Direct activation of endothelial NO pathway by Ba2+ in canine coronary artery

1. We have reported that Ba2+ causes endothelium-dependent relaxation of canine coronary arteries through NO synthesis in Ca2+-free and depolarizing solution. To determine the cellular mechanisms by which the endothelium-dependent relaxation occurs, we used fura-2 fluorometry (F350 and F390; excitation wavelengths, 350 and 390 nm, respectively) and estimated the intracellular Ba2+ concentration in endothelial and vascular smooth muscle cells. 2. Ba2+ (10(-3) M) increased the fura-2 ratio (F350/F390) recorded from a combined preparation of smooth muscle and endothelium (0.445+/-0.073, n = 4) and contracted the arteries in the presence of 80 mM K+ (0.22+/-0.06 g, n = 4). 3. Diltiazem (3 x l0(-6) M) blocks Ba2+ entry into vascular smooth muscle cells via L-type Ca2+ channels. In this condition, Ba2+ increased the fura-2 ratio in endothelial cells (0.141+/-0.014, n = 5) and relaxed the underlying smooth muscle (0.08+/-0.01 g, n = 5) by a mechanism which was sensitive to 10(-4) M NG-methyl-L-arginine (L-NMMA). 4. Ba2+-induced relaxation was not attenuated with repeated application and was elicited even after endothelium-dependent relaxations in response to 10(-6) M bradykinin were abolished due to tachyphylaxis. Neither 10(-2) M caffeine nor 10(-6) M thapsigargin had effect upon Ba2+-induced relaxation. 5. To further rule out changes in intracellular Ca2+ as a mechanism of Ba2+-induced relaxation, fura-2 fluorescence was measured at the isosbestic wavelengths for Ca2+ (360 nm) and Ba2+ (370 nm) in endothelium-intact arteries. Ba2+ altered F360, but not F370, suggesting little or no contribution of intracellular Ca2+ to the phenomenon of Ba2+-induced relaxation. 6. These results suggest that the Ba2+-induced relaxation is due to its direct activation of endothelial NO synthesis without mobilization of intracellular Ca2+.