Endothelin (ET)-1, a potent
vasoconstrictor and smooth muscle
mitogen, is produced from its precursor, preproET-1, by
endothelin-converting enzyme (ECE)-1 activity. ET-1 may bind to two receptors, ETA and ETB, that mediate vasoconstriction and vasodilation in the ovine fetal lung, respectively. ET-1 contributes to high pulmonary vascular resistance in experimental perinatal
pulmonary hypertension induced by
ligation of the ductus arteriosus in the fetal lamb. Physiological studies in this model have demonstrated enhanced ETA- and diminished ETB-receptor activities and a threefold increase in lung immunoreactive ET-1
protein content. We hypothesized that increased ET production and an imbalance in receptor expression would favor vasoconstriction and smooth muscle cell
hypertrophy in
pulmonary hypertension and may be partially due to alterations in gene expression. To test this hypothesis, we studied lung
mRNA expression of preproET-1, ECE-1, and the ETA and ETB receptors in normal and hypertensive fetal lambs. Total
RNA was isolated from whole lung tissue in normal late-gestation fetuses (135 +/- 3 days; 147 days = term) and from animals with
pulmonary hypertension after ductus arteriosus
ligation for 8 days (134 +/- 4 days). Ductus arteriosus
ligation increased
right ventricular hypertrophy [control 0.56 +/- 0.02 vs.
hypertension 0.85 +/- 0.05; right ventricle/(left ventricle + septum); P < 0.05]. Northern blot analysis was performed using
cDNA probes and was normalized to the signal for
18S rRNA. We found a 71 +/- 24% increase in steady-state preproET-1
mRNA (P < 0.05) and a 62 +/- 5% decrease in ETB
mRNA (P < 0.05) expression in ductus arteriosus
ligation. ECE-1 and ETA-receptor
mRNA expression did not change. We conclude that chronic intrauterine
pulmonary hypertension after ductus arteriosus
ligation increases steady-state preproET-1
mRNA and decreases ETB-receptor
mRNA without changing ECE-1
mRNA or ETA-receptor
mRNA expression. These findings suggest that increased ET-1 production and decreased ETB-receptor expression may contribute to increased
vasoconstrictor tone in this experimental model of neonatal
pulmonary hypertension.