several username/password

Is there a way to "update" the username/password between the TestCases? I want to test a webservice with several different usernames/passwords but it seems that soapUI only reuses the first username/password in the testsuite. Even if I run the testcases one by one, soapUI uses the first used pair.
Only if I close and restart soapUI between each testcase, the username/password saved in the request is used.