Fig. 5. Illustration of the agonistic effect of EPI on a1-AR and antagonizing effect of TA. (A) EPI interacts with a1-AR at the orthosteric site and activates the Gq protein. The activation of Gq protein is followed by the phospholipase C in a cascade reaction and produces inositol triphosphate (IP3). IP3 then activates the Ca++ channel. The accumulation of Ca++ eventually leads to an increase in cell proliferation. (B) PTL, TRY, TYM, PEA and DOP act as a1-AR antagonists. While PTL, TRY and TYM bind to a1-AR orthosteric site and thus acting as competitive inhibitors, PEA and DOP bind to the allosteric site of a1-AR, thus acting as allosteric inhibitors.