Every job description, implicitly or explicitly, includes "other duties as assigned." If your manage menumanagement thinks something Isis part of your responsibilities, and upper management doesn't disagree, it's part of your responsibilities... unless it's illegal, immoral, of otherwise something they really can't ask anyone to do.
If you're overloaded, ask for help prioritizing things if necessary, give them your best estimates of when things will actually be completed on that schedule at a less-than-insane rate of work, and ask if they're OK with that. If not, they can adjust work distribution or target dates appropriately.
If they insist everything is top priority and due yesterday, they'rethere isn't much you can do but look for a better manager.