Up-regulated neuronal nitric oxide synthase compensates coronary flow response to bradykinin in endothelial nitric oxide synthase-deficient mice

It has been reported that endothelium-dependent relaxations are preserved in isolated coronary arteries of endothelial nitric oxide synthase-deficient (eNOS^-/-) mice with a possible involvement of nNOS. However, it remains to be examined whether nNOS compensates coronary flow response in a beating heart of eNOS^-/- mice and if so, whether and where nNOS is up-regulated. Coronary flow response to bradykinin was examined in Langendorff-perfused hearts from WT and eNOS^-/- mice. Bradykinin-induced coronary flow was greater in eNOS^-/- mice than in WT mice, and indomethacin had no inhibitory effect on it. Bradykinin receptor antagonist HOE-140 abolished the bradykinin response in both strains. Nonselective NOSs inhibitor L-NNA inhibited the bradykinin-induced coronary flow in both strains, whereas specific inhibitors of nNOS, SMTC, and 7-NI, significantly attenuated the coronary flow response only in eNOS^-/- mice. A guanylate cyclase inhibitor ODQ also attenuated the bradykinin response in eNOS^-/- mice. Immunohistochemistry revealed the presence of nNOS mainly in coronary vascular smooth muscle cells (VSMCs) in both strains and Western blot analysis demonstrated a marked increase in cardiac nNOS expression in eNOS^-/- mice. These results indicate that nNOS compensates coronary flow response to bradykinin in eNOS^-/- mice, for which upregulation of nNOS in VSMCs may be involved.