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Background and objectives: In prior studies we demonstrated that meconium-induced lung cell death by apoptosis was associated with activation of the local pulmonary renin-angiotensin system (RASL). Alveolar epithelial apoptosis requires the authocrine synthesis and proteolytic processing of angiotensinogen (AGT) to Angiotensin II (ANG II). Inhibitors of angiotensin converting enzyme (ACE) block meconium-induced apoptosis. ANG II plays an essential role in vascular homeostasis and lung injury. The objectives of this study were to evaluate expression of AGT, ANG II and Caspase 3 in meconium and saline treated newborn lungs and to study mechanisms of its inhibition by a selective antagonist of the AT1 receptor. Methods: Two week old rabbits were studied. Three treatment groups were studied (six rabbit pups in each group): Group 1: rabbits instilled with saline; Group 2: rabbits instilled with 10% meconium; Group 3: Losartan pretreated followed by meconium-instillation. Three groups of A549 human lung epithelial cells were studies as well. Group 4: AGT pretreated and then meconium-exposed cells; Group 5: ANG II pretreated and then meconium exposed cells and Group 6: Caspase 3 inhibitor ZVAD-fmk pretreated and then meconium exposed cells. AGT, ANG II and Caspase 3 were evaluated and compared with and without inhibitors in meconium and control groups. Results: In Situ End Labeling (ISEL) and Caspase 3 assays showed that purified ANG II induced dose dependent apoptosis in rabbit lung lavage cells and the human A549 lung epithelial cell line. Apoptosis also was induced by purified AGT. The increase in apoptotic cells was accompanied by increases in ANG II and Caspase 3 activities. In both airway epithelium and alveolar wall cells, measures of apoptosis were attenuated by Losartan or by the Caspase 3 inhibitor ZVAD-fmk. Conclusions: These data demonstrate the presence of a functional ANG II and Caspase 3 dependent apoptotic pathways in newborn meconium-instilled lungs. They also imply that meconium-induced apoptosis is modulated by the pulmonary RAS<sub>L</sub> system in which ANG II plays a critical role. Both losartan and the Caspase 3 inhibitor ZVAD-fmk pretreatment significantly decreased meconium-induced Caspase-3 activation and lung cell apoptosis

Angiotensin Receptor Type I Blockade Inhibits Apoptosis in Meconium-Instilled Rabbit Lungs