HMOX1 and pulmonary hypertension: Similarly, Hartsfield, et al. reported that although inhibition of HO-1 activity with tin protoprorphyrin did not worsen pulmonary hypertension in rats exposed to hypoxia for 5 weeks, the RV did not hypertrophy appropriately but was dilated and had worsened function as compared with untreated hypoxic controls [36].