Add tiny pinch of DTT into the 3 muscle sample tubes. Gently shake tubes and let samples sit for 5 minutes. In the meantime, get 3 new microfuge tubes and label them 'skeletal', 'smooth', and 'cardiac'. DTT denatures proteins by breaking disulfide bonds, helping unfold the proteins so they can move freely through the agarose during electrophoresis. Thus, the proteins will separate by molecular weight and charge, and not by 3-dimensional structure.
Heat also helps denature the proteins.
Samples are ready to load into the gels. Be sure to keep track of which samples are loaded into which wells.