Myoepithelial cell contraction initiates glandular release from most exocrine glands developed from the ectoderm. These are the cells that exist in the glandular epithelium.
At the distal end of the glanduar coil, an isotonic plasma filtrate gets actively transported into its lumen. As you move down the coil, close to where it connects to the duct, absorptive cells remove NaCl to make sure the sweat is hypotonic when it reaches the surface of the skin. If you view these cells where staining for alpha tubulin or beta actin, you will see they have abundant levels of myofillaments, something uncharacteristic of epithelial cells. Hence Myoepithelial.