The distribution of Ca2'-dependent and &"-independent nitric oxide synthase (NOS) was studied in rabbits and in conteol and endotoxin-treated rats and guinea-pigs. There was a widespread localization of NOS which differed for the two forms of the enzyme and which showed marked differences between species. Endotoxin induced the activity of the Ca2'-independent NOS in many tissues and also inceeased the activity of Ca2'-dependent NOS in the rat ileum and caecum. These results demonstrate the differential distribution of NOSs in control and endotoxin-treated animals and emphasize the widespread biological role of nitric oxide (NO).

Nitric oxide (NO) synthase in rat brain was found to be constitutive and Ca2(+)-dependent. The enzyme in rat lung or liver (predominantly in parenchymal cells) was not constitutive, but was induced by endotoxin treatment and was Ca2(+)-independent. The NO synthases in rat brain and liver or lung are therefore distinct both in their properties and in their regulation.

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