Tokyo stocks were slightly lower Tuesday morning as investors locked in gains after recent 34-year highs of key indexes, while some export-related issues were sought on a weaker yen. The 225-issue Nikkei Stock Average fell 31.74 points, or 0.08 percent, from Monday to 38,438.64. The broader Topix index was down 5.50 points, or 0.21 percent, at 2,634.19. The U.S. dollar remained firm in the lower 150 yen range, supported partly by buying from domestic importers, dealers said. At noon, the dollar fetched 150.30-35 yen compared with 150.10-20 yen in London at 4 p.m. and 149.97-99 yen in Tokyo at …