I believe you may find the following KB on the matter useful: http://www.telerik.com/support/kb/aspnet-ajax/splitter/load-on-demand-radslidingpane.aspx
. You could also use an iframe in the pane if you need to load an entire page and change its src in the same event instead of loading a user control. Of course, a global variable could be useful as a flag so that the content is loaded only once (or according to some custom business logic).
the Telerik team
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