Sildenafil and an early stage of chronic hypoxia-induced pulmonary hypertension in newborn piglets

Effect of a phosphodiesterase-5A (PDE5A) gene polymorphism on response to sildenafil therapy in canine pulmonary hypertension

Pulmonary hypertension (PH) is a common clinical condition associated with morbidity and mortality in both humans and dogs. Sildenafil, a phosphodiesterase-5 (PDE5) inhibitor causing accumulation of cGMP, is frequently used for treatment of PH. The authors previously reported a PDE5A:E90K polymorphism in dogs that results in lower basal cyclic guanosine monophosphate (cGMP) concentrations than in wild-type dogs, which could contribute to variability in the efficacy of sildenafil. In this study, response to sildenafil therapy was evaluated in dogs with PH by comparing echocardiographic parameters, quality-of-life (QOL) score, and plasma cGMP concentrations before and after sildenafil therapy. Overall, tricuspid regurgitation estimated systolic pressure gradient (PG) and QOL score were significantly improved after sildenafil therapy, and the plasma cGMP concentration was significantly decreased. Dogs that had a heterozygous PDE5A status had a significantly worse QOL score when compared to the wildtype group after sildenafil treatment. The simple and multiple regression analyses revealed a significant but weak prediction for the percent reduction in QOL score with sildenafil treatment by plasma cGMP level and by the PDE5A:E90K polymorphic status. This study showed that sildenafil treatment improved PH in dogs, and the PDE5A:E90K polymorphism blunted the efficacy of sildenafil in terms of QOL improvement.

Cardiac Therapeutics in Horses

Many cardiac therapeutics lack significant evidence of benefit in the horse, and in many cases their use is based on extrapolation of evidence from other species. In recent years there has been a push to develop a better understanding of both the pharmacodynamics and pharmacokinetics of these drugs. Recent data have described the use of antiarrhythmic agents including sotalol, flecainide, and amiodarone. Data about the use of ACE inhibitors in the management of congestive heart failure are encouraging and support their use in certain cases, wheras evidence for other medicines, such as pimobendan, remain speculative.

Changes in the nitric oxide pathway of the pulmonary vasculature after exposure to hypoxia in swine model of neonatal pulmonary vascular disease

Neonatal pulmonary vascular disease (PVD) is increasingly recognized as a disease that complicates the cardiopulmonary adaptations after birth and predisposes to long-term cardiopulmonary disease. There is growing evidence that PVD is associated with disruptions in the nitric oxide (NO)-cGMP-phosphodiesterase 5 (PDE5) pathway. Examination of the functionality of different parts of this pathway is required for better understanding of the pathogenesis of neonatal PVD. For this purpose, the role of the NO-cGMP-PDE5 pathway in regulation of pulmonary vascular function was investigated in vivo, both at rest and during exercise, and in isolated pulmonary small arteries in vitro, in a neonatal swine model with hypoxia-induced PVD. Endothelium-dependent vasodilatation was impaired in piglets with hypoxia-induced PVD both in vivo at rest and in vitro. Moreover, the responsiveness to the NO-donor SNP was reduced in hypoxia-exposed piglets in vivo, while the relaxation to SNP and 8-bromo-cyclicGMP in vitro were unaltered. Finally, PDE5 inhibition-induced pulmonary vasodilatation was impaired in hypoxia-exposed piglets both in vitro and in vivo at rest. During exercise, however, the pulmonary vasodilator effect of PDE5 inhibition was significantly larger in hypoxia-exposed as compared to normoxia-exposed piglets. In conclusion, the impaired endothelium-dependent vasodilatation in piglets with hypoxia-induced PVD was accompanied by reduced responsiveness to NO, potentially caused by altered sensitivity and/or activity of soluble guanylyl cyclase (sGC), resulting in an impaired cGMP production. Our findings in a newborn animal model for neonatal PVD suggests that sGC stimulators/activators may be a novel treatment strategy to alleviate neonatal PVD.

The principal pathways involved in the in vivo modulation of hypoxic pulmonary vasoconstriction, pulmonary arterial remodelling and pulmonary hypertension

Hypoxic pulmonary vasoconstriction (HPV) serves to optimize ventilation-perfusion matching in focal hypoxia and thereby enhances pulmonary gas exchange. During global hypoxia, however, HPV induces general pulmonary vasoconstriction, which may lead to pulmonary hypertension (PH), impaired exercise capacity, right-heart failure and pulmonary oedema at high altitude. In chronic hypoxia, generalized HPV together with hypoxic pulmonary arterial remodelling, contribute to the development of PH. The present article reviews the principal pathways in the in vivo modulation of HPV, hypoxic pulmonary arterial remodelling and PH with primary focus on the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways. In summary, endothelin-1 and thromboxane A₂ may enhance, whereas nitric oxide and prostacyclin may moderate, HPV as well as hypoxic pulmonary arterial remodelling and PH. The production of prostacyclin seems to be coupled primarily to cyclooxygenase-1 in acute hypoxia, but to cyclooxygenase-2 in chronic hypoxia. The potential role of adenine nucleotides in modulating HPV is unclear, but warrants further study. Additional modulators of the pulmonary vascular responses to hypoxia may include angiotensin II, histamine, serotonin/5-hydroxytryptamine, leukotrienes and epoxyeicosatrienoic acids. Drugs targeting these pathways may reduce acute and/or chronic hypoxic PH. Endothelin receptor antagonists and phosphodiesterase-5 inhibitors may additionally improve exercise capacity in hypoxia. Importantly, the modulation of the pulmonary vascular responses to hypoxia varies between species and individuals, with hypoxic duration and age. The review also define how drugs targeting the endothelin-1, nitric oxide, cyclooxygenase and adenine nucleotide pathways may improve pulmonary haemodynamics, but also impair pulmonary gas exchange by interference with HPV in chronic lung diseases.

