NO in the Perinatal Pulmonary Circulation Experimental Aspects

Although NOS is present early in the fetal pulmonary circulation, NOS activity appears to increase with maturation (3, 58). To examine matura-tional changes in pulmonary vascular reactivity, we performed a series of 

Parallel in vivo studies of fetal pulmonary vasoreactivity appear to support these in vitro observations that exogenous NO or agents which directly increase smooth muscle GMP content cause more effective fetal 

FIGURE I Maturational changes in pulmonary vascular responsiveness to acetylcholine (an endothelium-dependent agonist) and sodium nitroprusside (an endothelium-independent agonist). PE, phenylephrine BL, baseline. [From Abman et at. (3).] 

and rhythmic distention of the lung (18), act, in part, by stimulating NO activity. NOS activity is present at least as early as 0.78 term in the premature fetal lung, as reflected by the presence of a hypertensive response to NOS inhibition and attenuation of pulmonary vasodilation during ventilation and increased oxygen at delivery (35) (Fig. 2). 

To further examine the ability of the developing lung circulation to respond to exogenous NO, we studied the hemodynamic effects of inhaled NO in near-term fetal lambs during ventilation with hypoxic gas mixtures 

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