📚 Hormonal Communication | 激素通讯
Hormonal communication is a major cell signalling system that enables an animal to coordinate slow but sustained responses throughout the body.
激素通讯是一种重要的细胞信号传递系统，使动物能够在全身范围内协调缓慢但持久的反应。
This article covers endocrine glands, hormone-receptor interactions, second messenger cascades, blood glucose regulation and diabetes.
本文将介绍内分泌腺、激素与受体的相互作用、第二信使级联反应、血糖调节以及糖尿病。
1. Overview of hormonal communication | 激素通讯概述
Hormones are chemical messengers secreted by endocrine glands directly into the blood. They travel throughout the body, but only influence target cells that possess complementary receptors. This ensures specificity and avoids unnecessary responses in other tissues.
激素是由内分泌腺直接分泌到血液中的化学信使。它们在全身运输，但只影响具有互补受体的靶细胞。这确保了特异性，并避免其他组织产生不必要的反应。
Compared with nervous communication, hormonal signalling is generally slower because hormones travel in the circulatory system. However, the responses often last longer and can affect multiple tissues at once.
与神经通讯相比，激素信号通常较慢，因为激素随血液循环运输。然而，这种反应往往持续更久，并能同时影响多个组织。
2. Endocrine glands and hormones | 内分泌腺与激素
Endocrine glands are ductless organs that release hormones into the surrounding blood capillaries. Examples include the pituitary, thyroid, adrenal glands, pancreas and gonads.
内分泌腺是无导管的器官，将激素释放到周围的毛细血管中。例如垂体、甲状腺、肾上腺、胰腺和性腺。
This contrasts with exocrine glands, such as salivary glands, which secrete substances through ducts to a body surface or cavity. Endocrine glands therefore have a rich blood supply.
这与外分泌腺（如唾液腺）不同，后者通过导管将物质分泌到体表或体腔。因此，内分泌腺具有丰富的血液供应。
Hormones can be classified chemically as peptide/protein hormones, steroid hormones, or amine derivatives such as adrenaline. Each class has a different solubility and therefore a different mechanism of action.
激素按化学结构可分为肽/蛋白质激素、类固醇激素以及胺类衍生物如肾上腺素。每一类的溶解性不同，因此作用机制也不同。
3. Target cells and receptors | 靶细胞与受体
A hormone is a first messenger. It binds to a specific receptor protein on or within the target cell. The receptor has a binding site complementary to the hormone, just as an enzyme has a specific active site.
激素是第一信使。它与靶细胞表面或内部的特定受体蛋白结合。受体具有与激素互补的结合位点，如同酶有特定的活性位点。
Cells that do not express the matching receptor are unaffected, even though they are exposed to the same hormone concentration in the blood. This explains why one hormone can have different effects in different organs.
即使细胞暴露在血液中相同浓度的激素下，如果不表达相匹配的受体，也不会受到影响。这解释了为什么同一种激素在不同器官中可能产生不同效应。
The hormone-receptor binding triggers a signal transduction pathway inside the cell, leading to the cellular response. This may involve enzyme activation, altered membrane permeability, or changes in gene expression.
激素与受体结合后，会在细胞内启动信号转导通路，最终引起细胞反应。这可能涉及酶活化、膜通透性改变或基因表达的变化。
4. Protein/peptide hormones: second messenger model | 蛋白质/肽类激素：第二信使模型
Protein and peptide hormones, such as insulin, glucagon and adrenaline, are hydrophilic. They cannot pass through the phospholipid bilayer of the plasma membrane.
蛋白质和肽类激素（如胰岛素、胰高血糖素和肾上腺素）是亲水性的。它们不能穿过细胞膜的磷脂双分子层。
They bind to transmembrane receptors on the cell surface. This activates a G protein, which in turn activates the membrane enzyme adenylate cyclase.
它们与细胞表面的跨膜受体结合。这会激活 G 蛋白，G 蛋白再激活膜上的腺苷酸环化酶。
Adenylate cyclase converts ATP into cyclic AMP (cAMP), a second messenger:
腺苷酸环化酶将 ATP 转化为环磷酸腺苷（cAMP），即第二信使：
ATP → cAMP + PPᵢ
cAMP activates protein kinase A, which phosphorylates specific enzymes. Each step activates many enzyme molecules, so the signal is amplified dramatically. This is called a cascade.
cAMP 激活蛋白激酶 A，后者使特定酶磷酸化。每一步都会激活大量酶分子，因此信号被显著放大。这称为级联反应。
5. Steroid hormones: gene regulation | 类固醇激素：基因调控
Steroid hormones such as oestrogen, testosterone and cortisol are lipid-soluble. They can diffuse directly across the plasma membrane into the cytoplasm.
类固醇激素如雌激素、睾酮和皮质醇是脂溶性的。它们可以直接跨过细胞膜扩散到细胞质中。
Inside the cell, the steroid binds to an intracellular receptor. The hormone-receptor complex moves into the nucleus and acts as a transcription factor.
