Abstract 252 Poster Session III, Monday, 5/3 (poster 104)

N-methyl-D-aspartic acid (NMDA) been shown to increase pulmonary edema via a nitric oxide dependent mechanism in a isolated perfused rat lung. NMDA also has been shown to alter the vascular response to hypoxia in this model. We demonstrated that NMDA does not in itself effect hypoxic pulmonary vasoconstriction (HPV), but significantly attenuated the effect of NOS inhibition by L-NAME (a NOS inhibitor) in the face of hypoxia. In this current study, we measured NOS activity within the lung in response to six experimental conditions: (a) baseline control, (b) baseline + 1mM NMDA, (c) hypoxic challenge, (d) hypoxia + 2 mM L-NAME, (e) 1mM NMDA + hypoxia, and (f) 1mM NMDA + 2 mM L-NAME + hypoxia. For these experiments, the lungs of adult, male, Sprague-Dawley rats (300-450g) were isolated and perfused using Earl's balanced salt solution with sodium bicarbonate and bovine albumin added. Pulmonary artery pressures were 12-18 mm Hg and left atrial pressures were 1-2 mm Hg. One of the six experimental conditions were applied. After 20 minutes, (+ 6 minutes of hypoxic challenge (0.95 N2, 0.05 CO2) in groups b-f) the perfusion was stopped. The lungs were removed, and frozen in liquid N2. The NOS activity (pmol/min/mg protein) in the lung tissue was determined by a modification of the method of Bredt, et al, measuring the conversion rate of 3H-L-arginine to 3H-L-citrulline. *=p<0.05 vs control by ANOVA. (Table) Hypoxia alone caused a doubling of NOS activity within the lung. This effect was erased by the addition of L-NAME to the perfusate. NMDA alone caused a significant, 3 fold increase in NOS activity, which was not further affected by hypoxic challenge. L-NAME added to the perfusate did not depress the increased NOS activity in the hypoxia + NMDA group. We have shown that NMDA receptor activation does indeed result in increased NOS activity and presumably increased NO production. The ability to maintain NOS activity, and presumably, production of nitric oxide in spite of NOS inhibition may be especially important during ongoing physiologic dysfunction, as can occur in hypoxic ischemic insult, and other pathologic states. These findings serve to increase our understanding of the functions of excitatory amino acids and their receptors outside the central nervous system.

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