Medtronic PLC said Wednesday it is restarting clinic studies on controlling chronic high blood pressure by severing nerves near the kidneys, following a similar announcement from Boston Scientific Corp. in February.

High blood pressure takes a heavy toll on America's public health, and all three big med-tech companies in Minnesota are developing devices for a therapy known as renal denervation. Analysts have predicted a potential multibillion-dollar market, because uncontrolled hypertension can induce atrial fibrillation, stroke, heart failure and kidney failure.

But renal denervation's promise seemed to collapse last year after a highly publicized clinical study failed to document that patients who had the nerves in their renal-arteries severed had larger drops in blood pressure than those who took hypertension drugs alone.

Now Medtronic and Boston Scientific Corp., each of which employs thousands of Minnesotans, say they've refined their research studies and are restarting them. Medtronic is testing its Symplicity Spyral system and Boston Scientific is testing its Vessix system. Both expect to enroll their first patients by the end of June.

A spokeswoman for St. Jude Medical in Little Canada said the company continues to monitor patients outside the United States who have been treated with its EnligHTN ­system. The device doesn't yet have investigational approval in the U.S.

With renal denervation, a doctor inserts a long device studded with electrodes into the main arteries leading to the kidneys. The electrodes fire and burn away tissue in a corkscrew pattern, severing nerves in the arteries that are thought to carry fight-or-flight signals from an hyperactive sympathetic nervous system that cause chronic high blood pressure. Although past testing has found the therapy safe, Medtronic's Symplicity-3 trial failed to show it works better than drugs.

"We were all very surprised by the result of the trial," Sean Salmon, president of Medtronic's renal-denervation division, said in an interview.

Medtronic spent more than $800 million in 2010 to acquire California renal denervation company Ardian, Inc., only to learn last year that results of clinical testing would leave the device many years from full approval from the Food and Drug Administration.

Since then, the companies have spent time and money figuring out why the trial failed to validate anecdotal stories from physicians who say the therapy seems to work well in the patients they've tried it on.

"In retrospect, we've really peeled apart what went on in the trial," Salmon said. "We've done a huge battery of work in both animals and cadavers to better understand what we are trying to get done here. And we have teased out things that were highly variable [and] may have led to the differences we saw."

The initial study included groups of patients who, in retrospect, clearly would not benefit from the therapy. They will now be excluded. Also, Symplicity-3 required patients to go on the maximum-tolerated doses of hypertension drugs, which may have artificially cut blood pressure in the study's control population while also introducing uncertainty for all participants because some patients had trouble reliably taking as many as 17 pills per day.

Medtronic is launching two new randomized 100-person clinical trials in response, one with patients on medication and one without. Both have control populations. Boston Scientific plans to enroll 100 patients in a randomized, controlled study. The National Institutes of Health website on Boston Sci's Reduce-HTN: Reinforce study lists an expected completion date in 2019.

Twitter: @_JoeCarlson