Otto Loewi performed an experiment in which he electrically stimulated the vagus nerve connected to one frog heart and let the fluid bathing this first heart pass to a second chamber containing a second frog heart not connected to a nerve. What did Loewi observe?

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Loewi observed that stimulating the vagus nerve of the first heart caused it to slow down, and when the fluid from the first heart's chamber was transferred to the second heart, the second heart also slowed down. This experiment demonstrated that nerve impulses affected heart rate through chemical means, leading to the discovery of the first neurotransmitter, which Loewi named Vagusstoff (later identified as acetylcholine).

Otto Loewi Performed an Experiment in Which He Electrically Stimulated