Iowa’s taxable retail sales were up 5.6 percent in the first quarter of 2015 compared with the first quarter of 2014, according to a report released Tuesday by the Iowa Department of Revenue. Taxable sales for the quarter were $8.3 billion, with a total computed tax of $496.2 million. Polk County businesses accounted for 21 percent of the state’s total taxable revenue for the quarter, with taxable sales of $1.75 billion. Statewide, other than the “miscellaneous” category, which had the largest percentage gain in sales at 11 percent, the three categories with the largest percentage increase compared with the year-ago quarter were building materials, eating and drinking establishments, and wholesale. Only one category, home furnishings, had a decrease in sales compared with first-quarter 2014.