Diaphragmatic pressure-flow relationship during hemorrhagic shock: role of nitric oxide

In the vascularly isolated resting and contracting (3 Hz) canine hemidiaphragm, we studied the effect of intra-arterial infusion of the nitric oxide (NO) inhibitor NG-nitro-L-arginine (LNA) on the relationship between phrenic arterial perfusion pressure (Pphr) and blood flow (Qphr). In separate groups of animals, either saline or LNA (final concn 6 x 10(-4) M) was infused into the phrenic artery over 20 min. The diaphragm was then autoperfused by diverting flow from the left femoral artery. Arterial blood pressure was reduced in stages by controlled hemorrhage. The Pphr-to-Qphr relationship was plotted for each animal, and the third-order polynomial of best fit was determined by least squares regression. The inflection point of this relationship was determined for each animal. In the contracting and resting diaphragms, the inflection point corresponded to Pphr values of 83.6 +/- 4.7 and 72.5 +/- 6.8 mmHg, respectively, in the saline-treated group compared with 86.2 +/- 2.7 and 76.8 +/- 5.1 mmHg, respectively, in the LNA-treated group. In the contracting diaphragm, LNA reduced Qphr uniformly across the entire range of perfusion pressures. In the resting diaphragm, the effect of LNA was not uniform. At perfusion pressures below the inflection point, the flow was reduced in proportion to the reduction in inflection point flow. At higher perfusion pressures, Qphr was decreased to a greater extent than could be accounted for by the change in inflection point flow.(ABSTRACT TRUNCATED AT 250 WORDS)