老鼠腎上腺髓質嗜鉻細胞分泌方式之重新探討

Abstract

嗜鉻細胞，位於腎上腺的髓質區域，在體內專司腎上腺素與正腎上腺素的分泌，尤其是當動物處於緊張狀態下時，嗜鉻細胞會進行旺盛的分泌作用，釋放出大量的荷爾蒙，藉以調控適當的生理反應。長久以來，胞吐作用一直被認為是嗜鉻細胞唯一的分泌方式，但實際觀察到分泌顆粒與細胞膜癒合的數量太少，與細胞在緊急時必須釋放出大量荷爾蒙的現象不符，且腎上腺素與正腎上腺素在血液中極容易被酵素氧化而失去活性，造成即使分泌出大量的荷爾蒙，也無法達到、甚至維持生理反應所需之濃度。 以電子顯微鏡觀察受驚嚇老鼠的腎上腺，我們意外發現嗜鉻細胞會擠出血管內皮細胞上的開口，將分泌顆粒、細胞質甚至完整的細胞核直接釋放至血液中，達成大量分泌的目的。以生化方法分析受刺激老鼠腎上腺靜脈的血液，則發現在以Triton X-100作用或冷凍再解凍的方式處理的血液中，所測得的多巴胺脫羥?（嗜鉻細胞分泌顆粒中特有的酵素）活性明顯高於未處理者，另外在超高速離心（100,000g/lhr）後所得的沉澱物中，多巴胺脫羥?的活性也較上清液高，顯示完整的分泌顆粒確實存在於血液中。而以免疫酵素呈色分析也發現苯乙醇胺轉甲基?（嗜鉻細胞細胞質中特有的酵素）在血液中出現，乳酸脫氫?的含量也明顯增高，證明有大量嗜絡細胞的細胞質流入血液的情形。此外受驚嚇刺激後的老鼠在恢復時期，常可見血小板聚集在破損血管的管壁上，而且嗜鉻細胞細胞分裂數目也有增加的現象。 由上述的研究結果顯示，嗜鉻細胞在緊急狀態下所採取的分泌方式，並非以一般認為的胞吐作用來進行，而是採取了一種特殊的分泌方式—細胞自毀。此種分泌方式，不僅解釋了許多胞吐作用無法說明的現象，也為內分泌學開?了一個全新的研究領域。

Chromaffin cells of adrenal gland in animal are controlling several physiological responses by secreting epinephrine and norepinephrine during stress circumstance. For several decades, exocytosis has been regarded as the only secretory pattern of chromaffin cells, however, there still has some questions remained unsolved. For example, the fusion of secretory vesicles with cell membrane is seldom observed under electron microscope, and it seems not quite corresponding with large amount hormone releasing during stress condition. Besides, free from hormones are vary easily to be destroyed by enzymes appearing in blood, therefore, the maintenance of physiological concentration of hormone in blood would be difficult. In our study, some chromaffin cells are found penetrating or bursting out through openings on the endothelia of blood vessels in case of emergency under electron microscope investigation. The intact vesicles, cytoplasm and even whole nuclei are released into blood stream directly. Moreover, the dopamine-β-hydroxylase activity in blood collected from adrenal vein in stress situation after Triton X-l00 or frozen and thawed treatment is always higher than that without treated. If the blood was centrifuged in 100,000g for one hour, the dopamine-β-hydroxylase activity can only be detected in pellets. In addition, some vesiclelike structures similar to secretory granules of chromaffin cells can also be found in these pellets. Furthermore, phenylethanolamine N-emthyltransferase can be detected in the blood by using enzyme-linked immunosorbent assay and lactate dehyrogenase activity is also greatly increased in the plasma of stress-stimulated rats. These results indicate that cytoplasmic materials of chromaffin cells actually be released into blood stream during stress condition, and intact secretory vesicles of chromaffin cells do exist in the blood. According to our findings, a special secretory pattern by means of cell sacrificed is suggested to be the major secretory way in adrenal medulla instead of exocytosis under stress condition. This important discovery not only solves lots of questions which could not explain by exocytosis, but also provides a new era for further research in endocrinology.