NATURE AND SITE OF ACTION OF ENDOGENOUS NITRIC-OXIDE IN VASCULATURE OF ISOLATED PIG LUNGS

The site of action of endogenous and exogenous nitric oxide (NO) in is olated pig lungs was investigated by using arterial, double, and venou s occlusion, which allowed precapillary, postcapillary, and venous seg ments to be partitioned into arterial, precapillary, postcapillary, an d venous segments. N-G-nitro-L-arginine (L-NNA; 10(-5) M) increased re sistance in the arterial (35 +/- 6.6%, P = 0.003), precapillary (39.3 +/- 5.1%, P = 0.001), and venous (18.3 +/- 4.8%, P = 0.01) segments, r espectively. Sodium nitroprusside (10(-5) M) and NO (80 parts/million) reversed the effects of L-NNA. Total pulmonary vascular resistance fe ll with increasing flow, due to a fall in precapillary resistance and dynamic resistance, and was significantly lower than mean total resist ance. L-NNA increased the resistances but did not alter the pattern of the pressure-flow relationships. It is concluded that, in isolated pi g lungs, the effect of endogenous NO seems to be dependent on flow in the arterial segment and independent of flow in the precapillary segme nt, but variation of its release does not appear to be fundamental to accommodation to changes in steady flow.