Thornburg Mortgage said today that it has received more than $300 million in margin calls on a portfolio of securities backed by Alt-A loans. According to published reports, shares of Thornburg dipped $3.09, or 26.8%, to $8.45 in premarket trading today. Shares have traded between $7.49 and $28.40 over the past year. The mortgage company stated in a regulatory filing with the Securities and Exchange Commission that it is facing margin calls as result of the plummeting of the value of its Alt-A RMBS between 10% and 15% since the end of last month. As of Feb. 15, the mortgage originator said it that it had $2.9 billion of exposure to these troubled mortgages.