RELAXIN ACTIVATES THE L-ARGININE NITRIC-OXIDE PATHWAY IN VASCULAR SMOOTH-MUSCLE CELLS IN CULTURE

The peptide hormone relaxin (RLX) has been shown to elicit a powerful vasodilatory response in several target organs. This response is media ted by the stimulation of intrinsic nitric oxide (NO) generation. The present study was designed to clarify whether RLX directly promotes th e relaxation of vascular smooth muscle cells through stimulation of NO generation. Vascular smooth muscle cells from bovine aortas were incu bated with RLX at concentrations ranging from 1 nmol/L to 1 mu mol/L. The expression and activity of NO synthase, production of NO, and the intracellular levels of cGMP and Ca2+ were determined. The cell morpho logy and signal transduction mechanisms of these bovine aortic smooth muscle cells in response to RLX were also studied. RLX stimulated the expression of immunoreactive inducible NO synthase and increased signi ficantly and in a concentration-related fashion inducible NO synthase activity, NO generation, and intracellular cGMP levels. Concurrently, RLX significantly decreased cytosolic Ca2+ concentrations and caused c hanges in cell shape and the actin cytoskeleton that were consistent w ith cell relaxation. The signal transduction mechanisms leading to the enhanced expression of inducible NO synthase protein and activity cau sed by RLX involve the activation of tyrosine kinase, phosphatidylchol ine-phospholipase C, and the transcription factor nuclear factor-KB, s imilar to bacterial endotoxins' and proinflammatory cytokines. This st udy suggests that RLX is an endogenous agent capable of regulating vas cular tone by activation of the L-arginine-NO pathway in vascular smoo th muscle cells.