American Renal Associates Holdings Inc. shares jumped 70% in premarket trade Friday, after the provider of outpatient dialysis services said it has reached an agreement to be acquired by private-equity firm Nautic Partners in a deal with an enterprise value of about $853 million. Under the terms of the deal, American Renal Associates shareholders will receive $11.50 per share in cash, equal to a premium of about 66% over the company's closing price on Oct. 1.