The development of chronic pulmonary hypertension (CPH) is often secondary to other chronic lung diseases, such as congestive heart failure, obstructive lung disease, lung fibrosis and the acute respiratory distress syndrome. To develop more effective treatment for these patients, its pathogenesis must be more fully understood. In this application, we propose biochemical, physiological, cellular and molecular studies to examine the role of the endothelin-1/endothelin converting enzyme (ET-1/ECE) system of pulmonary artery smooth muscle cells to the pathogenesis of CPH. We will use a chronically catheterized model of CPH, the sheep receiving continuous air embolization (CAE) into the pulmonary artery for these studies and smooth muscle cells isolated from the intimal (L1) and inner medial (L2) layers of the main and mid-region pulmonary artery from control and hypertensive animals. The following hypotheses will be tested: a) local levels of ET-1 contribute to the onset of CAE-induced CPH; b) cell-and site-specific differences in the ET-1/ECE system and ET-1 receptors of normal main and mid-region pulmonary artery modulate smooth muscle cell function; c) cell and site-specific alterations in the ET-1/ECE system and ET-1 receptors contribute to the structural and functional changes of CPH; d) the L1 cells are more synthetically active than the L2 cells and L2 cells are more responsive to exogenous ET-1; e) ECE gene expression and activity in L1 and L2 cells is modulated by ET-1; and f) local synthesis of growth factors modulates the ET-1-stimulated ET-1/ECE system. We propose two specific aims to address these hypotheses. The first will determine whether alterations in cell- and site-specific differences in the ET-1/ECE system and ET-1 receptor populations in normal main and mid-region pulmonary artery contribute to the onset of CAE-induced CPH. The second will determine whether the ET-1/ECE system and ET-1 receptors are distinct in L1 and L2 cells isolated from main and mid-region pulmonary artery from control and hypertensive sheep, and whether exogenous ET-1 modulates the ET-1/ECE system and, in turn, whether growth factors, e.g., transforming growth factor-beta and insulin-like growth factor-1 modulate ET-1 stimulated ET-1 synthesis. Such studies will contribute to our understanding of the pathogenesis of CPH and ultimately to the development of new and novel therapies for this devastating disease.