Fig. 6. Effects of 10^-4moll^-1 ouabain on (A) fluid secretion rate and (B) K⁺ flux in serotonin-stimulated tubules. At t=10 min, ouabain was added to the Malpighian tubules in the experimental group (open circles) and an equal volume of saline was added to the control group (filled circles). Data are expressed as means ± S.E.M. The experimental and control tubules did not differ significantly; N=10 experimental tubules, N=17 for control fluid secretion and N=11 for control K⁺ flux.