The present study examines the role of serum growth factors in the proliferative response and Na-K-Cl cotransport activity of vascular smooth muscle cells from Milan normotensive (MNS) and hypertensive (MHS) rats. Cells from thoracic aorta of both strains were cultured in 10% serum medium and made quiescent by 72 hours in 0.3% serum medium. MHS cells grown with 10% serum had a shorter population doubling time than MNS cells between passages 8 and 12 (13.8±1.7 versus 20.1±1.6 hours, P<.01, n=4). MHS cells also exhibited a higher response of thymidine incorporation into nucleic acid to serum, epidermal, and platelet-derived growth factor BB. In MHS cells epidermal (100 ng/mL) and platelet (50 ng/mL) growth factors increased thymidine incorporation 2-and 10-fold, respectively. In MNS cells epidermal factor did not induce a significant response, and that of platelet factor was twofold lower than in MHS cells. Binding curves revealed a higher number of receptors for platelet than epidermal growth factor in both strains and a similar number of both receptors in MHS and MNS cells. Quantitative immunoblots of these recep-V ascular smooth muscle cells (VSMCs) within the tunica media of adult arteries are in quiescent state of growth, with the cell cycle reversibly arrested in the Gi/G 0 phase. In rats, the replication rate is approximately 0.02%, but this slow growth rate can be altered by mechanical injury and the release of growth factors by platelets, monocytes, and endothelial cells. The initiation of the proliferative cycle and the rate of growth are controlled by autocrine and paracrine secretion of growth factors and the expression of these receptors.1 Platelet-derived growth factor (PDGF), isoform BB, is the most powerful mitogenic for the initiation of the proliferative cycle in VSMCs, 2 and it may play a role in the hyperplasia of the tunica media in hypertension. Stimulation of ion fluxes is among the earliest changes induced by growth factor addition to Go-arrested cells. Activation of Na + -H + exchange by several mitogens has been well characterized in various cell types, 3 and stimulation of Ca 2+ -Na + exchange by PDGF BB 4 and of Na-K-Cl cotransport by epidermal growth factor (EGF) has been reported in VSMCs 5 and fibroblasts.6 Previous investigations have shown thatFrom the Endocrine-Hypertension Division, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Mass (M.C., G.V.), and Departamento de Inmunologia Ginica y Reumatologia, Facultad de Medicina, Universidad Catolica de Chile, Santiago (G.S., E.W., A.G.).Correspondence to Mitzy Canessa, PhD, Endocrine-Hypertension Division, Brigham and Women's Hospital, 221 Longwood Ave, Boston, MA 02115. tor proteins confirmed the observation that the greater proliferation of MHS cells could not be related to a higher number of growth factor receptors. Cotransport activity (bumetanide-sensitive M Rb influx in nanomoles per milligram protein per 5 minutes) was found to be significantly higher in MHS cells (16±3, n=18) t...