Interactions between indomethacin, noradrenaline and vasodilators in the fetal rabbit ductus arteriosus

Spontaneous Rhythmic Contractions (Vasomotion) of the Isolated, Pressurized Ductus Arteriosus of Preterm, but Not Term, Fetal Mice

The mechanisms that regulate relaxation of the fetal ductus arteriosus (DA) and its postnatal constriction are the subject of ongoing studies. Using pressure myography, a pattern of rhythmic oscillatory contractions termed vasomotion was observed in the isolated DA of preterm (day 15) fetal mice. Vasomotion was enhanced by oxygen-induced DA constriction and other contractile agents, and diminished by vasodilatory stimuli or inhibition of chloride channels. The DA of late preterm (day 17) or term (day 19) gestation fetal mice did not exhibit vasomotion. These studies establish the stage-specific presence of vasomotion in the DA of fetal mice and suggest that complex events contribute to intrinsic mechanisms for control of fetal DA tone.

Basics and dynamics of neonatal and pediatric pharmacology

Understanding the role of ontogeny in the disposition and actions of medicines is the most fundamental prerequisite for safe and effective pharmacotherapeutics in the pediatric population. The maturational process represents a continuum of growth, differentiation, and development, which extends from the very small preterm newborn infant through childhood, adolescence, and to young adulthood. Developmental changes in physiology and, consequently, in pharmacology influence the efficacy, toxicity, and dosing regimen of medicines. Relevant periods of development are characterized by changes in body composition and proportion, developmental changes of physiology with pathophysiology, exposure to unique safety hazards, changes in drug disposition by major organs of metabolism and elimination, ontogeny of drug targets (e.g., enzymes, transporters, receptors, and channels), and environmental influences. These developmental components that result in critical windows of development of immature organ systems that may lead to permanent effects later in life interact in a complex, nonlinear fashion. The ontogeny of these physiologic processes provides the key to understanding the added dimension of development that defines the essential differences between children and adults. A basic understanding of the developmental dynamics in pediatric pharmacology is also essential to delineating the future directions and priority areas of pediatric drug research and development.

Regulation of vascular tone and remodeling of the ductus arteriosus

The ductus arteriosus (DA), a fetal arterial connection between the main pulmonary artery and the descending aorta, normally closes immediately after birth. The DA is a normal and essential fetal structure. However, it becomes abnormal if it remains patent after birth. Closure of the DA occurs in two phases: functional closure of the lumen within the first hours after birth by smooth muscle constriction, and anatomic occlusion of the lumen over the next several days due to extensive neointimal thickening in human DA. There are several events that promote the DA constriction immediately after birth: (a) an increase in arterial oxygen tension, (b) a dramatic decline in circulating prostaglandinE(2) (PGE(2)), (c) a decrease in blood pressure within the DA lumen, and (d) a decrease in the number of PGE(2) receptors in the DA wall. Anatomical closure of the DA is associated with the formation of intimal thickening, which are characterized by (a) an area of subendothelial deposition of extracellular matrix, (b) the disassembly of the internal elastic lamina and loss of elastic fiber in the medial layer, and (c) migration into the subendothelial space of undifferentiated medial smooth muscle cells. In addition to the well-known vasodilatory role of PGE(2), our findings uncovered the role of PGE(2) in anatomical closure of the DA. Chronic PGE(2)-EP4-cyclic AMP (cAMP)-protein kinase A (PKA) signaling during gestation induces vascular remodeling of the DA to promote hyaluronan-mediated intimal thickening and structural closure of the vascular lumen. A novel target of cAMP, Epac, has an acute promoting effect on smooth muscle cell migration without hyaluronan production and thus intimal thickening in the DA. Both EP4-cAMP downstream targets, Epac and PKA, regulate vascular remodeling in the DA.

Fishing for prostanoids: deciphering the developmental functions of cyclooxygenase-derived prostaglandins

Prostaglandin G/H synthases (PGHS), commonly referred to as cyclooxygenases (COX-1 and COX-2), catalyze a key step in the synthesis of biologically active prostaglandins (PGs), the conversion of arachidonic acid (AA) into prostaglandin H(2) (PGH(2)). PGs have important functions in a variety of physiologic and pathologic settings, including inflammation, cardiovascular homeostasis, reproduction, and carcinogenesis. However, an evaluation of prostaglandin function in early development has been difficult due to the maternal contribution of prostaglandins from the uterus. The emergence of zebrafish as a model system has begun to provide some insights into the roles of this signaling cascade during vertebrate development. In zebrafish, COX-1 derived prostaglandins are required for two distinct stages of development, namely during gastrulation and segmentation. During gastrulation, PGE(2) signaling promotes cell motility, without altering the cell shape or directional migration of gastrulating cells. During segmentation, COX-1 signaling is also required for posterior mesoderm development, including the formation of vascular tube structures, angiogenesis of intersomitic vessels, and pronephros morphogenesis. We propose that deciphering the role for prostaglandin signaling in zebrafish development could yield insight and ultimately address the mechanistic details underlying various disease processes that result from perturbation of this pathway.

