Variable values in load tests

Hi,
I' m testing an application that creates some users in a DB. The requests sent by SOAPUI have to use different values for a given parameter (userID) that is primary key for the DB table involved.
Unfortunatelly when I launch the load tests I violate this constraint, for all requests created during the load test use the same value as userID.

Please, can somebody suggest me how I could manage this situation?

Thanks in advance for the support.