Impaired Inhibitory Function of Presynaptic A1-Adenosine Receptors in SHR Mesenteric Arteries

Abstract

Abstract. In hypertension, vascular reactivity alterations have been attributed to numerous factors, including higher sympathetic innervation/adenosine. This study examined the modulation of adenosine receptors on vascular sympathetic nerves and their putative contribution to higher noradrenaline spillover in hypertension. We assessed adenosine receptors distribution in the adventitia through confocal microscopy, histomorphometry, and their regulatory function on electrically-evoked [3 H]-noradrenaline overflow, using selective agonists/antagonists. We found that: i) A 1 -adenosine receptor agonist (CPA: 100 nM) inhibited tritium overflow to a lower extent in SHR (25% ± 3%, n = 14) compared to WKY (38% ± 3%, n = 14) mesenteric arteries; ii) A 2A -adenosine receptor agonist (CGS 21680: 100 nM) induced a slight increase of tritium overflow that was similar in SHR (22% ± 8%, n = 8) and WKY (24% ± 5%, n = 8) mesenteric arteries; iii) A 2B -and A 3 -adenosine receptors did not alter tritium overflow in either strain; iv) all adenosine receptors were present on mesenteric artery sympathetic nerves and/or some adventitial cells of both strains; and v) A 1 -adenosine receptor staining fractional area was lower in SHR than in WKY mesenteric arteries. We conclude that there is an impaired inhibitory function of vascular presynaptic A 1 -adenosine receptors in SHR, likely related to a reduced presence of these receptors on sympathetic innervation, which might lead to higher levels of noradrenaline in the synaptic cleft and contribute to hypertension in this strain.

Endothelial and Neuronal Nitric Oxide Activate Distinct Pathways on Sympathetic Neurotransmission in Rat Tail and Mesenteric Arteries

“…[ 3 H]-noradrenaline release experiments were carried out as previously described [23,24,27–29]. Briefly, the arterial segments were pre-incubated in 2 ml Krebs-Henseleit solution containing 0.1 μmol/L [ 3 H]- noradrenaline (for 60 min at 37°C) and transferred to superfusion chambers, superfused with [ 3 H]-noradrenaline-free medium (1 ml.min-1; constant rate: Krebs-Henseleit solution with desipramine 400 nmol/L to inhibit noradrenaline’s neuronal uptake).…”

“…At the end of the experiments (t = 130 min), tritium was measured in superfusate samples and solubilized arteries (sonicated for 1h with 2.5 ml perchloric acid (0.2 mol/L)) by liquid scintillation spectrometry (LS 6500, Beckman Instruments, Fullerton, USA) after adding 6 ml of a scintillation mixture (OptiPhase ‘Hisafe’ 3, PerkinElmer, I.L.C., Lisboa, Portugal) to each sample. Tissue labelling with [ 3 H]-noradrenaline and evaluation of electrically-evoked tritium overflow changes was performed as previously described [23,24]. …”

“…Immunohistochemistry procedures were performed as previously described [24]. From each artery four segments were obtained, immediately placed in cold phosphate buffer solution (PBS; in g/L): NaCl 8.0, Na2HPO4.2H2O 0.77, KCl 0.20, KH2PO4 0.19 (pH 7.2), longitudinally opened and fixed (paraformaldehyde 4% in PBS; 50 min; room temperature, RT).…”

“…For this purpose, in the present study, two different vessels were used, mesenteric and tail arteries, which have been extensively used as models for the study of neuromodulation exerted by many substances in the vasculature [5,7,8,31–33] and where we have previously described the presence of adenosine receptors on sympathetic nerves [24,27]. …”

Lack of Endogenous Adenosine Tonus on Sympathetic Neurotransmission in Spontaneously Hypertensive Rat Mesenteric Artery

“…In vessels, the involvement of adenosine receptors in sympathetic modulation has been described both in arteries[6]–[13] and in veins [12]. A reduced effect mediated by selective adenosine A 1 , but not A 2A receptor agonists in sympathetic vascular neurotransmission in hypertensive state has been reported [13].…”