Vascular endothelium constitutively generates nitric oxide (NO) in large vessels and induces a relaxation of smooth muscle cells. However, little is known about the production of NO in microvessels, where smooth muscle layers are thin or absent. In this study, we have compared the constitutive production of NO in bovine brain microvascular endothelial cells (BBECs) with that in bovine aortic endothelial cells (BAECs). ATP, acetylcholine (ACh) and A23187 induced Ca2+ transients both in BBECs and BAECs. In contrast, although ATP and A23187 evoked a similar degree of [Ca2+]i increase in both types of cell, they failed to induce NO production in BBECs, as measured with an NO-sensitive fluorescent dye DAF-2, whereas in BAECs there was an increase in DAF-2 fluorescence. Hypotonic stress induced ATP release and subsequent NO production in BAECs, but not in BBECs. We have developed an in vitro model vessel system that consists of aortic smooth muscle cells embedded in a collagen gel lattice and overlaid with endothelial cells. Precontracted gels showed relaxation in response to ACh, when BAECs were overlaid. However, ACh-induced relaxation was not observed in BBEC-overlaid gels. Expression of eNOS protein as well as cellular uptake of L-[3H] arginine were significantly lower in BBECs than in BAECs. These results indicate that Ca2+-dependent NO production is at an undetectable level in BBEC, for which at least two factors, i.e. low levels of eNOS expression and L-arginine uptake, are responsible.
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