All of the following are involved in the transmission of regulatory signals through the ECF, EXCEPT:

Gap junctions are a narrowed poion of the intercellular space containing channels linking adjacent cells and through which pass ions, most sugars, aminoacids, hormones, and cyclic AMP. In electrically excitable tissues these, gap junctions transmit electrical impulses ionic currents and are known as electronic synapses. Gap junctions are involved in transmission of regulatory signals without entering ECF. Gap junctions causes the action potential to transmit from one cardiac muscle to the next cardiac muscle cell. It also causes the transmission of action potentials in neurons and smooth muscle cells. Ref: Ganong's Review of Medical Physiology, 22nd Edition, Page 36; Textbook of Medical Physiology By Guyton and Hall, 10th Edition, Pages 96, 515, 718