Abstract
The release of nitric oxide (NO) by NG-hydroxy-L-arginine (L-OHArg) from endothelial cells (ECs) grown in culture was assessed by two bioassays. The first was a cascade bioassay system, in which the effluent from a column of ECs grown on beads superfused a cascade of isolated endothelium-denuded rabbit aortic strips. Exclusion of calcium from the Krebs' buffer perfusing the ECs (but not from the cascade) diminished the relaxant effects of L-OHArg. Growing the ECs in the presence of dexamethasone did not affect the relaxations of the bioassay cascade induced by L-OHArg. However, these relaxations were potentiated by superoxide dismutase (SOD) and inhibited by haemoglobin, but not inhibited by NG-nitro-L-arginine methyl ester (NO2 ArgMeE). The second bioassay was a transfer assay, in which accumulation of cGMP was measured in detector, RFL-6 fibroblast cells. The accumulation of cGMP induced by the basal release of NO from ECs was increased by L-OHArg and by SOD. However, L-OHArg, unlike SOD, did not scavenge superoxide (O2-) anions, as assessed spectrophotornetrically by the rate of reduction of ferricytochrome C caused by O2- anions. ECs were also incubated, in the presence of SOD, with adenosine diphosphate (ADP) or with ADP plus L-OHArg. The accumulation of cGMP in the detector cells, due to NO release from ECs by ADP in the presence of L-OHArg was significantly greater than the release evoked from ECs by ADP alone. Preincubation of the ECs with NO2ArgMeE inhibited accumulation of cGMP with [C50 values of 5.3 μM (against ADP) and 3.4 μM (against L-OHArg). Preincubation with haemoglobin blocked the cGMP accumulation. Thus, L-OHArg can be metabolised in ECs by a constitutive eNOS or by an alternative, calcium-dependent enzyme which is not affected by dexamethasone or NO2 ArgMeE. The common product of these enzymes is NO or a closely related molecule increasing the concentration of cGMP in intact cells.