Shares of Coinbase climbed 12% in premarket trading on Thursday, a day after the cryptocurrency platform became the most valuable U.S. exchange Wednesday amid strong demand for its newly listed stock. Shares of Coinbase made its debut at $381 a share on Wednesday, after a $250 reference price was established Tuesday afternoon. The stock pushed as high as $429.54 before ending at $328.28. At the closing price, Coinbase was valued at $85.8 billion.