There are some tentative signs of improving business confidence, although credit demand is yet to respond, according to National Australia Bank's chief executive, Cameron Clyne, at the release of the group's results for the half-year to March 2014.Overall, NAB's business lending franchise has seen its net interest margin decline half-on-half by 7 basis points, from 2.30 per cent in September 2013 to 2.23 per cent in the six months to 31 March 2014. In the same period, the business loan portfolio shifted towards higher quality, with the proportion of "investment grade equivalent" loans on NAB's books lifting from 45 to 48 per cent.Clyne said that despite this active de-risking, NAB was still, out of the major banks, "the most open for business during the GFC". In recent times the bank has again started noticing that "some of the risk appetite of its customers wind back", possibly because people are now looking at the impacts on capital, and the nature of the risk they're taking on with loans.Joseph Healy, NAB's group executive of business banking, told analysts the big priority for NAB has been margin protection, given the relative lack of volume in the market"In recent times we've seen an easing of the margin pressure, but it's too soon to 'call that out' as sustainable," said Healy. Healy said the trend has been flat quarter to quarter. "Margin management has been a big priority for us so revenue impact has not been material," he said.He added that the bank's Asian lending was largely trade related, supporting Asian Australian and New Zealand businesses moving into Asia.