Effects of Ba2+ on norepinephrine-induced contraction of rat thoracic aorta in vitro

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 mu mol/l). In the aortic preparation pretreated with nicardipine (1 mu mol/l), exogenous BaCl2 (0.1-3 mmol/l) did not elevate the nor epinephrine-induced sustained contraction, but the high concentration of BaCl2 (10 mmol/l) slightly inhibited the norepinephrine-induced tone. In a Ca2+-free Krebs bicarbonate solution (KBS) containing norepinephrine (1 mu mol/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 mu mol/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 mu mol/l), but such a sustained contraction was partly inhibited by nicardipine (3 mu mol/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. Copyright (C) 2000 S. Karger AG, Basel.