Pulmonary capillary endothelium -bound angiotensin -converting enzyme in acute thromboembolic pulmonary hypertension

Abstract

Pulmonary embolism (PE) imposes a significant hemodynamic burden upon the pulmonary circulation, compromising the functional integrity of pulmonary vascular endothelium. The relative contribution of mechanical occlusion alone or together with endothelial dysfunction in the lung vasculature is unclear. By indicator-dilution techniques we assessed relative changes in the activity and mass of pulmonary capillary endothelium-bound (PCEB)-angiotensin converting enzyme (ACE) in acute PE followed by thrombolysis with streptokinase in an autologous clot model of New Zealand White rabbits placed on heart by-pass and perfused with donor blood in Kreb's buffer at a constant flow. We estimated the percent metabolism (%M) and hydrolysis (v) of the ACE-specific substrate 3H-benzoyl-Phenylalanine-Alanine-Proline; 3H-BPAP, representing ACE activity within individual capillary groups. Changes in ACE mass, expressed as the Capillary Perfusion Index (CPI), reflected the dynamically perfused capillary surface area since ACE is uniformly distributed along the vascular endothelial surface. We evaluated the relationship between these parameters and effects of PE and thrombolysis on mean pulmonary arterial pressure (PAP) and employed on-line measurements of peak exhaled nitric oxide levels.

Acute PE-induced increases in PAP 100% above baseline were immediately followed by significant decrease in all indices of PCEB-ACE activity. Without thrombolysis, they remained at low levels, while in the presence of thrombolysis, they significantly increased inversely proportionally to the decreasing PAP suggesting increasing recruitment of previously non-perfused capillary segments. Separate measurements of enzyme affinity constant and the product of enzyme mass with a kinetic constant ratio, showed stable enzyme function but suggested changing enzyme mass as the source of altered enzyme-substrate kinetics. Treating bovine aortic endothelial cells with either thrombin or perfusate from in vivo experiments with acute PE, failed to alter endothelial cell ACE activity. Streptokinase had no effect, both in vivo and in vitro. ETA-receptor blockade did not attenuate acute pressor responses to PE, but in the absence of thrombolysis, all enzyme-substrate kinetic parameters were significantly improved after the first hour of PE. Since this was abolished in the presence of immediate thrombolysis, ET-1-mediated endothelial dysfunction may be superimposed on the hemodynamic burden post-PE, later in the pathophysiological course. Animal pretreatment with superoxide dismutase, catalase and dimethylurea (known to ameliorate endothelial ACE dysfunction in vitro), did not have any effect. Exhaled NO gradually and steadily declined even in the absence of any intervention, but the decrease was more pronounced and immediate in the presence of acute PE. Thrombolysis, although partly reversing the post-thromboembolic decrease in ACE activity indices and lowering PAP, did not have any effect upon peak exNO levels. ETA-blockade transiently increased exNO levels 2.5 min post-PE and exNO remained in significantly higher levels compared to non-ET A-blocker pre-treated groups.

This study demonstrates that acute PE followed by thrombolysis, decreases and subsequently increases and normalizes PCEB-ACE activity inversely proportionally to altered pulmonary hemodynamics. This phenomenon is due to mechanical occlusion rather than endothelial dysfunction, which, however, could complicate vascular luminal loss in the absence of thrombolysis, an effect which may be ET A-receptor mediated. ExNO significantly decreases immediately after PE—an effect reversed by ETA-receptor blockade—independently of restoration in hemodynamics or endothelial ACE activity.