Does it claim memory of Object when I use one reference for other objects?

[GoodMorningSky]

string strSql;
DataTable tbl;
strSql = "Select * from tbl1";
tbl = this.GetRemoteObj().ExecuteDataSet(strSql, cnnStr).Tables[0]; // A.
..... do something with tbl..
strSql = "Select * from tbl2";
tbl = this.GetRemoteObj().ExecuteDataSet(strSql, cnnStr).Tables[0]; // B
..... do something with tbl..

.... repeat several times using tbl...

In this example, tbl is assigned on B. tbl now points to new object.
I thought that the object created on A lost reference, so it is marked as
eligible for garbage collection. But, I'm not sure about it since there is
lots of memory loss from my application when running the part.
I didn't call Dispose or set tbl=null; .
Then Does garbage collect reclaim the memory that tbl object created on A?
If so, when does it happen?
What is best pattern for the case that using one reference for different
objects repeatly?