PROVIDENCE, R.I. (AP) _ Claus von Bulow's retrial on charges of attempting to murder his multimillionaire wife moves into its seventh week with prosecutors expected to call the last of their witnesses after putting two medical experts on the stand.

A defense attorney on Friday challenged a second medical expert called by the prosecution who said Martha ''Sunny'' von Bulow's comas were triggered by insulin injections.

Dr. Robert F. Bradley, president of the Joslin Diabetes Center in Boston, testified he was 100 percent certain Mrs. von Bulow's temporary coma in 1979 was caused by insulin injections and 99 percent sure an irreversible coma suffered a year later was also caused by the sugar-regulating hormone.

But attorney Thomas P. Puccio, using medical textbooks, articles and various hypothetical scenarios, tried to erode Bradley's testimony through a battery of questions during two hours of cross-examination.

Prosecutors say Von Bulow, driven by greed for his wife's multimillion dollar fortune and love of another woman, twice tried to murder the Pittsburgh utilities heiress with insulin injections, plunging her into the comas.

His conviction on attempted murder charges in 1982 was overturned by the Rhode Island Supreme Court last year on state constitutional grounds.

Friday's defense strategy was the same one employed Thursday against Dr. George F. Cahill, a Harvard Medical School professor and blood-sugar expert.

Like Bradley, Cahill testified that both comas were caused by insulin injections.

The defense maintains Mrs. von Bulow brought on the comas herself by overindulging in alcohol, drugs and sweets, which aggravated a low blood-sugar condition.

Bradley said he reached his conclusions after studying Mrs. von Bulow's medical records for 40 to 50 hours. Neither he nor Cahill has treated the heiress.

Bradley did not testify at the first trial, but Cahill did and was considered the state's top medical witness.

One of the state's final witnesses next week, the seventh week of the retrial, is expected to be Princess Annie-Laurie Kneissl, Mrs. von Bulow's daughter from her first marriage to a titled Austrian.

She and her brother, Prince Alexander von Auersperg, hired a lawyer whose investigation led to their stepfather's 1981 attempted-murder indictment.

Superior Court Judge Corinne P. Grande also is expected to hear a prosecution motion early next week to allow the 1982 testimony of von Bulow's former lover into the retrial.

Alexandra Isles, a former soap opera actress who testified at the first trial that she pressured the Danish-born socialite to leave his wife and marry her, is out of the country in an apparent attempt to avoid again testifying against her ex-lover.