LAUSR

We examined the effects of exogenous dopamine on gill (Na+, K+)-ATPase activity in vitro in microsomal preparations from juvenile or adult freshwater shrimp, Macrobrachium amazonicum. Dopamine had no effect on enzyme activity in juveniles but stimulated activity in adult shrimp gills by ≈35%. Stimulation of the gill (Na+, K+)-ATPase in adult shrimps by 100 mmol L-1 dopamine was characterized kinetically by varying ATP, MgATP, and Na+ and K+ concentrations, together with inhibition by ouabain. Dopamine stimulated ATP hydrolysis by ≈40% obeying Michaelis-Menten kinetics, reaching VM = 190.5 ± 15.7 nmol Pi min-1 mg-1 protein, KM remaining unaltered. Stimulation by Na+ (≈50%) and K+ (≈25%) revealed distinct kinetic profiles: although KM values were similar, Na+ stimulation followed cooperative kinetics, contrasting with the Michaelian kinetics seen for K+. Stimulation by MgATP increased activity by ≈30% with little change in KM. Similar saturation profiles were seen for ouabain inhibition with very similar calculated KI values. Our findings suggest that dopamine may be involved in hemolymph sodium homeostasis by directly binding to the gill (Na+, K+)-ATPase at a site different from ouabain, thus stimulating enzyme activity in an ontogenetic stage-specific manner. However, dopamine binding does not affect enzyme affinity for cations and ouabain. This is the first report of the direct action of dopamine in stimulating the crustacean gill (Na+, K+)-ATPase.