在细胞内，类固醇与胞内受体结合。激素-受体复合物进入细胞核，作为转录因子发挥作用。
It binds to specific DNA sequences, promoting or inhibiting the transcription of target genes. This changes protein synthesis and cell activity over a longer timescale than the cAMP pathway.
它结合到特定的 DNA 序列上，促进或抑制靶基因的转录。与 cAMP 通路相比，这需要更长的时间尺度来改变蛋白质合成和细胞活动。
6. Adrenaline and cAMP | 肾上腺素与 cAMP
Adrenaline is released from the adrenal medulla in response to stress or danger, preparing the body for ‘fight or flight’.
肾上腺素在应激或危险时由肾上腺髓质释放，使身体做好 ‘战斗或逃跑’ 的准备。
In liver cells, adrenaline binds to a specific cell-surface receptor and triggers the cAMP second messenger pathway described above.
在肝细胞中，肾上腺素与特定的细胞表面受体结合，触发上述 cAMP 第二信使通路。
The cascade activates glycogen phosphorylase, which catalyses the breakdown of glycogen to glucose. This glucose enters the blood, raising the blood glucose concentration for rapid respiration in muscles and other tissues.
级联反应激活糖原磷酸化酶，催化糖原分解为葡萄糖。这些葡萄糖进入血液，提高血糖浓度，以供肌肉和其他组织快速呼吸。
7. Regulation of blood glucose: insulin and glucagon | 血糖调节：胰岛素与胰高血糖素
The pancreas contains clusters of endocrine cells called the islets of Langerhans. Alpha cells secrete glucagon, and beta cells secrete insulin.
胰腺含有成群的内分泌细胞，称为胰岛。α 细胞分泌胰高血糖素，β 细胞分泌胰岛素。
When blood glucose rises, beta cells release insulin. Insulin increases glucose uptake by cells, stimulates glycogenesis in liver and muscle, and increases the use of glucose in respiration.
当血糖升高时，β 细胞释放胰岛素。胰岛素增加细胞对葡萄糖的摄取，刺激肝脏和肌肉中的糖原生成，并增加呼吸作用对葡萄糖的利用。
The hormonal control of blood glucose can be summarised:
血糖的激素调节可概括为：
High glucose → insulin → glycogenesis and glucose uptake
When blood glucose falls, alpha cells release glucagon. Glucagon stimulates glycogenolysis (breakdown of glycogen) and gluconeogenesis (formation of glucose from non-carbohydrate sources such as amino acids and glycerol).
当血糖下降时，α 细胞释放胰高血糖素。胰高血糖素刺激糖原分解和糖异生（从氨基酸、甘油等非碳水化合物来源生成葡萄糖）。
The glucagon response can be summarised:
胰高血糖素的反应可概括为：
Low glucose → glucagon → glycogenolysis and gluconeogenesis
8. Negative feedback in glucose homeostasis | 血糖稳态中的负反馈
Blood glucose concentration is maintained near a set point, typically around 5 mmol dm⁻³ in humans, by negative feedback.
人类血糖浓度通过负反馈维持在调定点附近，通常约为 5 mmol dm⁻³。
Beta and alpha cells continuously monitor the blood glucose level. A rise above the set point is corrected by insulin; a fall below the set point is corrected by glucagon.
β 细胞和 α 细胞持续监测血糖水平。高于调定点时由胰岛素纠正；低于调定点时由胰高血糖素纠正。
This homeostatic loop keeps glucose supply balanced with cellular demand, especially for brain cells that rely heavily on glucose as a respiratory substrate.
这种稳态回路使葡萄糖供应与细胞需求保持平衡，特别是对依赖葡萄糖作为呼吸底物的脑细胞尤为重要。
9. Diabetes mellitus | 糖尿病
Diabetes mellitus is a condition in which blood glucose concentration remains persistently high, often because of a failure to produce or respond to insulin.
糖尿病是一种血糖浓度持续升高的疾病，通常是由于无法产生胰岛素或对胰岛素无反应。
Type 1 diabetes is an autoimmune disease in which the body’s immune system destroys pancreatic beta cells. Patients produce little or no insulin and need regular insulin injections.
1 型糖尿病是一种自身免疫性疾病，免疫系统破坏胰腺 β 细胞。患者几乎不产生胰岛素，需要定期注射胰岛素。
Type 2 diabetes is associated with insulin resistance: cells have fewer or less responsive receptors, and beta cells may reduce insulin output over time. Risk factors include obesity, inactivity and a family history.
2 型糖尿病与胰岛素抵抗有关：细胞受体减少或敏感性下降，β 细胞随时间可能减少胰岛素分泌。风险因素包括肥胖、缺乏运动和家族史。
Common symptoms include hyperglycaemia, glucose in urine, excessive thirst, frequent urination, tiredness and unexplained weight loss.
常见症状包括高血糖、尿糖、过度口渴、尿频、疲倦和不明原因的体重下降。
10. Comparison of hormonal and nervous communication | 激素通讯与神经通讯的比较
Hormonal and nervous systems both coordinate body functions, but they operate on different timescales and routes.
激素系统和神经系统都能协调身体功能，但它们的时间尺度和传递途径不同。
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