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The ductus venosus is actively regulated in the fetus, but questions remain on the presence of a functional sphincter at its inlet. Using fetal sheep (0.6–0.7 gestation onwards), we have examined the morphology of the vessel and have also determined whether endothelin-1 (ET-1) qualifies as a natural constrictor being modulated by prostaglandins (PGs). Masson's staining and α-actin immunohistochemistry showed a muscular, sphincter-like formation at the ductus inlet and a muscle layer within the wall of the vessel proper. This muscle cell component increased with age. ET-1 contracted dose-dependently isolated sphincter and extrasphincter preparations of the ductus from term fetus. This ET-1 effect also occurred in the premature, but its threshold was higher. BQ123 (1 μM) caused a rightward shift in the ET-1 dose–response curve, while indomethacin at a threshold concentration (28 nM) tended to have an opposite effect. Big ET-1 also contracted the ductus sphincter but differed from ET-1 for its lesser potency and inhibition by phosphoramidon (50 μM). The ductus sphincter (term and preterm) and extrasphincter (term) released 6-keto-PGF1α (hence PGI2) and, to a lesser degree, PGE2 at rest and their release increased dose-dependently upon ET-1 treatment. Both basal and stimulated release was curtailed by endothelium removal. BQ123 and phosphoramidon reduced slightly the contraction of ductus sphincter to indomethacin (2.8 μM). We conclude that the ductus contains a contractile mechanism in the sphincter and extrasphincter regions. ET-1 lends itself to a role in the generation of contractile tone and its action may be modulated by prostaglandins. Keywords: Ductus venosus, endothelin, prostaglandin I2, prostaglandin E2, fetal and neonatal physiology Introduction The ductus venosus is a fetal shunt that, in connecting the portal sinus with the inferior vena cava, allows a large proportion of well-oxygenated umbilical vein blood to bypass the liver and reach rapidly the central circulation, specifically heart and brain (Edelstone & Rudolph, 1979; Bellotti et al., 2000; Kiserud et al., 2000b). After birth, the ductus ceases its function and undergoes closure, but the timing of this process varies individually and between species so that the vessel may persist for a certain period as a portocaval shunt (Zink & Van Petten, 1980; Momma et al., 1992; Fugelseth et al., 1997). The issue of whether prenatal patency and postnatal closure are actively or passively determined has been debated through the years, the specific point of contention being the functional significance of a tissue ridge at the junction of the ductus with the portal sinus (Coceani, 1993; Kiserud, 1999; Mavrides et al., 2002; Ailamazyan et al., 2003; Tchirikov et al., 2003). According to some investigators, including us, this structure is a true sphincter with autonomous regulation of its muscle (see Coceani, 1993; Kiserud, 1999). We have shown, in particular, that the vessel wall is endowed with a noradrenergic innervation (Coceani et al., 1984) along with relaxing and contractile non-neural mechanisms that are, respectively, prostaglandin (conceivably PGI2)- and cytochrome P450 (CYP450)-linked (Adeagbo et al., 1982, 1985, 1990). In contrast to our position, however, Rudolph and his associates (Paulick et al., 1990; Rudolph et al., 1991) have argued that patency and closure are passive processes merely denoting an adjustment of the vessel wall to the prevailing hemodynamic state. To compound difficulties, there is evidence suggesting that any active regulation is not limited to the inlet region, but may extend to the whole ductus (Momma et al., 1992; Mavrides et al., 2002). Recent advances in ultrasound imaging have shed light on this problem (Bellotti et al., 1998, 2000; Kiserud, 1999; Kiserud et al., 2000b). While confirming the importance of extrinsic factors – resistance of the hepatic vascular district, magnitude of the umbilicocaval pressure gradient, blood viscosity – for the control of ductal flow (see Kiserud, 1999), this new line of investigation has yielded direct evidence, both in animals and humans, of an active tone regulation in the putative sphincter region as well as in the remainder of the vessel (Bellotti et al., 1998; Kiserud et al., 2000a). Coincidentally, it has demonstrated that ductus closure is delayed in preterm compared to term infants and, moreover, has ascertained that this sign of prematurity may subside after antenatal treatment with corticosteroids (Fugelseth et al., 1998; Loberant et al., 1999; Kondo et al., 2001). The latter observation is consistent with a gestation-linked maturation of local mechanism(s) that are amenable to upregulation. The present study originates from this premise and from allied research on the ductus arteriosus where a functional complex, linking a CYP450 with endothelin-1 (ET-1), has been implicated in the generation of contractile tension and the postnatal closure of the vessel (Coceani et al., 1999; Coceani, 2000; see also Taniguchi et al., 2001). Within this complex, CYP450 is regarded as the sensing site for stimuli and ET-1 as the effector (Coceani, 2000). Since a CYP450-based contractile mechanism has also been found in the ductus venosus (Adeagbo et al., 1990), we aimed to determine whether ET-1 lends itself to an equivalent effector role. Any such function for ET-1 would accord with the broader role being assigned to this peptide in the perinatal circulation (Perreault & Coceani, 2003). An additional aim was to ascertain whether ET-1, besides exerting a direct constrictor effect, may stimulate the release of a vasorelaxant prostaglandin with modulator properties. The latter occurrence not only would agree with findings in other cell systems (De Nucci et al., 1988; Suzuki et al., 1992), but could also reconcile an effector function of ET-1 with the observed variability in the timing of ductus venosus closure. In the process, we have also examined the morphology of the vessel, both term and preterm, to resolve the questions raised by recent findings reportedly arguing against the existence of a muscle sphincter in the inlet region (Mavrides et al., 2002; Tchirikov et al., 2003). |
