forearm pump

when doing lat pull downs my forearms get really pumped and fail before my back.
also have problems supinate my forearms, can't curl a striaght bar, and find db curls difficult. any ideas?

Weak Point Training.  The sme thing can happen with shoulders, biceps, triceps and other smaller muscles.  The larger muscle is capable of more work and on some is a problem because you can't work the big muscles if your small muscles let you down.  I would work on the smaller muscles for awhile to build strength in them so they can keep up with the larger ones down the road.  To help your wrist with range of motion and strength, try these exercises:
http://www.shapefit.com/forearms-exercises-dumbbell-wrist-flippers.html
wrist flippers work great if the dumbbell is only weighted on one side, but you can cheat your hand to one side if you only have access to premade dumbbells.