How to represent a call being made in a loop in a sequence diagram?

You can use a box enclosing the message send arrow (and whatever else is inside the same repetitive construct).

See this tutorial for an example.

link to larger image (archived)

Just adding a clearer picture because this one at @joel.tony's answer is damn blur.

As you can see the loop happens inside the frame called loop n. There is a guard, array_size, which controls the loop's iterations.

In conclusion the sequence of the messages inside the loop n frame (those between DataControl and DataSource objects) will happen array_size times.