Antenatal betamethasone therapy augments isoproterenol and prostaglandin E2-mediated relaxation of preterm ovine pulmonary veins.

Antenatal glucocorticoid therapy improves pulmonary function in preterm newborns. We have determined the effect of antenatal glucocorticoid therapy on isoproterenol and prostaglandin (PG) E2-mediated relaxation in preterm ovine pulmonary veins after birth. Ovine fetuses (121 and 126 d of gestation; term = 150 d) received an ultrasound guided intramuscular injection of betamethasone, 0.5 mg/kg, or saline. Lambs were delivered 15 or 48 h later, ventilated for 3 h, and killed. Isolated fourth generation pulmonary veins were suspended in organ chambers filled with modified Krebs-Ringer solution (95% O2, 5% CO2) at 37 degrees C, and their isometric tension was recorded. During contractions to U46619, isoproterenol and PGE2 induced greater relaxations of pulmonary veins of betamethasone-treated lambs than those of control. Forskolin, an activator of adenylate cyclase, caused greater relaxation in veins of betamethasone-treated lambs than in those of controls. A greater relaxation of veins treated with betamethasone than that of control veins also occurred in the presence of isobutylmethylxanthine, an inhibitor of phosphodiesterases. All vessels relaxed similarly to 8-bromo-cAMP, a cell membrane-permeable analog of cAMP. When stimulated with isoproterenol, PGE2, and forskolin, adenylate cyclase activity of crude membrane preparations of pulmonary veins treated with betamethasone was greater than that of controls. These results demonstrate that antenatal betamethasone therapy potentiates isoproterenol and PGE2-mediated relaxation of pulmonary veins of preterm lambs; an enhanced adenylate cyclase activity explain in part the effect of antenatal glucocorticoid therapy on pulmonary veins of preterm lambs.