OBJECTIVE	Both mechanisms are mediated by the efflux transporters ABCC4 ( MRP4 ) , ABCC5 ( MRP5 ) , and ABCC11 ( MRP8 ) , which are involved in cellular efflux of endogenous signaling molecules ( e.g. , cyclic adenosine 3 ' , 5 ' - monophosphate and cyclic guanosine 3 ' ,5 ' - monophosphate ) and nucleoside analogues .\