Adenosine relaxation in isolated rat aortic rings and possible roles of smooth muscle Kv channels, KATP channels and A2a receptors.

Aryadi Arsyad,Geoffrey P Dobson

doi:10.1186/s40360-016-0067-8

Abstract

BackgroundAn area of ongoing controversy is the role adenosine to regulate vascular tone in conduit vessels that regulate compliance, and the role of nitric oxide (NO), potassium channels and receptor subtypes involved. The aim of our study was to investigate adenosine relaxation in rat thoracic aortic rings, and the effect of inhibitors of NO, prostanoids, Kv, KATP channels, and A2a and A2b receptors.MethodsAortic rings were freshly harvested from adult male Sprague Dawley rats and equilibrated in an organ bath containing oxygenated, modified Krebs-Henseleit solution, 11 mM glucose, pH 7.4, 37 °C. Isolated rings were pre-contracted sub-maximally with 0.3 μM norepinephrine (NE), and the effect of increasing concentrations of adenosine (1 to 1000 μM) were examined. The drugs L-NAME, indomethacin, 4-aminopyridine (4-AP), glibenclamide, 5-hydroxydecanoate, ouabain, 8-(3-chlorostyryl) caffeine and PSB-0788 were examined in intact and denuded rings. Rings were tested for viability after each experiment.ResultsAdenosine induced a dose-dependent, triphasic relaxation response, and the mechanical removal of the endothelium significantly deceased adenosine relaxation above 10 μM. Interestingly, endothelial removal significantly decreased the responsiveness (defined as % relaxation per μM adenosine) by two-thirds between 10 and 100 μM, but not in the lower (1–10 μM) or higher (>100 μM) ranges. In intact rings, L-NAME significantly reduced relaxation, but not indomethacin. Antagonists of voltage-dependent Kv (4-AP), sarcolemma KATP (glibenclamide) and mitochondrial KATP channels (5-HD) led to significant reductions in relaxation in both intact and denuded rings, with ouabain having little or no effect. Adenosine-induced relaxation appeared to involve the A2a receptor, but not the A2b subtype.ConclusionsIt was concluded that adenosine relaxation in NE-precontracted rat aortic rings was triphasic and endothelium-dependent above 10 μM, and relaxation involved endothelial nitric oxide (not prostanoids) and a complex interplay between smooth muscle A2a subtype and voltage-dependent Kv, SarcKATP and MitoKATP channels. The possible in vivo significance of the regulation of arterial compliance to left ventricular function coupling is discussed.

Highlights

An area of ongoing controversy is the role adenosine to regulate vascular tone in conduit vessels that regulate compliance, and the role of nitric oxide (NO), potassium channels and receptor subtypes involved
We report in isolated rat thoracic rings that adenosine vasodilation was: 1) triphasic and partially dependent on an intact endothelium, 2) regulated predominately by endothelial NO, not prostanoids, 3) dependent on opening smooth muscle Kv, SarcKATP and MitoKATP channels, 4) ouabain-insensitive (Na+/K+ ATPase), and 5) activated by the A2a subtype, not A2b
Contributions of SarcKATP and MitoKATP channels to adenosine relaxation, and A2a receptor activation We further showed that the SarcKATP channel contributed to 14 to 63 % of adenosine relaxation up to 100 μM adenosine (Figs. 3b and 5b), and MitoKATP channels contributed to 22 to 70 % relaxation up to 1000 μM adenosine in intact and denuded aortic rings (Figs. 3c and 5c)

Summary

Introduction

An area of ongoing controversy is the role adenosine to regulate vascular tone in conduit vessels that regulate compliance, and the role of nitric oxide (NO), potassium channels and receptor subtypes involved. The aim of our study was to investigate adenosine relaxation in rat thoracic aortic rings, and the effect of inhibitors of NO, prostanoids, Kv, KATP channels, and A2a and A2b receptors. An area of ongoing controversy is the role adenosine to regulate vascular tone in the arterial tree, and the receptor subtypes involved. There is ongoing debate on the relative importance of an intact endothelium to adenosine relaxation in these vessels, and the role of nitric oxide (NO) and interplay between voltagedependent transmembrane Na+, K+ and Ca2+ fluxes and signalling pathways. Adenosine vasodilation has been linked to A1 and A2a receptor activation of endothelial production of NO and prostanoids [21], hyperpolarising factors [4], and a complex interplay between endothelial and smooth muscle mitochondrial and sarcolemmal KATP channels [16, 22, 23], and Na+/K+ ATPase activation [4, 24]

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