Abstract

When ouabain and labeled serotonin (¹⁴C-5-HT) or norepinephrine (³H-NE) are simultaneously introduced into a suspension of isolated nerve endings (synaptosomes) in Krebsbicarbonate solution, the initial accumulation of ³H-NE by the synaptosomes proceeds for a few minutes, then abruptly stops. When ouabain is first incubated with synaptosomes in the absence of substrate, the initial accumulation of ¹⁴C-5-HT subsequently added is blocked. Thus, a lapse of time is required for the inhibitory effect of ouabain to develop. Moreover, Na⁺ must also be present with the ouabain. In contrast, ouabain inhibits (Na⁺ + K⁺)-ATPase activity immediately. It appears, therefore, that ouabain blocks the accumulation of amine by an indirect process resulting from the inhibition of (Na⁺ + K⁺)-ATPase. The indirect process requires Na⁺ and is probably the increase in the intracellular Na⁺ concentration which occurs in the presence of ouabain.