As Congress begins to grapple with health reform, an influential senator is looking hard at one of the medical insurance industry's most controversial practices and raising questions about how well insurers are regulated. Senator John D. Rockefeller IV, the West Virginia Democrat who is chairman of the committee on commerce, is holding hearings on whether health insurers have systematically short-changed patients when they use doctors outside of their health plan's network. The companies are accused of underpaying consumers by hundreds of millions of dollars over the last decade.