Stimulation of imidazoline receptors inhibits proliferation of human coronary artery vascular smooth muscle cells

Vascular smooth muscle cells of rat aorta express imidazoline receptors whose stimulation, by drugs or an endogenous ligand, agmatine, inhibits serum-stimulated proliferation. We investigated whether imidazoline receptors are expressed in human vascular smooth muscle cells if their stimulation is antiproliferative. Cultured human coronary artery vascular smooth muscle cells express a nonadrenergic binding site for H-3-idazoxan and an imidazoline receptor-binding protein as revealed by immunocytochemical and immunoblot analyses with a specific antibody. Idazoxan and agmatine dose-dependently inhibited serum-stimulated proliferation of these cells as measured by the incorporation of H-3-thymidine (IC50: 5 and 70 mu mol/L, respectively) and serum-stimulated expression of proliferating cell nuclear antigen and cell numbers. The agents inhibited proliferation of human and rat (aorta) smooth muscle cells stimulated by either norepinephrine (6560+/-440 disintegrations per minute norepinephrine versus 3345+/-220 norepinephrine and idazoxan), angiotensin II (7680+/-335 disintegrations per minute angiotensin II versus 3769+/-450 angiotensin II and idazoxan), or platelet-derived growth factor (IC50: 3 mu mol/L for idazoxan and 40 mu mol/L for agmatine), indicating inhibition of mitosis mediated by G-protein or receptor tyrosine kinase pathways. We conclude that human vascular smooth muscle cells express imidazoline-receptors whose activation inhibits proliferation by interacting at a distal step in an intracellular signal cascade common to G-protein and receptor tyrosine kinase mitogenic pathways. Agmatine, synthesized in endothelium, may act as a paracrine regulator of vascular smooth muscle cell proliferation through imidazoline receptors, and agents acting at this site may be useful in treating vascular hyperplasia.