Increasing pulmonary artery pulsatile flow improves hypoxic pulmonary hypertension in piglets

Effect of pulsatile stretch on unfolded protein response in a new model of the pulmonary hypertensive vascular wall

Persistent pulmonary hypertension of the newborn (PPHN) is characterized by hypoxemia and arterial remodeling. Dynamic stretch and recoil of the arterial wall during pulsation (in normal conduit arteries, stretch 20% above diastolic diameter) maintains homeostasis; a static arterial wall is associated with remodeling. PPHN is diagnosed by echocardiography as decreased pulmonary artery wall displacement during systole, causing decreased pulmonary arterial pressure acceleration time in a stiff artery.

We hypothesized that a ‘normal’ amplitude of pulsatile stretch is protective against ER stress, while the loss of stretch is a trigger for hypoxia-induced stress responses. Using a novel in vitro model of pulmonary arterial myocytes subject to repetitive stretch-relaxation cycles within a normoxic or hypoxic environment, we examined the relative impact of hypoxia (pulmonary circuit during unresolved PPHN) and cyclic mechanical stretch (diminished in PPHN) on myocyte homeostasis, specifically on signaling proteins for autophagy and endoplasmic reticulum (ER) stress.

Stretch induced autophagosome abundance under electron microscopy. Hypoxia, in presence or absence of pulsatile stretch, decreased unfolded protein response (UPR) hallmark BIP (GRP78) in contractile phenotype pulmonary arterial myocytes. Inositol requiring enzyme-1 α (IRE1α) was not activated; but hypoxia induced eif2α phosphorylation, increasing expression of ATF4 (activating transcription factor-4). This was sensitive to inhibition by autophagy inhibitor bafilomycin A1.

We conclude that in the pulmonary circuit, hypoxia induces one arm of the UPR pathway and causes ER stress. Pulsatile stretch ameliorates the hypoxic UPR response, and while increasing presence of autophagosomes, does not activate canonical autophagy signaling pathways. We propose that simultaneous application of hypoxia and graded levels of cyclic stretch can be used to distinguish myocyte signaling in the deformable pulmonary artery of early PPHN, versus the inflexible late stage PPHN artery.

Intrapulmonary artery balloon pulsation improves circulatory function after acute myocardial infarction in pigs

AIM

To examine whether pulmonary artery balloon pulsation (PABP) could improve circulatory function in acute myocardial infarction (AMI) in pigs.

METHODS/RESULTS

Ten downsize pigs were sedated and ventilated. AMI was induced by inserting a plug into the left anterior descending artery. A pulsation balloon was placed in the pulmonary artery in all animals. In the treatment group (TG), pulsations began when life-threatening arrhythmia or > 30% drop in mean blood pressure (MBP) or > 40% decrease in cardiac output compared to baseline occurred. Pulsation rate was 120/min, independent of the heartbeat, maintained for 10 min. The control group (CG) received no pulsation. In the TG (n = 5), mean BP after the AMI improved by 7 ± 12 mmHg after 150 min while in the CG, MBP decreased by 17 ± 25 mmHg, P < 0.05; coronary perfusion pressure improved by 8 ± 7 mmHg in the TG but decreased by 15 ± 12 in the CG (P < 0.05). In the CG, cardiac output did not change but in the TG it improved from 3.5 ± 0.9 after the AMI to 4.2 ± 1.1 l/min 150 min after AMI (P < 0.05). The TG required 1.8 ± 0.4 electric shocks for ventricular fibrillation versus 0.8 ± 0.4 in the pulsation group (P < 0.05).

CONCLUSION

PABP could be useful in the management of AMI due to improved mean arterial BP, coronary perfusion pressure, cardiac output and electrical stability. The mechanism of this effect remains to be determined.