Conductor James Levine has responded to a story that said he was suffering from ailments that increasingly make him an ineffective leader. He has signed an extension to his Metropolitan Opera contract to 2011, and will take over as music director of the Boston Symphony as scheduled. “I wouldn’t have agreed to it if I didn’t think I would be able to fulfill the contract. As I get older, no doubt I’ll keep changing in some ways, and I hope it’ll mean I keep doing better work. The perception of most of the people I know has been that even when I absorb or deal with something like my back or my tremor, the work gets better. And I think that’s true.”