Autocrine agents

Stimulated platelets release arachidonic acid rapidly from their phospholipids, apparently as a result of activation of phospholipase A2. The released arachidonate can in turn be metabolized to endoperoxides and thromboxane A2 (Chapter 21). These compounds are also potent activators of platelets and cause a self-activating or autocrine effect.1) While PAF has a beneficial function, it can under some conditions contribute in a dangerous way to inflammation and to allergic responses including anaphylaxis,) asthmag and cold-induced urticaria.1 Although the effect of PAF is separate from those of histamine and of leukotrienes, these agents may act cooperatively to induce inflammation.1... 

Disturbances of the metabolism of bone tissue that usually exhibit not very specific clinical signs and symptoms are induced by the effects of both systemic and local agents. The role of hormones has already been described and defined many times, but news and new views keep appearing all the time. Without doubt, practically all systemic agents, hormones, but also drugs act on the bone tissue only through mediators. Their effect is provided by local, paracrine, and autocrine substances effective on the level of the cell, and by cellular components and on molecular level. Some of the hormones and local agents will be mentioned below. 

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