Final answer:
The hormone insulin initiates a second messenger system that allows GLUT4 vesicles to move to the cell membrane, facilitating glucose transport into most cell types.
Step-by-step explanation:
The hormone responsible for initiating a second messenger system that moves GLUT4 vesicles to cell membranes in most cell types is insulin. When insulin binds to its receptor on the plasma membrane, it activates a cascade of events that cause the GLUT4-containing vesicles to fuse with the plasma membrane. This action increases the permeability of cells to glucose, allowing glucose to be transported into the cell. The second messenger systems involved in signal transduction can include cyclic adenosine monophosphate (cAMP) and calcium ions, with cAMP being the more common second messenger used by hormones. In the case of insulin, however, the process does not involve the production of cAMP, but rather the mediation of protein kinases and other signaling molecules that directly facilitate the fusion of GLUT4 vesicles with the membrane