Response of fetal rabbit ductus arteriosus to bradykinin: role of nitric oxide, prostaglandins, and bradykinin receptors

Nitric oxide plays a major role in vascular tone control. Increased blood levels of bradykinin (BK), which stimulates nitric oxide biosynthesis, occur at birth. BK effects on ductus arteriosus (DA) tone were investigated in fetal rabbit under fetal (2.5% O2 "low PO2") and neonatal (30% O2 "high PO2") conditions using in vitro isometric tension studies. Intact and endothelium-denuded DA, contracted with norepinephrine (ED75-90), showed a biphasic response to BK, with relaxation at 10(-9) to 10(-7) M BK and contraction at 10(-6) to 10(-5) M BK. BK (10(-6) to 10(-5) M) contracted intact DA from baseline tension, with greater contraction under high PO2. The B2-receptor antagonist D-Arg-[Hyp3,Thi5,D-Tic7,Oic8]-BK (Hoe-140, 10(-7) M) abolished relaxation, but not contraction, to BK in intact and denuded DA. The B1-receptor antagonist des-Arg9-[Leu5]-BK (10(-7) M) reduced BK-induced contraction but not relaxation in intact DA only. Nitric oxide synthase inhibitors, N(omega)-nitro-L-arginine methyl ester (10(-4) M) and N(omega)-monomethyl-L-arginine (10(-4) M) partially inhibited relaxation to BK in intact DA, with L-arginine (3 x 10(-4) M) reversing N(omega)-monomethyl-L-arginine inhibition. N(omega)-nitro-L-arginine methyl ester (10(-4) M) caused a small but significant inhibition of relaxation to BK in denuded DA. Indomethacin (2.8 x 10(-6) M), a cyclooxygenase inhibitor, abolished relaxation but not contraction to BK in intact and denuded DA. BK-induced relaxation of the DA acts through B2-receptors, releasing both nitric oxide and prostaglandins, whereas endothelial B1-receptors may mediate contraction. BK action on isolated DA changes from relaxation to contraction as its concentration increases, with greater contraction at neonatal PO2. Thus increased BK levels at birth may aid functional closure of the DA.

Cyclooxygenase-2 plays a significant role in regulating the tone of the fetal lamb ductus arteriosus

Nonselective cyclooxygenase (COX) inhibitors are potent tocolytic agents but have adverse effects on the fetal ductus arteriosus. We hypothesized that COX-2 inhibitors may not affect the ductus if the predominant COX isoform is COX-1. To examine this hypothesis, we used ductus arteriosus obtained from late-gestation fetal lambs. In contrast to our hypothesis, fetal lamb ductus arteriosus expressed both COX-1- and COX-2-immunoreactive protein (by Western analysis). Although COX-1 was found in both endothelial and smooth muscle cells, COX-2 was found only in the endothelial cells lining the ductus lumen (by immunohistochemistry). The relative contribution of COX-1 and COX-2 to PGE2 synthesis was consistent with the immunohistochemical results: in the intact ductus, PGE2 formation was catalyzed by both COX-1 and COX-2 in equivalent proportions; in the endothelium-denuded ductus, COX-2 no longer played a significant role in PGE2 synthesis. NS-398, a selective inhibitor of COX-2, was 66% as effective as the selective COX-1 inhibitor valeryl salicylate and the nonselective COX inhibitor indomethacin in causing contraction of the ductus in vitro. At this time, caution should be used when recommending COX-2 inhibitors for use in pregnant women.

