Vol. 173, No. 2, 1990 December 14, 1990 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages 541-547 DEXAMETHASONE PREVENTS THE INDUCTION BY ENDOTOXlN OF A NITRIC OXIDE SYNTHASE AND THE ASSOCIATED EFFECTS ON VASCULAR TONE: AN INSIGHT INTO ENDOTOXlN SHOCK D.D. Rees, S. Cellek, R.M.J. Palmer and S. Moncada Wellcome Research Laboratories Langley Court, Beckenham, Kent BR3 3BS, United Kingdom Received October 19, 1990 Summary: The relationship between vascular tone and the induction by endotoxin of a nitric oxide (NO) synthase was studied in vitro in rings of rat thoracic aorta. In rings with and without endothelium there was a time-dependent induction of NO synthase accompanied by both spontaneous and L-arginine-induced relaxation and by reduced contractility to phenylephrine. These effects, which were attributable to the presence of endotoxin in the Krebs' buffer, were attenuated by cycloheximide, polymyxin B and inhibitors of NO synthase. Furthermore, dexamethasone inhibited the induction of NO synthase and the consequent effects on vascular tone. These findings indicate that prevention of the induction of NO synthase by glucocorticoids may be an important component of their therapeutic action. ©1990 Academic Press, Inc. Nitric oxide (NO) synthesized from L-arginine by the vascular endothelium plays a role in the physiological regulation of blood flow and blood pressure (1). In this and other tissues, such as the brain, adrenal gland and platelets, NO is produced by a synthase which is constitutive, Ca 2+-, and NAD PH-dependent ( 1,2-4). However, in macrophages, neutrophils, Kupffer cells and hepatocytes (5-7) NO is produced by an 2+ enzyme which is Ca -independent, and is induced by endotoxin (LPS) and cytokines. This NO plays a role in host defence mechanisms, for it is cytotoxic for bacteria, protozoa, fungi and tumour cells (8). Other roles for this NO have not been defined, however, excessive production of immunologically derived NO may be responsible for the hypotension and the reduced responses to pressor agents (9) characteristic of endotoxin shock. L-arginine, which has little or no effect on the tone of freshly isolated rings of vascular tissue, produces a substantial relaxation of rings of rat aorta and bovine pulmonary artery incubated for 6 and 24 h respectively in vitro. This action, which is endothelium-independent, has been attributed to depletion of L-arginine during prolonged incubation (10, 11). We have, however, shown that the vascular endothelium expresses an inducible, Ca2+-independent NO synthase after activation in vitro with LPS 541 0006-291X/90 $1.50 Copyright © 1990 by Academic Press, Inc. All rights of reproduction in any form reserved.