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The gastrointestinal tract is the greatest endocrine organ in the body, containing a large number of biologically active peptides (1.2). Several of these peptides function both as circulating hormones and as neurotransmitter substances; some act solely as circulating hormones: some are localized to nerve terminals and probably function as neurotransmitter substances only, and others are most likely biologically active locally in the tissues, having mainly a paracrine effect.
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Jorde et al. (1985) studied this question.
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