WASHINGTON (AP) -- A pair of powerful senators have asked Johnson & Johnson to turn over information about its payments to physicians as part of a widening investigation into drugmakers' influence over medical professionals.

The health care conglomerate said Sens. Charles Grassley, R-Iowa, and Herb Kohl, D-Wis., are investigating its relationship with psychiatrists. J&J markets Risperdal, a blockbuster medication prescribed to patients with schizophrenia and bipolar disorder.

Calls placed to New Brunswick, N.J.-based J&J were not immediately returned Wednesday morning.

Grassley and Kohl have been investigating drug company payments to the American Psychiatric Association since July.

The senators also requested information about J&J's relationship with doctors who use its Cypher drug-coated stent, according to a regulatory filing made late Tuesday. Stents are tiny, mesh-wire tubes used to prop open arteries after they have been cleared of fatty plaque.

Grassley and Kohl last month questioned Columbia University about cardiology professors who received consulting payments from companies that market stents, including Medtronic Inc., Abbott Laboratories and J&J. Columbia is the latest university to come under scrutiny for allegedly failing to monitor corporate payments to professors who also receive federal research money.

Grassley is the ranking member of the Senate Finance Committee. Kohl chairs the Special Committee on Aging.

The pair have repeatedly criticized the influence wielded by pharmaceutical and medical device companies over physicians. A bill introduced by the lawmakers earlier this year would require companies to disclose all payments over $500 to physicians. Eli Lilly & Co. recently became the first drugmaker to volunteer to publish such information.

Shares of J&J fell $1.38 to $60.51 in Wednesday afternoon trading.