Lamotrigine inhibits depolarization-evoked Ca++ influx in dissociated amygdala neurons

Spectrophotometry with the Ca(++)-sensitive dye fura-2 was used to study the effect of lamotrigine (LAG) on the depolarization-evoked Ca++ influx in the acutely isolated basolateral amygdala neurons. Depolarization of the neurons with high K+ resulted in the elevation of intracellular Ca++ concentration [Ca++]i in a concentration-dependent manner. The K(+)-induced Ca++ influx was completely blocked in the Ca(++)-free solution or by Cd++, indicating that depolarization-induced increases in [Ca++]i were triggered largely, if not at all, by Ca++ entry from extracellular space and Ca++ entry occurred through voltage-dependent Ca++ channels. Application of LAG reduced the depolarization-evoked Ca++ influx in a concentration-dependent manner. The effect of LAG was markedly reduced in the presence of N-type Ca++ channel blocker omega-conotoxin-GVIA (omega-CgTX). These results suggest that the action of LAG is mediated, at least in part, by the modulation of N-type Ca++ channels.

Role of potassium channels and nitric oxide in the effects of iloprost and prostaglandin E1 on hypoxic vasoconstriction in the isolated perfused lung of the rat

1. The aims of this study were to compare in the rat isolated perfused lung preparation, the antagonist effects of iloprost, a stable analogue of prostacyclin, and prostaglandin E1 (PGE1) on the hypoxic pulmonary pressure response, and to investigate the possible involvement of KATP and KCa channels and of EDRF (NO) in the effects. In addition, iloprost and PGE1 effects were compared to those of adenosine and forskolin. 2. Isolated lungs from male Wistar rats (260-320 g) were ventilated with 21% O2 + 5% CO2 + 74% N2 (normoxia) or 5% CO2 + 95% N2 (hypoxia) and perfused with a salt solution supplemented with ficoll. Glibenclamide (1 microM), charybdotoxin (0.1 microM), NG-nitro-L-arginine methyl ester (L-NAME, 100 microM) were used to block KATP, KCa channels and NO synthesis, respectively. 3. Iloprost, PGE1, adenosine and forskolin caused relaxation during the hypoxic pressure response. The order of potency was: iloprost > PGE1 = forskolin > adenosine. EC50 values were 1.91 +/- 0.52 10(-9) M, 3.31 +/- 0.58 10(-7) M, 3.24 +/- 0.78 10(-7) M and 7.70 +/- 1.68 10(-5) M, respectively. Glibenclamide, charybdotoxin and L-NAME inhibited partially the relaxant effects of iloprost and forskolin but not those of PGE1. 4. It is concluded that in the rat isolated lung preparation, iloprost and forskolin but not PGE1 dilate pulmonary vessels partly through KATP channels, KCa and nitric oxide release. Furthermore our results suggest that the role of cycli AMP in these effects is not unequivocal.

alpha-Toxin-permeabilised rabbit fetal ductus arteriosus is more sensitive to Ca2+ than aorta or main pulmonary artery

OBJECTIVES

The Ca2+ sensitivity of contractile protein-generated tension production was measured in the smooth muscle of the rabbit ductus arteriosus and compared with two neighbouring fetal blood vessels (main pulmonary artery and aorta). The effect of prostaglandin E2 (PGE2), 3-isobutyl-1-methylxanthine (IBMX, a phosphodiesterase inhibitor), cyclic adenosine 3'5'-monophosphate (CAMP) and forskolin (an activator of adenylate cyclase) on Ca(2+)-activated force generated by preparations from ductus arteriosus was also examined.

METHODS

Strips of smooth muscle from the three vessels were permeabilised using crude alpha-toxin from Staphylococcus aureus. The relationship between [Ca2+] and force production was then measured in the three tissues and the effect of PGE2, cAMP, IBMX and forskolin was examined on submaximal Ca(2+)-activated force (0.3 microM Ca2+) in preparations from rabbit ductus arteriosus.

RESULTS

Permeabilised smooth muscle from fetal rabbit ductus arteriosus was significantly more sensitive to Ca2+ (EC50, 0.20 microM) than its two neighbouring blood vessels aorta (EC50, 0.52 microM) and main pulmonary artery (EC50, 0.72 microM). Submaximal Ca(2+)-activated force (0.3 microM Ca2+) was depressed by PGE2 (1 nM) in the presence of IBMX (10 microM), by cAMP (10 and 100 microM) and by forskolin alone (0.1 microM and 1 microM).

CONCLUSION

PGE2-mediated depression of Ca(2+)-activated force in the smooth muscle of the ductus arteriosus may play a role in the maintenance of a patent ductus arteriosus in the fetus. The intrinsically high Ca2+ sensitivity of smooth muscle contractile proteins may aid the sustained vasoconstriction of the ductus when the PGE2 levels fall after birth.