U.S. markets opened lower Thursday as investors digest a mixed bag of corporate earnings. Aetna (AET) topped earnings expectations and raised its guidance for the rest of the year. The health insurance company credited the good results to rising costs for medical care. Aetna agreed to buy Humana (HUM) in July as a flood of mergers in the industry take place. Aetna had the higher earnings even as it paid more in taxes and went through some expensive acquisitions. MasterCard (MA) though, saw its revenue hurt by rebates and incentives to card members. It's earnings however topped expectations. GoPro (GPRO) issued disappointing results as it's sales fell short. It says it took a $19 million charge for lowering the price of its Hero4 session camera, which was aimed at regular users not the daredevil clientele, it usually caters to.