first messenger

noun

Medical Definition of first messenger

: an extracellular substance (as the hormone epinephrine or the neurotransmitter serotonin) that binds to a cell-surface receptor and initiates intracellular activity — compare second messenger

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“First messenger.” Merriam-Webster.com Medical Dictionary, Merriam-Webster, https://www.merriam-webster.com/medical/first%20messenger. Accessed 5 Mar. 2021.

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