Circulating levels of ET-1 and NO. (a) Plasma levels of ET-1 were measured in patients with SSc with or without PAH. (b) Serum levels of NO metabolites were measured. (c) The ratios of NO/ET-1 are shown. Data are expressed as mean ± standard deviation. ET, endothelin; NO, nitric oxide; NS, not significant; PAH, pulmonary arterial hypertension; SSc, systemic sclerosis.