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Two groups of bacteria were determined as oligotrophs and eutrophs. The former can grow only on a poor medium and not on a rich medium, and the latter can grow on a rich medium. Based on such trophic nature of bacterial groups, a method was established for counting viable cell number of oligotrophs and eutrophs, employing poor and rich media. The present method was used to examine water samples of fresh and sea water areas including rivers and lakes in Toyama Prefecture, off the coast of Toyama Bay, and open sea area at Yamato Bank in the Japan Sea. Physical and chemical environmental factors in these areas were also estimated. Based on the result of an extensive survey on these oligotrophic and eutrophic water samples, two general conclusions were deduced: (a) Oligotrophs and eutrophs responded quite similarly to respective chemical environmental factors, and (b) the number of these trophic groups remained almost constant at even higher concentrations of these factors. In fresh water areas, the number of oligotrophs usually exceeded that of eutrophs, and limiting substances for the size of these bacterial populations were considered to be nitrate-nitrogen and dissolved organic nitrogen. From the effect of rainfall on the number and trophic flora of bacteria in a river, the source of river bacteria was suggested at least in part to be attributed to the soil bacteria.In conventional studies on emphasis has been laid on some microorganisms in a natural water system, extreme living conditions specific to water an en-

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Nitric oxide (NO)-generating compounds (NO donors) such as sodium nitroprusside, S-nitroso-N-acetylpenicillamine, S-nitroso-L-glutathione, 3-morpholinosydnonimine (SIN-i), (DL)-(E)-ethyl-2-[(E)-hydroxyimino] -5-nitro-3-hexenamide, and i -hydroxy-2-oxo-3-(N-methyl-3-aminopropyl)-3-methyl-i -triazene inhibited the Na~,K~-ATPase activity purified from porcine cerebral cortex. NO-reducing or -scavenging agents, such as superoxide dismutase or N-(dithiocarbamate)-Nmethyl-D-glucamine sodium salt, L-ascorbic acid, and sulfhydryl (SH) compounds, such as dithiothreitol or the reduced form of glutathione, but not a-tocopherol, prevented the inhibition of the enzyme activity by all NO donors except sodium nitroprusside. Enzyme inhibition could also be reversed by these SH compounds, but not by superoxide dismutase, L-ascorbic acid, and a-tocopherol. 2 -(4-Carboxyphenyl)-4, 4,5, 5-tetramethylimidazolin-i-oxyl 3-oxide (PTIO), which is able to scavenge NO radicals and generate nitrogen dioxide radicals (NO 2), potentiated the inhibition of this enzyme activity induced by all NO donors (except SIN-i). PTIO did not potentiate, but rather attenuated, the SIN-i -induced inhibition. SIN-i has been reported to release both NO and superoxide and thereby to rapidly form peroxynitrite (ONOO). These potentiated and attenuated inhibitions of the enzyme activity induced by PTIO plus all of the NO donors except sodium nitroprusside were prevented by SH compounds, but not by superoxide dismutase, Lascorbic acid, and es-tocopherol. These results suggest that NO donors may release NO or NO-derived products, presumably N02 and ONOO, and may inhibit the Na~,K~-ATPase activity by interacting with a SH group at the active site of the enzyme.