Nitric oxide synthase and arginase in cells isolated from the rat gastric mucosa

Nitric oxide (NO) synthase activity, which converts arginine to citrulline and NO, is present in homogenates of rat gastric mucosal cells. The aims of this study were to identify the form of NO synthase expressed in gastric cells isolated from fed rats, and to investigate the metabolism of arginine by suspensions of intact mucosal cells. Antibodies directed against the neuronal form of NO synthase recognised a protein of 160 kDa on immunoblots of extracts of gastric cells, and stained isolated cells of approx. 8 μm in diameter. NO synthase was enriched in a cell fraction which banded at high-density in a Percoll gradient, and was inhibited (IC₅₀ by N(G)-nitro-L-arginine (0.8 μM), N(G)-monomethyl-L-arginine (12.6 μM), L-canavanine (147 μM), trifluoperazine (140 μM) and by phosphorylation involving protein kinase C. Intact gastric cells converted exogenous arginine to ornithine and citrulline. Arginase was present in the cells, and was predominantly responsible for arginine metabolism because formation of ornithine and citrulline was reduced by the arginase inhibitors, N(G)-hydroxy-L-arginine and L-ornithine, but not by NO synthase inhibitors such as N(G)-nitro-L-arginine. In conclusion, NO synthase that resembles the neuronal isoform is present in gastric mucosal cells, but a pathway involving arginase seems to be largely responsible for citrulline formation from exogenous arginine in intact mucosal cells.