Broker Downgrades Sink Medtronic  CHICAGO (Reuters) - Shares of Medtronic Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MDT.N target=/stocks/quickinfo/fullquote"&gt;MDT.N&lt;/A&gt; on  Thursday sank 7 percent to their lowest level in more than  three months after the medical device maker posted  weaker-than-expected growth in one of its key business units,  sparking at least three broker downgrades.