For water-soluble hormone systems, there is an extracellular "first messenger" that interacts with a cell surface receptor. That receptor directly or indirectly activates an intracellular "second messenger" which causes some sort of biochemical change inside the target cell. Examples of second messengers include (mark all that apply): Group of answer choices cyclic AMP (cAMP) human growth hormone (hGH) oxytocin calcium (Ca++) epinephrine cyclic GMP (cGMP)