Developmental change in magnesium sulfate-induced relaxation of rabbit pulmonary arteries

Hypoxic pulmonary hypertension of the newborn

Hypoxic pulmonary hypertension of the newborn is characterized by elevated pulmonary vascular resistance and pressure due to vascular remodeling and increased vessel tension secondary to chronic hypoxia during the fetal and newborn period. In comparison to the adult, the pulmonary vasculature of the fetus and the newborn undergoes tremendous developmental changes that increase susceptibility to a hypoxic insult. Substantial evidence indicates that chronic hypoxia alters the production and responsiveness of various vasoactive agents such as endothelium-derived nitric oxide, endothelin-1, prostanoids, platelet-activating factor, and reactive oxygen species, resulting in sustained vasoconstriction and vascular remodeling. These changes occur in most cell types within the vascular wall, particularly endothelial and smooth muscle cells. At the cellular level, suppressed nitric oxide-cGMP signaling and augmented RhoA-Rho kinase signaling appear to be critical to the development of hypoxic pulmonary hypertension of the newborn.

Antenatal magnesium sulfate and the postnatal response of the ductus arteriosus to indomethacin in extremely preterm neonates

OBJECTIVE

The aim of this study is to evaluate the influence of antenatal magnesium sulfate (MgSO(4)) treatment on the clinical responsiveness of the ductus arteriosus to indomethacin prophylaxis and on that of symptomatic patent ductus arteriosus (sPDA) to indomethacin treatment in premature neonates.

STUDY DESIGN

This is a retrospective study of 160 consecutively admitted neonates with a gestational age of <28 weeks (41 MgSO(4) exposed and 119 controls) who received indomethacin prophylaxis.

RESULT

Incidence of early closure of the ductus arteriosus was lower in the MgSO(4)-exposed neonates than in the control group (59 vs 84%, respectively; P=0.002), whereas incidence of an sPDA was higher (46 vs 24%, respectively; P=0.006). Response to indomethacin treatment was similar between the two groups. Logistic regression analysis indicated increased risk of failure of early ductus arteriosus closure following antenatal MgSO(4) treatment (odds ratio, 4.03; P=0.002).

CONCLUSION

In extremely preterm neonates, antenatal MgSO(4) treatment reduces clinical responsiveness of the ductus arteriosus to indomethacin prophylaxis but not that of sPDA to indomethacin treatment.

Inhaled fluoride, magnesium salt and L-arginine reverse bronchospasma

In vitro studies showed that fluoride and magnesium salts relax bronchial smooth muscle cells. Their combined administration could have potential interest. Magnesium fluoride salt (MgF₂) is nearly insoluble. A soluble derivate can be obtained by introducing L-arginineinine (L-arginine) between the ions. L-arginine, being the substrate leading to the release of NO, might add another relaxing effect to this derivate. Relaxing effects of NaF, MgSO₄, L-arginine, NaF+MgSO₄ and MgF₂+L-arginine given via the inhaled route were studied on rats challenged with acetylmethylcholine (ACMCH) following eight successive doses. Tested salts were given at the fourth dose of ACMCH. Changes in bronchial resistances (R) were measured and compared to results obtained in a control group, receiving ACMCH alone. NaF, MgSO₄, and L-arginine led to significant bronchorelaxing effects (p < 0.05). The association NaF+MgSO₄ gave a greater decrease in bronchial resistance compared to that obtained with each salt (fluoride and magnesium) separately. (MgF₂, 2L-arginine) induced a significant fall in R at the fourth dose of ACMCH just after its inhalation. R values obtained with (MgF₂, 2L-arginine) were significantly lower than L-arginine alone at the sixth dose of ACMCH (p < 0.05). (MgF₂, 2L-arginine) is a triple combination able to induce a significant and constant bronchodilating effect through three different pathways. The effect looked partly additive.

Antenatal exposure to magnesium sulfate and the incidence of patent ductus arteriosus in extremely low birth weight infants

OBJECTIVE

Magnesium sulfate (MgSO(4)) is the most commonly used tocolytic agent in the US and is also employed as a prophylactic agent against seizures in pre-eclamptic women. MgSO(4) crosses the placenta and its concentration in the newborn usually exceeds that of maternal levels. The purpose of this study was to explore the relationship between antenatal exposure to MgSO(4) and the incidence of patent ductus arteriosus (PDA) in extremely low birth weight infants.

STUDY DESIGN

A total of 954 neonates with birth weights between 500 and 1000 g, born at the University of Miami/Jackson Memorial Hospital between January 1995 and December 2004 and surviving for more than 3 days, were followed until death or discharge from the hospital. The incidence of PDA in infants exposed to MgSO(4) was compared with those not exposed and comparisons were also made between infants exposed to different maternal doses of MgSO(4).

RESULTS

The incidence of PDA was significantly higher in the group of infants exposed to MgSO(4) compared with the unexposed control group (67 vs. 60%, P<0.018). When stratified by gestational age the differences were significant only in the group of infants with a gestational age of >or=26 weeks (58 vs. 49%, P<0.039). Logistic regression analysis to adjust for co-variables indicated an increased risk of PDA with higher doses of MgSO(4) (odds ratio 1.33 confidence interval (CI) 1.12 to 1.58, per 50 g of MgSO(4)).

