Constitutive expression of arginase in microvascular endothelial cells counteracts nitric oxide-mediated vasodilatory function.

Arginase shares a common substrate, L-arginine, with nitric oxide (NO) synthase (NOS). It is thought that arginase, if it is expressed in the endothelium, might play a pivotal role in the regulation of NO-mediated vasodilation by reducing the L-arginine availability to NOS. In the present study, we wanted to determine whether arginase is expressed and active in coronary arterioles and to demonstrate whether endothelial arginase can influence NO production and play a functional role in regulating NO-mediated dilation. In this regard, the expression of arginase mRNA and distribution of arginase protein in porcine coronary microvessels were determined by reverse transcription-polymerase chain reaction and immunohistochemistry, respectively. To assess the role of arginase in vasoregulation directly, porcine subepicardial coronary arterioles (60–110 μm in diameter) were isolated, cannulated, and pressurized for in vitro study under the conditions with and without arginase inhibition. Molecular evidence indicated that arginase I, but not arginase II, mRNA was expressed in the coronary arterioles. The constitutive expression of arginase I protein in the coronary arteriolar endothelial cells was revealed immunohistochemically. Adenosine and serotonin stimulated a threefold increase in NO release and produced dilation of isolated coronary arterioles. NOS inhibitor LNMMA abolished the stimulated NO release and attenuated the dilations in response to these agonists. In contrast to L-NMMA, arginase inhibitor α-difluoromethylornithine (DFMO) increased the NO release by about 80% and also enhanced vasodilations in response to adenosine and serotonin. DFMO inhibited arginase (by 80%), but not NOS, activity in these microvessels without affecting their dilation in response to sodium nitroprusside. Similar to DFMO, intraluminal application of L-arginine enhanced NO-mediated vasodilations. The DFMO-enhanced vasodilation was not observed in the presence of L-NMMA or after endothelial removal, which suggests a regulatory role of endothelial arginase in the NOmediated response. Collectively, this study provides novel findings that the arginase expressed in the endothelium plays a counteracting role in the stimulated NO production, and thus NO-mediated vasodilatory function.