Effects of Ba2+ on Norepinephrine-Induced Contraction of Rat Thoracic Aorta in vitro

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Abstract
The aim of this study was to examine the effect of exogenously applied BaCl2 on the norepinephrine-induced contraction of the rat thoracic aorta. Exogenously applied BaCl2 (0.3–1 mmol/l) slightly elevated the norepinephrine-induced sustained contraction of the rat thoracic aorta in the absence of nicardipine (1 μmol/l). In the aortic preparation pretreated with nicardipine (1 μmol/l), exogenous BaCl2 (0.1–3 mmol/l) did not elevate the norepinephrine-induced sustained contraction, but the high concentration of BaCl2 (10 mmol/l) slightly inhibited the norepinephrine-induced tone. In a Ca2+-free Krebs bi- carbonate solution (KBS) containing norepinephrine (1 μmol/l) or a Ca2+-free K+-rich (60 mmol/l) KBS, exogenously applied BaCl2 (1–30 mmol/l) caused a sustained contraction of the rat thoracic aorta, and this sustained contraction was completely inhibited by nicardipine (1 μmol/l). Exogenous CaCl2 (0.1–3 mmol/l) also caused a sustained contraction of the aortic preparation in a Ca2+- free KBS containing norephinephrine (1 μmol/l), but such a sustained contraction was partly inhibited by nicardipine (3 μmol/l). These results indicate that Ba2+ elevates the norepinephrine-induced tone of the rat isolated thoracic aorta by permeating voltage-dependent Ca2+ channels in the absence of nicardipine, but that Ba2+ has a minor modification on the norepinephrine-induced sustained contraction of the nicardipine-pretreated preparation.

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