Effect of a phosphodiesterase 5 inhibitor on pulmonary and cerebral arteries of newborn piglets with chronic hypoxia-induced pulmonary hypertension

BACKGROUND

The use of phosphodiesterase 5 (PDE5) inhibitors to treat newborns with pulmonary hypertension is increasing. The effect of PDE5 inhibitors on the neonatal cerebral circulation remains unknown. The neonatal piglet model of chronic hypoxia-induced pulmonary hypertension allows the study of the effects of PDE5 inhibitors on both the pulmonary and cerebral circulations.

OBJECTIVES

To determine whether the PDE5 inhibitor, zaprinast, causes dilation in pulmonary and middle cerebral arteries (MCA) of normoxic newborn piglets and those with chronic hypoxia-induced pulmonary hypertension, and to evaluate whether zaprinast alters responses to increased pressure (autoregulatory ability) of the MCA.

METHODS

Two-day-old piglets were raised in normoxia or hypoxia for 3 or 10 days. Pulmonary arteries and MCA were isolated and pressurized, after which changes in diameter to zaprinast were measured. MCA pressure-diameter relationships were determined.

RESULTS

Dilation to zaprinast was similar in pulmonary arteries from normoxic and hypoxic piglets. Zaprinast dilated MCA from all groups but the response was diminished in MCA from piglets raised in hypoxia for 10 days. MCA pressure-diameter relationships (autoregulation) did not differ between the groups.

CONCLUSIONS

Pulmonary artery dilation to zaprinast supports the use of PDE5 inhibitors to treat pulmonary hypertension in neonates. PDE5 inhibitors function as MCA dilators but do not impair the pressure-diameter behavior of the cerebral circulation of either normoxic newborn piglets or those with chronic hypoxia-induced pulmonary hypertension. These findings suggest that cerebral autoregulation is likely to be intact with acute PDE5 inhibitor treatment in infants with pulmonary hypertension in conditions associated with chronic hypoxia.

Over-the-wire intravascular ultrasound in an animal model of pulmonary hypertension

BACKGROUND

Intravascular ultrasound (IVUS) measures prognostically important pulsatile flow indexes in patients with pulmonary hypertension (PH). IVUS catheters traditionally require a guiding catheter for placement which can impact hemodynamics in small infants because the guiding catheter renders the atrioventricular valve incompetent.

METHODS

Domestic swine (1.4-2.2 kg) were raised in isobaric normoxia (n = 4) or hypoxia (n = 3, FiO(2) 10-12%) for 72 hr for induction of PH. Cardiac catheterization and intravascular imaging was performed using a 3.5-Fr 20-MHz Eagle Eye Gold catheter (Volcano Corp., CA, USA) over a 0.014'' guide wire. Intima-media thickness (IMT) was measured and relative area change and vascular pulsatility were calculated.

RESULTS

The IVUS probe was easily manipulated over a 0.014'' wire without hemodynamic compromise in all animals. The IMT was thicker in the hypoxic group than the normoxic group (0.19 ± 0.03 mm vs. 0.31 ± 0.04 mm, p = .067). Hypoxic animals had systolic PH (39.66 ± 2.51 vs. 21.75 ± 2.87 mmHg, p = .02). Systemic arterial pressures between the groups were the same (hypoxic 68 ± 10.44 vs. normoxic 79.75 ± 14.84 mmHg, p = .26). Vascular pulsatility was similar (hypoxic 24 ± 2.64 vs. 20.25 ± 0.57%, p = .18). However, the arterial wall distensibility was significantly different (0.98 ± 0.2 vs. 2.01 ± 1.38 %/mmHg, p = .04).

CONCLUSIONS

Monorail IVUS imaging without a guide catheter overcomes a major limitation for use in infants and small animal experimental models by avoiding hemodynamic compromise. This would be a valuable tool for assessment of PH in the research and clinical setting.

Sildenafil reverses hypoxic pulmonary hypertension in highland and lowland newborn sheep

Perinatal exposure to chronic hypoxia induces sustained hypertension and structural and functional changes in the pulmonary vascular bed. We hypothesized that highland newborn lambs (HLNB, 3600 m) have a higher pulmonary arterial pressure (PAP) due in part to a higher activity/expression of phosphodiesterase 5 (PDE5). We administered sildenafil, a PDE5 inhibitor, during basal and hypoxic conditions in the pulmonary hypertensive HLNB and compared them to lowland newborn lambs (LLNB, 580 m). Additionally, we compared the vasodilator responses to sildenafil in isolated small pulmonary arteries and the PDE5 mRNA expression and evaluated the vascular remodeling by histomorphometric analysis in these newborn lambs. Under basal conditions, HLNB had a higher PAP and cardiac output compared with LLNB. Sildenafil decreased the PAP during basal conditions and completely prevented the PAP increase during hypoxia in both groups. HLNB showed a greater contractile capacity and a higher maximal dilation to sildenafil. PDE5 mRNA expression did not show significant differences between HLNB and LLNB. The distal pulmonary arteries showed an increased wall thickness in HLNB. Our results showed that HLNB are more sensitive to sildenafil and therefore could be useful for treatment of pulmonary hypertension in high-altitude neonates.