The effects of dopamine on Na(+)?K(+)-ATPase activity in nerve ending membranes are prevented by N-ethylmaleimide

The activity of Na(+)?K(+)-ATPase in the membranes of nerve endings isolated from rat cerebral cortex was inhibited by dopamine. On the contrary, when the soluble fraction from cortical homogenates was added, dopamine stimulated enzyme activity. By varying the concentration of the soluble fraction present in the incubation medium for Na(+)?K(+)-ATPase assay, it was possible to establish that this fraction modulates those effects of dopamine on Na(+)?K(+)-ATPase. The preincubation of the membranes with N-ethylmaleimide under conditions in which the Na(+)?K(+)-ATPase activity was not inhibited (5 x 10(?5) M for 10 min at 37 degrees C), prevented both the inhibitory and the stimulatory effects of dopamine observed without or with the soluble fraction from brain respectively. These results suggest that dopamine probably acts on regions of the protein containing -SH groups, different from those sites responsible for the catalytic activity of the enzyme.