by diadenosine phosphates critically depends on the number of phosphate groups.

Presynaptic inhibition of norepinephrine release from sympathetic nerve endings by endogenous adenosine

ATP is coreleased with norepinephrine from sympathetic nerve endings and subsequently broken down to adenosine. In animal preparations, adenosine can inhibit norepinephrine release by stimulation of presynaptic receptors. We tested this feedback mechanism in humans by using a specific nucleoside transport inhibitor (draflazine) as a pharmacological tool to allow accumulation of endogenous adenosine in the synaptic cleft. In a dose-finding study on draflazine infusions into the brachial artery (n=10), we identified an optimal dose of 250 ng/min per deciliter of forearm tissue that induced considerable local nucleoside transport inhibition (approximately 40%) without systemic effects. In the main study, we investigated the effects of this draflazine dose on sympathetic-mediated norepinephrine spillover during lower body negative pressure (-25 mm Hg) by the use of the [3H]norepinephrine isotope dilution technique (n=25). Lower body negative pressure induced a significant increase in total body norepinephrine spillover, forearm norepinephrine appearance rate, forearm vascular resistance, and heart rate. During draflazine infusion into the brachial artery, the responses to lower body negative pressure were preserved for all parameters, with the exception of the median increase in forearm norepinephrine appearance rate, which was reduced from 54% to 2% (P <.05). We conclude that accumulation of endogenous adenosine in the synaptic cleft during sympathetic stimulation can inhibit norepinephrine release from sympathetic nerve endings.

Plasma adenosine concentrations are elevated in Dahl salt-sensitive rats

We measured plasma levels of adenosine in Dahl salt-sensitive rats (DS) and Dahl salt-resistant rats (DR) to examine the potential role of adenosine in cardiovascular regulation in this type of hypertension. Plasma adenosine concentrations were significantly higher in DS than in DR. The NaCl content in the diet did not affect plasma adenosine concentration in either DS or DR. Significant positive correlation was found between adenosine concentrations and systolic blood pressure when the data for DS and DR were analyzed together. These results suggest that adenosine may play an important role in the pathophysiology of hypertension in DS.

Hemodynamic and neurohumoral effects of various grades of selective adenosine transport inhibition in humans. Implications for its future role in cardioprotection

In 12 healthy male volunteers (27-53 yr), a placebo-controlled randomized double blind cross-over trial was performed to study the effect of the intravenous injection of 0.25, 0.5, 1, 2, 4, and 6 mg draflazine (a selective nucleoside transport inhibitor) on hemodynamic and neurohumoral parameters and ex vivo nucleoside transport inhibition. We hypothesized that an intravenous draflazine dosage without effect on hemodynamic and neurohumoral parameters would still be able to augment the forearm vasodilator response to intraarterially infused adenosine. Heart rate (electrocardiography), systolic blood pressure (Dinamap 1846 SX; Critikon, Portanje Electronica BV, Utrecht, The Netherlands) plasma norepinephrine and epinephrine increased dose-dependently and could almost totally be abolished by caffeine pretreatment indicating the involvement of adenosine receptors. Draflazine did not affect forearm blood flow (venous occlusion plethysmography). Intravenous injection of 0.5 mg draflazine did not affect any of the measured hemodynamic parameters but still induced a significant ex vivo nucleoside-transport inhibition of 31.5 +/- 4.1% (P < 0.05 vs placebo). In a subgroup of 10 subjects the brachial artery was cannulated to infuse adenosine (0.15, 0.5, 1.5, 5, 15, and 50 micrograms/100 ml forearm per min) before and after intravenous injection of 0.5 mg draflazine. Forearm blood flow amounted 1.9 +/- 0.3 ml/100 ml forearm per min for placebo and 1.8 +/- 0.2, 2.0 +/- 0.3, 3.8 +/- 0.9, 6.3 +/- 1.2, 11.3 +/- 2.2, and 19.3 +/- 3.9 ml/100 ml forearm per min for the six incremental adenosine dosages, respectively. After the intravenous draflazine infusion, these values were 1.6 +/- 0.2 ml/100 ml forearm per min for placebo and 2.1 +/- 0.3, 3.3 +/- 0.6, 5.8 +/- 1.1, 6.9 +/- 1.4, 14.4 +/- 2.9, and 23.5 +/- 4.0 ml/100 ml forearm per min, respectively (Friedman ANOVA: P < 0.05 before vs after draflazine infusion). In conclusion, a 30-50% inhibition of adenosine transport significantly augments the forearm vasodilator response to adenosine without significant systemic effects. These results suggest that draflazine is a feasible tool to potentiate adenosine-mediated cardioprotection in man.

