Direct activation of endothelial NO pathway by Ba 2+ in canine coronary artery

1 We have reported that Ba 2+ causes endothelium‐dependent relaxation of canine coronary arteries through NO synthesis in Ca 2+ ‐free and depolarizing solution. To determine the cellular mechanisms by which the endothelium‐dependent relaxation occurs, we used fura‐2 fluorometry (F 350 and F 390 ; excitation wavelengths, 350 and 390 nm, respectively) and estimated the intracellular Ba 2+ concentration in endothelial and vascular smooth muscle cells. 2 Ba 2+ (10 −3 m ) increased the fura‐2 ratio (F 350 /F 390 ) 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 m m K + (0.22±0.06 g, n = 4). 3 Diltiazem (3×10 −6 m ) blocks Ba 2+ entry into vascular smooth muscle cells via l ‐type Ca 2+ channels. In this condition, Ba 2+ 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 N G ‐methyl‐ l ‐arginine ( l ‐NMMA). 4 Ba 2+ ‐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 Ba 2+ ‐induced relaxation. 5 To further rule out changes in intracellular Ca 2+ as a mechanism of Ba 2+ ‐induced relaxation, fura‐2 fluorescence was measured at the isosbestic wavelengths for Ca 2+ (360 nm) and Ba 2+ (370 nm) in endothelium‐intact arteries. Ba 2+ altered F 360 , but not F 370 , suggesting little or no contribution of intracellular Ca 2+ to the phenomenon of Ba 2+ ‐induced relaxation. 6 These results suggest that the Ba 2+ ‐induced relaxation is due to its direct activation of endothelial NO synthesis without mobilization of intracellular Ca 2+ .British Journal of Pharmacology (1998) 124 , 1149–1158; doi: 10.1038/sj.bjp.0701948