The Sensex at the Bombay Stock Exchange again gave up all its initial gains, to stand 138 points, or 0.7 percent in the negative zone, at 19,154, in afternoon trade, a short while ago. But in the morning, the Sensex had opened 64 points, or 0.3 per cent in the green, at 19,357, on select buying in blue-chip stocks. The Sensex has already lost 310 points in the last four trading sessions.Stock markets in Singapore, South Korea, Taiwan, Indonesia and Hong Kong were down by between 0.2 percent and 0.7 percent, after a report showing a slowdown in the US economy sparked concern about the sustainability in improvement in corporate earnings.