Noradrenaline (NA) alpha-1 and alpha-2 receptors
With the noradrenaline (NA) neurotransmitter, for example, two of the receptors are called "alpha-1" and "alpha-2" .... The Alfa-1-receptors stimulate the conversion of liver molecules to sugar molecules (glucogenolysis), resulting in more glucose in the blood. Alpha-1 also stimulates the consumption of oxygen (O2) by the cells and stimulates the conversion of fatty acid in blood to energy. All of these alpha-1 items result in more energy for the person. If the alpha-1 receptors are clogged (e.g. by a pesky heavy metal molecule), one would feel fatigued. The following things can clog NA alpha-1 receptors: antagonist drugs, lead, mercury, cadmium, sulfur-toxins, over 70,000 manmade chemicals, and several natural toxins.
From: http://www.beatcfsandfms.org/html/BrainChem.html

There's also some links on that site to clinical trials that prove this (I assume they do anyway), but I've not looked at them.