Stimulation of endothelin mRNA and secretion in rat vascular smooth muscle cells: a novel autocrine function.

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Abstract
Endothelin (ET), a peptide originally isolated from the supernatants of cultured endothelial cells, exerts a wide variety of biological effects in different tissues. Endothelial-cell-synthesized ET-1 has been proposed to act in a paracrine manner on adjacent smooth muscle cells (SMC) in vivo, with effects that include both vascular reactivity (vasodilation/vasoconstriction) and mitogenesis. This study, by the use of immunocytochemically characterized SMC (rVSMC) isolated from the aortas of spontaneously hypertensive rats, has investigated a possible autocrine role for ET in regulation of the vasculature. Although quiescent cultures of rVSMC apparently did not constitutively express prepro ET-1mRNA, ET-specific transcripts could be induced by a variety of growth factors (transforming growth factor beta [TGF-beta]; platelet-derived growth factor-AA homodimer [PDGF-A chain]) and vasoactive hormones (angiotensin II [Ang II], arginine-vasopressin, and ET-1 itself). The kinetics for prepro ET-1mRNA induction in rVSMC were characteristically rapid in onset and transient. Down-regulation of protein kinase C by 48 h pretreatment of rVSMC with phorbol ester markedly reduced the subsequent ability of rVSMC to express ET-1 transcripts and secrete ET-1 peptide in response to Ang II. Inducible prepro ET-1mRNA expression was accompanied by a cycloheximide-inhibitable release of ET-1 peptide into the medium of rVSMC. ET-1 peptide was determined by both radioreceptor- and radioimmunoassay. Stimulated rVSMC accumulated ET-1 (approximately 200 pg.10(6) cells-1 x 4 h-1) at levels that attained biological relevance (approximately 10(-10) M). Sep-pak C18 extracts of medium from stimulated rVSMC elicited contraction of isolated endothelium-denuded rat mesenteric resistance vessels, and this response was characteristically protracted and difficult to "wash out." Synthetic (porcine) ET-1 promoted the expression of transcripts for PDGF-A chain, TGF-beta, and thrombospondin in quiescent rVSMC. Such effects of ET-1 on gene expression may be relevant to the mitogenic potential of ET-1 on VSMC. Our findings imply a role for ET-1 in the control of vascular function via both paracrine and autocrine regulatory mechanisms. The expression of prepro ET-1mRNA and peptide biosynthesis by rVSMC may have both short-term (e.g., vasoconstriction) and long-term (e.g., structural remodeling) consequences. A sustained loop of autocrine stimulation by ET-1 in SMC could contribute toward the pathogenesis of vasospasm and/or atherosclerosis.