Model Driven Architecture (MDA) is a software development approach that focuses on models. In order to support MDA, a lot of CASE tools have emerged; each of them provides a different set of modeling services (operations for automating model manipulation). We have proposed an open environment called ModelBus, which enables the integration of heterogeneous and distributed CASE tools. ModelBus enables tools to invoke the modeling services provided by other tools. In this paper, we focus on supporting a particular kind of modeling services: services that update models (i.e. they have inout parameters). Our contribution is to enable a tool to update models owned by another tool. We propose a parameter passing mechanism that hides the complexity of model update from tools. First, it enables a tool to update models transparently to heterogeneous model representations. Second, it enables a tool to update models located in the memory of another remote tool transparently, as if the models were local. Third, it ensures the integrity between the updated models and the tool that owns the models.