ROLE OF ENDOTHELIUM-DERIVED RELAXING FACTOR IN ACTIVE HYPEREMIA OF THE CANINE DIAPHRAGM

To assess the effect of endothelium-derived relaxing factor (EDRF) on diaphragmatic vascular resistance at rest and during contractions, we studied an in situ isolated diaphragm preparation in anesthetized and mechanically ventilated dogs. The arterial supply of the left diaphragm (phrenic artery) was catheterized and perfused with arterial blood at a fixed flow rate. Drugs were infused through a side port of the arterial catheter at 1/100th of the phrenic arterial flow. The inferior phrenic vein was catheterized to complete the isolation from the systemic circulation. Three sets of experiments were performed. In set 1 (n = 3), we infused endothelium-dependent (acetylcholine, ACh) and endothelium-independent (sodium nitroprusside, SNP) dilators at increasing concentrations. ACh and SNP infusion elicited a dose-dependent decline in phrenic vascular resistance (Rphr) at concentrations > 10(-8) M and 0.50-mu-g/ml, respectively. In set 2 (n = 15), we infused an inhibitor of EDRF synthesis and release, L-argininosuccinic acid (ArgSA), at increasing concentrations (10(-4), 3 X 10(4), and 6 X 10(-4) M). ArgSA produced a dose-dependent increase in Rphr. Infusion of another EDRF inhibitor (N(G)-nitro-L-arginine, LNA, 6 X 10(-4) M) elicited increase in Rphr similar to that induced by ArgSA. In set 3 (n = 25), we infused ArgSA or LNA (6 X 10(-4) M) simultaneously with ACh and SNP and during sustained (2-Hz) contractions of the diaphragm. Both ArgSA and LNA completely reversed ACh vasodilation, whereas SNP vasodilation was reversed by 26 and 11%, respectively. ArgSA or LNA infusion during contractions reversed vasodilation by 48 and 52%, respectively. These data suggest a basal release of EDRF in the diaphragm and an increased release during muscle contraction. However, the incomplete reversal of exercise-induced vasodilation with ArgSA and LNA indicates that EDRF is not the only mediator of exercise-induced hyperemia.