Prenatal programming of pulmonary hypertension induced by chronic hypoxia or ductal ligation in sheep

Abstract

Pulmonary hypertension of the newborn is caused by a spectrum of functional and structural abnormalities of the cardiopulmonary circuit. The existence of multiple etiologies and an incomplete understanding of the mechanisms of disease progression have hindered the development of effective therapies. Animal models offer a means of gaining a better understanding of the fundamental basis of the disease. To that effect, a number of experimental animal models are being used to generate pulmonary hypertension in the fetus and newborn. In this review, we compare the mechanisms associated with pulmonary hypertension caused by two such models: in utero ligation of the ductus arteriosus and chronic perinatal hypoxia in sheep fetuses and newborns. In this manner, we make direct comparisons between ductal ligation and chronic hypoxia with respect to the associated mechanisms of disease, since multiple studies have been performed with both models in a single species. We present evidence that the mechanisms associated with pulmonary hypertension are dependent on the type of stress to which the fetus is subjected. Such an analysis allows for a more thorough evaluation of the disease etiology, which can help focus clinical treatments. The final part of the review provides a clinical appraisal of current treatment strategies and lays the foundation for developing individualized therapies that depend on the causative factors.

Keywords: chronic hypoxia; ductal ligation; pulmonary hypertension; sheep prenatal programming.

Figure 1

Pulmonary vasoconstriction is orchestrated. Neural, hormonal, and humoral mediators, as well as tissue hypoxia, constrict the lung vasculature in utero and maintain a high level of fetal pulmonary vascular resistance. Many of these agonists work through a G_q-coupled receptor pathway to activate a number of intracellular signaling systems that lead to pulmonary vascular smooth muscle cell contraction. These pathways are detailed in the text and ultimately cause simultaneous activation of myosin light-chain kinase (MLCK) and inhibition of myosin light-chain phosphatase (MLCP). The mechanisms associated with hypoxia-induced pulmonary vasoconstriction are controversial but overlap those of the other mediators. Adult animal studies illustrate that the hypoxia-induced pulmonary vasoconstriction response includes a combination of activation of L-type Ca²⁺ channels (Ca_L), ryanodine receptors (RyRs), Rho-kinase, nonselective cation channels, and inhibition of K⁺ channels. A dashed green arrow indicates an activation pathway; a dashed red line with a bar indicates an inhibition pathway; a dashed blue arrow indicates movement of calcium. CaM/CAMKII: calmodulin and Ca²⁺/calmodulin-dependent protein kinase II; Cl_Ca: calcium-activated chloride channel; CPI-17: C-kinase potentiated protein phosphatase-1 inhibitor; DAG: diacylglycerol; Em: membrane potential; ET-1: endothelin-1; G_q: q alpha subunit of G protein–coupled receptor; InsP₃/InsP₃R: inositol 1,4,5 triphosphate and associated receptor; K_v: voltage-dependent potassium channel; MLC₂₀/MLC_20-P: 20-kDa myosin light chain and phosphorylated moiety; NE: norepinephrine; NSCC: nonselective cation channel; PAF: platelet-activating factor; PGF_2α: prostaglandin F_2α; PKC: protein kinase C; PLC: phospholipase C; PP1c: protein phosphatase-1c; RhoA/ROCK: Ras homolog gene family, member A, and associated kinase; SERCA: sarco/endoplasmic reticulum Ca²⁺-ATPase; 5-HT: serotonin.

Figure 2

Pulmonary vasodilation is a synchronized process. A variety of substances, shear stress, and membrane stretch work together to increase prostacyclin, nitric oxide (NO) production, and other pathways defined broadly as endothelial-derived hyperpolarizing factors that collectively cause vascular dilation. The signaling molecules released from the endothelium and activated pathways work in conjunction with epinephrine and other neurohumoral substances to increase the activity of protein kinases A and G (PKA and PKG, respectively). As discussed in the text, these kinases phosphorylate many targets that reduce the level of vascular contraction, pathways that have not been fully examined in the fetus. A dashed green arrow indicates an activation pathway; a dashed red line with a bar indicates an inhibition pathway; a dashed blue arrow indicates movement of calcium. AC: adenylate cyclase; ACh: acetylcholine; AMP: adenosine monophosphate; BK: bradykinin; BK_Ca: calcium- and voltage-activated potassium channel; Ca_L: L-type Ca²⁺ channel; CaM/CAMKII: calmodulin and Ca²⁺/calmodulin-dependent protein kinase II; cAMP: cyclic adenosine monophosphate; cGMP: cyclic guanosine monophosphate; CO: carbon monoxide; CPI-17: C-kinase potentiated protein phosphatase-1 inhibitor; EDHF: endothelial-derived hyperpolarizing factors; Em: membrane potential; eNOS: endothelial nitric oxide synthase; Epi: epinephrine; G_q/G_s: q and s alpha subunits of G protein–coupled receptor; GMP: guanosine monophosphate; HO-1/2: heme oxygenase 1 and 2; MLC₂₀/MLC_20-P: 20-kDa myosin light chain and phosphorylated moiety; MLCK: myosin light-chain kinase; MLCP: myosin light-chain phosphatase; PDE3/4/5: phosphodiesterases 3, 4, and 5; PGI₂: prostacyclin; PLA₂: phospholipase A₂; PP1c: protein phosphatase-1c; RhoA/ROCK: Ras homolog gene family, member A, and associated kinase; RyR: ryanodine receptor; SERCA: sarco/endoplasmic reticulum Ca²⁺-ATPase; sGC: soluble guanylate cyclase.