Constitutive NOS expression in cultured endothelial cells is elevated by fluid shear stress

The role of chronic fluid shear stress on endothelial constitutive nitric oxide synthase (cNOS) levels may have an important role in vessel diameter control. We subjected primary human umbilical vein endothelial cells (HUVEC) or bovine aortic endothelial cells (BAEC, passages 2-14) to steady laminar shear stress. In both cell types, the intracellular level of cNOS was elevated within 3 h of flow exposure at 25 dyn/cm2 and remained elevated at 6 and 12 h of flow exposure, compared with stationary controls, as indicated by digital immunofluorescence microscopy. Shear stress exposure for 6 h caused a 2.2 +/- 0.3- and 2.8 +/- 0.3-fold elevation of cNOS protein levels in BAEC (n = 3, P < 0.01) and HUVEC (n = 3, P < 0.01), respectively, in the presence or absence of 1 microM dexamethasone. Dexamethasone suppresses induction of the inducible NOS gene, indicating that cNOS was elevated by fluid shear stress. Flow exposure at 4 dyn/cm2 caused no enhancement of cNOS levels in either cell type. The flow induction of the cNOS protein levels was not blocked by preincubation of BAEC with 100-400 microM of NG-nitro-L-arginine methyl ester, indicating that flow-induced NO (or guanosine 3',5'-cyclic monophosphate) was not involved in the elevation of cNOS levels. Protein kinase C inhibitor H-7 (10 microM) had no effect on induction of NOS protein in BAEC exposed to 25 dyn/cm2. The cNOS mRNA levels were found to be elevated by two- to threefold in BAEC after 6 or 12 h of flow exposure at either 4 or 25 dyn/cm2, and this induction of NOS mRNA occurred in the presence of dexamethasone. The elevation of cNOS levels by chronic flow exposure may provide a mechanism for chronic regulation of vessel diameter by endothelial response to prevailing blood flow.