Fig. 9.

(A) pSmad levels in response to fluctuating TGF-β levels with a fixed EGF level (E = 0.08 nM). High (T = 400 pM; arrows) and low (T = 1 pM) levels of TGF-β were imposed in a periodic fashion. The pSmad level rises quickly immediately after a step increase of TGF-β, and decays slowly to the low state in response to small input of TGF-β. (B) pSmad levels in response to fluctuating EGF levels with the fixed TGF-β level (T = 100 pM). High (E = 4 nM; arrows) and low (0.01 nM) levels of EGF were imposed in a periodic fashion. The high level of pSmad decreases rapidly to a low level in response to a high EGF level, but the pSmad level slowly creeps up to a high level in response to low EGF. Initial conditions are as in Fig. 8