Final answer:
Acetylcholine (ACh) has a negative inotropic effect on the heart, reducing the force of the heart's contraction, unlike the other options which are positive inotropes.
Step-by-step explanation:
The agent or change with a negative inotropic effect on the heart among the options provided is (D) Acetylcholine (ACh). Negative inotropic agents include hypoxia, acidosis, hyperkalemia, and a variety of synthetic drugs. These include numerous beta blockers and calcium channel blockers. Early beta blocker drugs include propranolol and pronethalol. Negative inotropic agents reduce the force of the heart's contraction.
Parasympathetic stimulation, where ACh is released, leads to hyperpolarization of the cardiac muscle cell membranes, ultimately decreasing the force of contraction and stroke volume. On the other hand, sympathetic stimulation, norepinephrine (NE), and cardiac glycosides are positive inotropes, increasing the heart's contractility. Beta blockers are also known negative inotropes; however, they are not listed in the options provided.