Differences in nitric oxide production in porcine resistance arteries and epicardial conduit coronary arteries

The purpose of this study was to test the hypothesis that endothelial cells from resistance arteries and epicardial conduit coronary arteries differ in their expression of nitric oxide synthase (NOS) and calcium metabolism, and that these differences contribute to the mechanism underlying disparate physiological vasodilator responses observed between the two populations of vessels. The functional vasodilator responses of isolated resistance arteries and epicardial conduit coronary arteries were compared in vitro using both the receptor‐independent agonist A23187 ionophore to increase intracellular calcium and the receptor‐dependent agonist bradykinin. Constitutive NOS (cNOS) activity in monocultures of endothelial cells derived from resistance arteries and conduit arteries was assayed using a fibroblast‐reporter cell method. Intracellular calcium concentration was assessed using fura‐2 microfluorometry. Nitric oxide production was determined using a chemiluminescence technique, while cNOS protein was quantitated by Western blot analysis. A23187 was a less potent vasodilator of resistance arteries studied in vitro, compared to epicardial conduit arteries (EC50 = 1.6 μM, resistance artery vs. EC50 = 0.03 μM, conduit artery); however, bradykinin was more potent in resistance arteries (EC50 = 0.3 nM, resistance artery vs. EC50 = 2 nM, conduit artery). In pure monocultures of endothelium, nitric oxide production measured by chemiluminescence both basally and in response to A23187 was significantly less in resistance arteries (6.1 ± 0.5, basal vs. 10.80 ± 0.55, stimulated nmol/μg protein), compared to conduit arteries (7.7 ± 0.5, basal vs. 17.00 ± 1.52, stimulated nmol/μg protein; P < 0.05 resistance artery endothelium vs. conduit artery endothelium). cNOS enzyme activity assessed by cGMP production in reporter cell fibroblasts was also lower in resistance arteries compared to conduit arteries (0.17 ± 0.03 vs. 0.33 ± 0.05 fmol cGMP/μg protein, respectively; P < 0.05 resistance artery endothelium vs. conduit artery endothelium). Conduit arteries expressed 2.1 × more cNOS protein than resistance arteries, as assessed by Western blotting of cellular homogenates. No significant differences were found with microfluorimetry in either basal or ionophore‐stimulated intracellular calcium concentrations. The results signified that porcine resistance arteries expressed less NOS and produced less nitric oxide than epicardial conduit arteries both basally and in response to an increase in intracellular calcium. This difference was reflected functionally as a decreased vasodilatory response to increased intracellular calcium in resistance arteries that could not be explained on the basis of differences in the metabolism of intracellular calcium. In contrast, the functional vasodilator response of intact vessels to a receptor‐mediated agonist was enhanced in resistance arteries compared to conduit arteries, suggesting an important role of signal transduction mechanisms in specific physiological responses. Thus, the ability of the endothelium to regulate on a regional basis the expression of NOS and integrate receptor‐mediated responses with these differences may provide a mechanism for diverse vasomotor responses in different populations of vessels. © 1996 Wiley‐Liss, Inc.