Flex :: Transform A String In Function And Concatenate With An Object?

I have the following code in actionscript 3:

var async:AsyncToken;
async = bridge.retornamenu();

The bridge is a remote object, instantiated. The retornamenu() is the function that I want the remote object open in amfphp.

However the retornamenu() is a dynamic function, which turns another function, but I can not run it at runtime,

example

var stringfunction:String = "retornamenu()" // this name is dynamic.
var async:AsyncToken;
async = bridge.stringfunction;

But this way does not work, not perform the function retornamenu();