Purpose: The historical background towards establishing the existence of chemical signaling. To understand this experiment, one needs to understand its importance and why it mattered. Before this experiment, it was not clear that signals between cells were chemical or electrical in nature.
Background information on the effect of neurotransmitters on heart beat:
How to slow down the heart
  • Stimulation of the vagus nerve releases acetylcholine (ACh). Muscarinic receptors on the heart bind ACh. This slows down the heart.
  • A short digression: The muscarinic receptors are metabotropic receptors. What we mean by metabotropic is that it involves a cascade of secondary messenger effects following binding of a neurotransmitter (which, in this case, is ACh). Commonly, this might be G-protein coupled. This is in contrast to an ionotropic receptor, which binds a neurotransmitter, and opens up and allows an ion to flow in. Thus, usually in metabotropic receptors, it takes longer for the effect of the NT binding to manifest vs. an ionotropic receptor binding a NT. I will review this idea again in a future post.

How to speed up the heart

  • Stimulating the accelerans nerve releases norepinephrine (NE). B-adrenergic receptors bind NE. This speeds up the heart. As an aside, B-adrenergic are also metabotropic receptors.
Experimental setup: Loewi setup 2 hearts bathing in solutions. Heart 1 had its vagus nerve stimulated… which slowed down Heart 1. Heart 1 was then taken out and Heart 2 dropped into the solution that Heart 1 was bathed in. Because the signaling mechanism was chemical in nature, Heart 2 also slowed down, even though its vagus nerve was never stimulated!
Result: Placing heart 2 into the same solution in which heart 1’s vagus nerve was stimulated in… slowed down heart 2. Proof that there was some chemical messenger! Vagusstuff!
Important notes: Loewi was very lucky that he chose the heart, where there was not the presence of Acetylcholinesterase, which breaks down Ach. If he had chosen another tissue or area, there would have been the breakdown of ACh, and there would have been no proof of chemical signaling. 😦
Next post, we will review ionotropic vs. metabotropic receptors!