Ductus venosus (DV) sparing means the maintenance of blood flow through the DV following reduction of liver venous blood supply during fetal hypoxia. The present study compared the reactions of the isthmic portion of the DV and intrahepatic veins (IHVs) to catecholamines in vitro. Vessel rings of 1 mm width and 3 mm diameter were obtained from 17 fetal sheep (88-136 days gestational age, median 120 days). The immunohistochemical examination of the DV and IHV was performed in eight cases using an antibody against alpha-smooth muscle actin and an antibody against alpha-adrenergic receptors. Five vessel rings of the DV in early gestation (median 95 days) did not respond to KCl-induced depolarisation. Force development in response to KCl of both vessel types increased with gestational age (P <0.05). The IHV required 4.1 +/- 0.8 min (mean +/- S.E.M.) and the DV 14.5 +/- 4.0 min to reach the maximum tension in response to KCl, which was 5.0 +/- 4.0 mN in the IHV and 2.2 +/- 1.9 mN in the DV (n = 12, P <0.05). The maximum forces developed in response to noradrenaline (norepinephrine; 42 microM, n = 9) and adrenaline (epinephrine; 100 microM, n = 12) were about sixfold higher in the IHV rings than in the DV rings (P <0.05). The EC50 values of the DV and the IHV rings to noradrenaline were 5.9 +/- 1.3 microM and 5.0 +/- 1.3 microM, respectively (P = 0.03). The EC50 values of the adrenaline responses were 2.5 +/- 0.5 microM for the DV and 2.2 +/- 0.7 microM for the IHV (not significant). The alpha-adrenergic receptors were present in the well-structured media of IHVs, but were less distinctive in the wall of the DV. DV sparing can be attributed to an increased resistance of IHVs to catecholamines compared with the DV. The different responses can be explained by different anatomical and functional properties of the two vessel types.
