Caffeine leads to a vasoconstriction, raises the epinephrine levels, and increases nervous signaling. a metabolite of caffeine blocks phosphodiesterase. what is the most likely impact of caffeine on cell signaling? a. caffeine directly binds and activates the protein kinases which signal the described effects, enhancing the effects of caffeine. b. inhibition of the phosphodiesterase results in an elevated camp levels and pka activation, leading to increased glycogen breakdown and increased phosphorylation of events controlling the physiology of caffeine addiction. c. phosphodiesterase leads to increased pip2 breakdown, inhibiting akt signaling. this alters the glycogen metabolism and glucose uptake in the liver after caffeine ingestion. d. the vascular effects of caffeine are long term, indicating a steroid-enhanced activity supported by loss of phosphodiesterase action