Mechanism of norepinephrine growth induction. Panel A is a western blot of Tf binding to D39 cultures (+) compared with non-Tf supplemented control cultures (-); the influence of growth phase on Tf binding is also shown: log-phase (L), stationary phase (S). Tf represents purified transferrin and is the positive control, indicated by the arrow. Panel B shows that uptake of Tf-complexed Fe (in the form of 55Fe) is increased in the presence of NE, P < 0.05; Panel C shows 3H-NE is internalised by D39. The values shown in panels B and C are means of triplicate counts; n = 3.