Role of arterial chemoreceptors in mediating the effects of endogenous adenosine on sympathetic nerve activity

BACKGROUND

Exogenous adenosine has been shown to increase muscle sympathetic nerve activity (MSNA), blood pressure, heart rate, and ventilation in conscious humans, effects attributed to peripheral chemoreceptor activation.

METHODS AND RESULTS

To determine whether endogenous adenosine has similar effects and whether they are mediated through chemoreceptor activation, we examined the effects of dipyridamole, an inhibitor of adenosine reuptake, on sympathetic nerve activity and ventilation. Twenty studies were conducted on separate days in 15 healthy volunteers. We examined responses to dipyridamole 0.56 mg/kg during room air breathing (n = 7), during hyperoxia (100% O2, n = 6), and during room air breathing after pretreatment with aminophylline (n = 7). During room air breathing, dipyridamole increased MSNA from 231 +/- 42 to 504 +/- 136 U/min, heart rate from 65 +/- 3.8 to 96 +/- 4.7 beats per minute, and systolic blood pressure from 129 +/- 3.5 to 140 +/- 4.8 mm Hg; central venous pressure decreased from 5.5 +/- 0.4 to 4.5 +/- 0.3 mm Hg (P < .01), and minute ventilation increased from 7.8 +/- 0.6 to 9.1 +/- 0.5 L/min (P < .01). During peripheral chemoreceptor suppression (with hyperoxia), there was a dissociation of the effects of dipyridamole on ventilation and sympathoexcitation. Effects on ventilation were attenuated, but sympathoexcitatory effects were not. Pretreatment with aminophylline, an adenosine receptor antagonist, either abolished (blood pressure, minute ventilation, and end-tidal CO2) or markedly attenuated (MSNA and heart rate) the effects of dipyridamole during room air breathing.

CONCLUSIONS

Augmentation of endogenous adenosine with dipyridamole increases sympathetic nerve activity and ventilation in conscious humans. The ventilatory effects of endogenous adenosine are mediated predominantly by chemoreceptor activation, but the sympathetic and hemodynamic responses to endogenous adenosine are probably mediated by an additional afferent mechanism that is independent of peripheral chemoreceptor activation.

Role of adenosine in the sympathetic activation produced by isometric exercise in humans

Isometric exercise increases sympathetic nerve activity and blood pressure. This exercise pressor reflex is partly mediated by metabolic products activating muscle afferents (metaboreceptors). Whereas adenosine is a known inhibitory neuromodulator, there is increasing evidence that it activates afferent nerves. We, therefore, examined the hypothesis that adenosine stimulates muscle afferents and participates in the exercise pressor reflex in healthy volunteers. Intraarterial administration of adenosine into the forearm, during venous occlusion to prevent systemic effects, mimicked the response to exercise, increasing muscle sympathetic nerve activity (MSNA, lower limb microneurography) and mean arterial blood pressure (MABP) at all doses studied (2, 3, and 4 mg). Heart rate increased only with the highest dose. Intrabrachial adenosine (4 mg) increased MSNA by 96 +/- 25% (n = 6, P < 0.01) and MABP by 12 +/- 3 mmHg (P < 0.01). Adenosine produced forearm discomfort, but equivalent painful stimuli (forearm ischemia and cold exposure) increased MSNA significantly less than adenosine. Furthermore, adenosine receptor antagonism with intrabrachial theophylline (1 microgram/ml forearm per min) blocked the increase in MSNA (92 +/- 15% vs. 28 +/- 6%, n = 7, P < 0.01) and MABP (38 +/- 6 vs. 27 +/- 4 mmHg, P = 0.01) produced by isometric handgrip (30% of maximal voluntary contraction) in the infused arm, but not the contralateral arm. Theophylline did not prevent the increase in heart rate produced by handgrip, a response mediated more by central command than muscle afferent activation. We propose that endogenous adenosine contributes to the activation of muscle afferents involved in the exercise pressor reflex in humans.

