Vasoactive intestinal polypeptide (VIP) is a peptide consisting of 28 amino acid residues and belongs to the glucagon/secretin superfamily. VIP was first isolated as a potent vasodilator from the porcine duodenum in 1970. It is widespread in many internal organs and systems, functioning as a neurotransmitter and/or neuromodulator in both the central and peripheral nervous systems. VIP exerts its effects through two types of G protein-coupled receptors, VPAC1 and VPAC2, which stimulate cellular adenylyl cyclase activity and the protein kinase A pathway, and may also regulate other cellular pathways involving nitric oxide, phospholipase C, and MAP kinases.

In the carotid body, VIP-positive nerve fibers have been identified in numerous species including mammals, birds, and amphibians. These nerve fibers are characterized by numerous varicosities and are primarily located in connective tissue between glomus cell clusters and near blood vessels. The distribution of VIP in the carotid body shows clear interspecies differences, with VIP found in nerve fibers in most species, though in humans it has also been described in glomus cells. The number of VIP-containing nerves in the carotid body can change under various physiological and pathological factors, including hypoxia. VIP is known to impact glomus type I cells, causing changes in their spontaneous discharge, but its main impact on the carotid body is likely connected with vasodilatory effects and influence on blood flow and oxygen delivery.

• Gonkowski S. Vasoactive Intestinal Polypeptide in the Carotid Body-A History of Forty Years of Research. A Mini Review. Int J Mol Sci. 2020 Jun 30;21(13):4692. doi: 10.3390/ijms21134692. PMID: 32630153; PMCID: PMC7370131.
• Mutt V, Said SI. Structure of the porcine vasoactive intestinal octacosapeptide. The amino-acid sequence. Use of kallikrein in its determination. Eur J Biochem. 1974;42(2):581-9.
• O'Donnell M, Garippa RJ, O'Neill NC, Bolin DR, Cottrell JM. Structure-activity studies of vasoactive intestinal polypeptide. J Biol Chem. 1991 Apr 5;266(10):6389-92. PMID: 1848854.

三氟乙酸（Trifluoroacetic acid，简称 TFA）是高效液相色谱（简称 HPLC）纯化过程中常用的反离子。TFA 的存在可能会对肽的净重量、外观及溶解度产生影响，具体如下：

对净重量的影响：TFA 盐会增加产物的总质量。在大多数情况下，肽的含量占总重量的 80% 以上，剩余部分则为 TFA。

对溶解度的影响：TFA 盐通常能提高肽在水溶液中的溶解度。

在生物检测中的影响：对于大多数标准的体外检测，残留的 TFA 水平一般不会造成干扰。但需注意，在高灵敏度的细胞研究或生化研究中，需关注其存在可能带来的影响。

干品多肽的重量中不仅仅包含多肽，还包含有一些非肽的组份，如水、被吸收的溶剂、配位离子和盐等。肽的净含量是指肽在其中的重量百分比，这个百分比的数值范围很大，可能从50%到90%，取决于纯度、序列以及合成和纯化的方法，不要将肽的净含量和肽的纯度混为一谈，他们是两个完全不同的概念。纯度通常是通过HPLC测定的。纯度定义的是多肽样品中含正确序列的组分的百分比，而肽的净含量是指样品中肽类物质相对于总物质所占的百分比，肽的净含量通常是氨基酸组分分析或紫外分光法测定的，这个信息主要是在一些对肽的浓度很敏感的实验中，对计算肽的浓度是很重要的。