The deficit in the broadest measure of U.S. trade narrowed to $135.3 billion last year for the best showing since 1985, despite a steep deterioration in the balance on investment earnings, the government reported Tuesday. The Commerce Department said the deficit in the current account, also known as the balance of payments, shrank 12.1 percent from the rec-ord imbalance of $153.96 billion set in 1987. The current account is the most important trade statistic because it measures not only trade in merchandise but also transactions in investments and other services. The big improvement last year came entirely from merchandise trade, where the deficit shrank by 21.1 percent to $126.5 billion. This reflected a 28 percent surge in American Export sales, which offset a 9 percent rise in imports.