i am not sure that i followed your question. I have not used ExtJS but working with JQuery. With MVC preview 3, the JSON method would return the objects in json format so, i  think the below line is not needed. (with your code, i think you are JSONing a JSONized string - i need to send those 2 words to Webster :) )

String medicos =  ExtJsJsonSerializer.ToExtJsJson(medicoDataObjects, "medicos");

once you have the JSON data on the client side, what you do with it is up to you... 

hope it helps