The Reagan administration plan to reduce the number of new medical research grants from 6,526 a year to 5,000 could be "devastating" to research in highly promising areas ranging from the use of a new drug against juvenile diabetes to the possible transplant of insulin-producing cells, leaders of the nation's health research establishment told Congress yesterday.

"The budget proposes to freeze scientific progress in its tracks," charged Rep. Henry A. Waxman (D-Calif.), chairman of the House Energy and Commerce subcommittee on health and the environment.

Congress last year approved funds to cover 6,526 research grants by the National Institutes of Health in fiscal 1985, but the administration said that to save money it would award only 5,000.

Rep. William B. Richardson (D-N.M.) said the administration plan amounts to impoundment. Waxman, as well as Sen. Lowell P. Weicker Jr. (R-Conn.), chairman of the Senate Appropriations subcommittee that oversees the NIH, have begun a campaign to force the awarding of all 6,526 grants.

Waxman said he has more than 150 cosponsors for a resolution to order the Department of Health and Human Services to award all the grants.

Rep. Doug Walgren (D-Pa.) said the administration is seeking to hold new grants to 5,000 for 1986 as well as 1985. Rep. Thomas J. Bliley Jr. (R-Va.), defending the administration, said the nation has "serious budget problems" and argued that the 6,500 grants approved for 1985 was "an unprecedented increase" over a previous standard level of 5,000.

Tufts medical school Professor Sheldon Wolff, representing the Infectious Diseases Society, said that 33 grants on the immune mechanism and allergic responses and 14 on new vaccines would go unfunded, as well as 25 on "ways to cure and combat sexually transmitted diseases."

Dr. Karl Sussman, president of the American Diabetic Association, said major NIH-supported projects involving use of a new drug in juvenile diabetes research would go unfunded under the cutback.

Dr. Randall Zusman, of the American Federation for Clinical Research, said that biomedical research is advancing rapidly and that an increase to 6,500 grants is a logical way to take advantage of major public health improvements that can be achieved with intense effort. He said the funding cuts can't be made up by industry and nonprofit groups, "since these sources provide less than 10 percent of the academically based research . . . . "

Dr. Thomas Ryan, president of the American Heart Association, said $1 spent on biomedical research is estimated "to return $10 to $16 in reduced illness and medical costs."