HETEROGENEITY IN ROLE OF ENDOTHELIUM-DERIVED NO IN PULMONARY-ARTERIESAND VEINS OF FULL-TERM FETAL LAMBS

Endothelium-derived nitric oxide (EDNO) modulates fetal pulmonary vaso activity. The role of EDNO in regulation of vasomotor tone in fetal pu lmonary arteries vs. that in veins is not known. We have investigated the role of EDNO in the responses of pulmonary arteries and veins of f ull-term fetal lambs. Fourth-generation pulmonary arterial and venous rings were suspended in organ chambers filled with modified Krebs-Ring er bicarbonate solution (95% O-2-5% CO2 at 37 degrees C), and their is ometric force was measured. N-omega-nitro-L-arginine had no effect on the resting tension of pulmonary arteries with endothelium but caused contraction of pulmonary veins with endothelium. The basal level of in tracellular guanosine 3',5'-cyclic monophosphate (cGMP) of pulmonary v eins with endothelium was higher than that of arteries with endotheliu m. In pulmonary arteries, bradykinin, but not acetylcholine, induced e ndothelium-dependent relaxation and an increase in cGMP content. In pu lmonary veins, acetylcholine, but not bradykinin, induced endothelium- dependent relaxation and an increase in cGMP content. Agonist-induced maximal relaxation and increases in cGMP content were smaller in pulmo nary arteries than in veins. All these endothelium-dependent responses were abolished by N-omega-nitro-L-arginine. In tissues without endoth elium, nitric oxide induced significantly less relaxation and less inc rease in cGMP content in pulmonary arteries than in pulmonary veins. A ll vessels relaxed similarly to 8-bromoguanosine 3',5'-cyclic monophos phate. Our data suggest that the role of EDNO in modulating tone diffe rs between pulmonary arteries and veins in full-term fetal lambs. This difference is in part dependent on the type of agonist used and on di fferences in the activity of soluble guanylate cyclase in vascular smo oth muscle.