Vasodilation And Mechanism Of Action Of Amiloride On Thoracic Aorta In Rats

Objective:Although the exciting effects of amiloride (AM) have been studied in rat isolated thoracic aorta rings, its pharmacological properties in vascular beds and underlying mechanism(s) are still not well clarified. The present study was designed to observe the effects and the possible mechanism of AM on the on isolated thoracic aorta rings pre-contracted by norepinphrine(NE) or potassium chloride(KCl).Methods:The rats were sacrificed by luxation and the thoracic aorta was quickly removed from each animal and placed in physiological salt solution (PSS). The aorta was carefully cleaned of adherent fat and connective tissue. The vessel was then cut into rings of about 2-3mm length. Two stainless-steel wires were passed through the lumen of each ring. One stirrup was connected to an isometric force transducer to measure tension in the vessels. The rings were placed in a 10ml organ chamber containing PSS. The rings were stretched until they exerted an optimal basal tension of 2g, and then were allowed to equilibrate for 1 hour with the bath fluid being changed every 15 minutes. The solution was bubbled with 100%O2. Rings of the isolated rat thoracic aorta were mounted in organ baths and isometric contractile force was measured. Isotonic tension of thoracic aortic rings precontracted by potassium chloride (KCl,60 mol/L) or norepinephrine (NE,1×10-6 mol/L) was recorded. The response action of AM and effect of various drugs were observed in the rings. The effects of AM on the extracellular Ca-dependent contraction and intracellular Ca-dependent contraction were also observed.Results:(1) AM(1×10-6-3×10-5 mol/L) had no effect on the resting tone.(2) AM (1×10-6-3×10-5 mol/L) respectively concentration-dependently reduced the contraction induced by NE (1×10-*6 mol/L) and KCl (60 mmol/L). The logEC50of AM respectively was-4.35±0.01 and-4.28±0.04. (3) In the PSS free Na+, AM (1×10-6-3×10-5mol/L) respectively concentration-dependently reduced the contraction induced by NE (1×10-6 mol/L) and KCl (60 mmol/L). The logEC50 of AM respectively was-4.24±0.02 and-4.13±0.01. There was no significant difference between them that with or without Na+(P>0.05). (4)AM (1×10-6-3×10-5 mol/L) respectively concentration-dependently reduced the contraction of denude-endothlium aortic rings which induced by NE (1×10-6 mol/L) and KCl (60 mmol/L).The relaxation scale was less than the intact endothium. The logEC50 of AM respectively was-4.18±0.02 and-4.16±0.01. There was significant difference between the rings intact and denude endothelium (P<0.01-0.05).(5) On NE-induced precontraction, 1×10-6-3×10-5 mol/L AM's vasodilation can be inhibited by 1×10-4 mol·L MG-nitro-L-arginine methyl ester (L-NAME, a nitric oxide synthetase inhibitor)(P<0.01).(6) On NE-induced precontraction,1×10-6-3×l0-5 mol/L AM's vasodilation can't be inhibited by 1×10-5 mol/L indomethacin (a cyclo-oxygenase inhibitor)(P>0.05).(7) On NE-induced precontraction, 1×10-6-3×10-5mol/L AM's vasodilation can be inhibited by TEA and BaCl2 (P＜0.05), can't be inhibited by Gli and 4-AP (P>0.05). (8) AM can inhibit the contraction induced by additions of NE and CaCl2,in Ca-free medium.Conclusion:AM could produce remarkable vasodilatation on NE or KCl pre-contracted aorta rings in adult rats, while it didn't affect the resting tone. It relaxed via endothelium-dependent and independent mechanisms. The endothelium-dependent effect is probably via NO release from the endothelium. The endothelium-independent effect is probably via Kir channel and BKca channel.The vasodilatation effect is probably via inhibiting of extracelluar Ca2+ influx and intracellular Ca2+ release and the vasodilatation effect has nothing to do with the Na+/H+...