14. ★ Know the characteristics of prostaglandins.
→ Prostaglandins are a group of lipid molecules that serve important and widespread integrative functions in the body but do not meet the usual definition of a hormone.
→ Although prostaglandins may be secreted directly into the bloodstream, they are rapidly metabolized, so that circulating levels are extremely low.
→ The term tissue hormone
is appropriate because the secretion is produced in a tissue and diffuses only a short distance to other cells within the same tissue. Whereas typical hormones integrate activities of widely separated organs, prostaglandins tend to integrate activities of neighboring cells.
→ There are at least 16 different prostaglandins
, falling into nine structural classes—prostaglandin A through prostaglandin I.
- Differentiation of endocrine (blood traveling hormones) from “local” or tissue hormones can be further refine by:
- Paracrine hormones – that regulate activity in nearby cells within the same tissue as their source.
- Autocrine hormones – that regulate activity in the secreting cell itself.
- In addition to prostaglandins, which have diverse local (paracrine/autocrine) effects such as inflammation and muscle contraction in blood vessels, there are:
- Thromboxane, a blood regulator important in blood clotting.
- Leukotrienes, which are regulators of immunity.