Influence of aortic baroreceptor denervation on adenosine receptor-mediated relaxation of isolated rat aorta

The effect of aortic baroreceptor denervation on the vasorelaxant activity of the adenosine analogue, 5'-N-ethylcarboxamidoadenosine (NECA) was evaluated in the isolated thoracic aortic rings from rats. The responses were evaluated at 3 h after baroreceptor denervation when the blood pressure was significantly higher than that of control (149 +/- 3 vs. 112 +/- 2 mmHg) and at 48 h after aortic baroreceptor denervation when blood pressure returned to control level. Sham operation had no effect on blood pressure at either time interval. A concentration-dependent relaxation of rat aorta elicited by NECA was observed in all groups. However, the responsiveness to NECA was reduced in aortic baroreceptor-denervated rats. The nitric oxide (NO) synthase inhibitor, L-monomethyl-L-arginine (30 microM) shifted the dose-response curve for NECA to the right in all groups suggesting that the vascular response to NECA is partially mediated through the release of NO. Removal of the endothelium abolished the differences in the response to NECA in sham and aortic baroreceptor-denervated rats suggesting that the decrease in the responsiveness of aortic smooth muscle to NECA is dependent on the release of NO. Vasorelaxant responses to acetylcholine were not altered by aortic baroreceptor denervation. The ability of aortic baroreceptor denervation to attenuate vasorelaxant responses to NECA but not to acetylcholine indicated the possibility of functional changes involving adenosine receptor-mediated relaxation in endothelium rather than structural changes due to aortic baroreceptor denervation. These findings suggest that short-term elevation of blood pressure following aortic baroreceptor denervation modulates the endothelium-dependent release of NO mediated by adenosine receptor activation.

Natriuretic and hypotensive effect of adenosine-1 blockade in essential hypertension

We studied the effects of a single dose (100 mg orally) and repeated administration (100 mg o.d. for 7 days) of FK453, a novel adenosine-1 receptor antagonist, on renal sodium handling and blood pressure in eight patients with essential hypertension. Within 60 minutes after administration of FK453, sodium excretion increased threefold. This occurred in the absence of a change in renal hemodynamics, assessed from inulin and para-aminohippurate clearance, and was accompanied by increased fractional excretion of lithium, phosphate, and uric acid and by increased excretion of calcium and magnesium. Maximal free water clearance data showed an increase in maximal urine flow and distal delivery term and a decrease in the diluting segment reabsorption term. FK453 also decreased blood pressure and increased heart rate, but this did not occur until about 3 hours after ingestion, that is, when the natriuresis was already over. The natriuretic effect of FK453 was short-lasting, and continued use of FK453 was in fact accompanied by some net sodium retention. Blood pressure on the seventh day before FK453 treatment was not different from blood pressure before administration of the first dose of FK453. Again an acute natriuretic response followed, although less than after the first dose. Changes in intrarenal sodium handling parameters, blood pressure, and heart rate were similar to those seen after the first dose. The natriuretic and hypotensive effects of FK453 indicate that adenosine-1 receptor activity plays a role in the regulation of blood pressure and renal sodium handling in patients with essential hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)