CONCLUSION

Antenatal exposure to MgSO(4) is associated with a higher risk of PDA in extremely low birth weight infants and this effect is more significant and dose-related in more mature infants.

Role of membrane potential in endothelium-dependent relaxation of isolated mouse main pulmonary artery

The physiology of smooth muscle and endothelial cells of a particular vascular bed and from different species differs from each other. Acetylcholine causes an endothelium-dependent relaxation of preconstricted pulmonary arteries from the rat. This relaxation is mediated by nitric oxide (NO) plus a yet-unidentified endothelium-derived hyperpolarizing factor, which relaxes the smooth muscles by hyperpolarizing them. Our aim is to test whether these observations could be generalized to the smooth muscle cells from the mouse pulmonary artery. Smooth muscle or endothelial cell membrane potential of strips of murine pulmonary artery were measured simultaneously with the force developed by the strip. Acetylcholine hyperpolarized the endothelial cells. However, acetylcholine did not induce an endothelium-dependent hyperpolarization of the smooth muscle, while it relaxed the strip in an endothelium-dependent manner. This relaxation was abolished by an inhibitor of NO synthesis, nitro-L-arginine. Moreover, nitroglycerin relaxed the strips without changing the membrane potential of the smooth muscle cells. Injection of Lucifer yellow into the endothelial cells and the smooth muscle cells did not show heterocellular dye coupling. Furthermore, electron microscopy did not show gap junction plate at the myoendothelial junctions. We conclude that in the mouse main pulmonary artery, NO alone is responsible for the acetylcholine-induced endothelium-dependent vasodilatation, whereas the phenomenon called endothelium-derived hyperpolizing factor is not present. Therefore, caution should be taken when comparing different animal models to study pulmonary circulation and its reactivity.

Extracellular Mg2+ blocks endothelin-1-induced contraction through the inhibition of non-selective cation channels in coronary smooth muscle

This study investigated the effects of changing the extracellular [Mg(2+)] ([Mg(2+)](o)) on endothelin-1 (ET-1)-induced contraction of rabbit coronary artery smooth muscle and the involvement of non-selective cation (NSC) channels in this response. Increased [Mg(2+)](o) shifted the concentration/contraction relationship curve of ET-1 to the right. In whole-cell patch clamp recordings, ET-1 (10(-7) M) induced a long-lasting inwards current (94.7+/-7.2 pA) that was inhibited by 8 mM [Mg(2+)](o) (45.3+/-4.4%) and NSC channel blockers (10(-3) M streptomycin and 10(-3) M La(3+)), but not by the voltage-dependent Ca(2+) channel blocker nicardipine. The current/voltage (I/V) curve was linear. Furthermore, in pressurized arteries, the ET-1-induced contraction was also inhibited by La(3+) and streptomycin, but not by nicardipine. U-73122, a selective phospholipase C (PLC) inhibitor and staurosporine and GF 109203X, which block protein kinase C (PKC), reduced ET-1-activated NSC currents by 54.2+/-5.1%, 60.3+/-5.5% and 48.5+/-2.9%, respectively. The inwards current was increased by 1-oleoyl-2-acetyl-sn-glycerol (OAG) and phorbol 12,13-dibutyrate (PDBu), which activate PKC selectively. Like transient receptor potential channel (TRPC3) currents, ET-1-activated NSC currents had a linear I/V relationship, were blocked by flufenamate and activated by a diacylglycerol analogue. These results suggest that [Mg(2+)](o) blocks ET-1-induced contraction of coronary arteries by inhibiting NSC channels. Activation of PLC and PKC might be involved in activation of NSC channels.

Differential responses of newborn pulmonary arteries and veins to atrial and C-type natriuretic peptides

Atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) are important dilators of the pulmonary circulation during the perinatal period. We compared the responses of pulmonary arteries (PA) and veins (PV) of newborn lambs to these peptides. ANP caused a greater relaxation of PA than of PV, and CNP caused a greater relaxation of PV than of PA. RIA showed that ANP induced a greater increase in cGMP content of PA than CNP. In PV, ANP and CNP caused a similar moderate increase in cGMP content. Receptor binding study showed more specific binding sites for ANP than for CNP in PA and more for CNP than for ANP in PV. Relative quantitative RT-PCR for natriuretic peptide receptor A (NPR-A) and B (NPR-B) mRNAs show that, in PA, NPR-A mRNA is more prevalent than NPR-B mRNA, whereas, in PV, NPR-B mRNA is more prevalent than NPR-A mRNA. In conclusion, in the pulmonary circulation, arteries are the major site of action for ANP, and veins are the major site for CNP. Furthermore, the differences in receptor abundance and the involvement of a cGMP-independent mechanism may contribute to the heterogeneous effects of the natriuretic peptides in PA and PV of newborn lambs.