Figure 1. Illustration of the modes of delivery of hormones (H) to their sites of action. Specific receptors (R) are involved in the cellular response to a hormone and these may span the plasma membrane or may be cytoplasmic and intranuclear with genomic interactions. Based on O’Malley (1989).

Figure 2. Biosynthesis of steroid hormones (top panel) and the interrelationships between adrenocortical, oestrogenic and androgenic steroids (lower panel). The substrate, cholesterol, may be synthesized de novo or be supplied in the diet. After side-chain cleavage, the pathways towards the various steroid types are embarked upon. Cholesterol is also the substrate for vitamin D metabolism.

Figure 3. Biosynthesis of catecholamines from the substrate tyrosine. PNMT, phenylethanolamine N-methyltransferase; Dopa, dihydroxyphenylalanine. Dopamine, noradrenaline and adrenaline are all biologically active and have their own specific receptors.

Figure 4. Biosynthesis of arachidonic derivatives: prostaglandins (PG), prostacyclins (PS), thromboxanes (TX) and leucotrienes (LT). The key enzymes are cyclooxygenase (towards prostaglandins, thromboxanes and prostacyclins), thromboxane synthetase, prostacyclin oxycyclase and 5-lipoxygenase (towards leucotrienes).

Figure 5. Biosynthesis of thyroid hormones from the substrate tyrosine.

Figure 6. Biosynthesis of melatonin from tryptophan. Serotonin is an important intermediate metabolite that has its own specific receptors.

Figure 7. The biosynthesis of peptide/protein hormones, from DNA to the hormone. A gene or gene family may be selectively transcribed. There then follow differential processing and differential proteolysis and/or chemical modifications, such as phosphorylation or glycosylation, to generate the final hormone.

Figure 8. The processing of the preprohormone proopiomelanocortin (POMC). Genes express the full POMC molecule within which sequences for a variety of biologically active fragments are present. These are selectively cleaved to release adrenocorticotrophin (ACTH), -endorphin (-END), -lipotrophin (-LPH), corti cotrophin-like intermedia peptide (CLIP), -LPH, and -melanotrophin (-MSH). These posttranslational events take place in different regions of the pituitary gland. The amino acid sequences cleaved are shown on the top line.

Figure 9. Simplified scheme to illustrate the intracellular topographic distributions of corticosteroid-forming events. Lipoproteins containing cholesterol enter the cell to be first taken up by mitochondria, which produce pregnenolone, which in turn moves into the smooth endoplasmic reticulum where 17--hydroxyprogesterone or progesterone is produced. These metabolites then re-enter the mitochondria, which depending on the species and circumstances will generate aldosterone, cortisol or corticosterone for secretion. Full pathways are given in Figure 3. Equivalent intracellular processing and trafficking occurs in the production of gonadal steroids.

Figure 10. A flow diagram to illustrate the general sequence of events associated with G protein reactions of intracellular second messengers after a hormone has occupied a G protein-coupled cell surface receptor. (a) The receptor complex spanning the membrane. (b) The events that occur within or closely associated with the membrane. (c) The intracellular results. AC, adenylyl cyclase; DAG, diacylglycerol; PLC, phospholipase C.

Figure 11. Flow diagram summarizing the sequence of events that follow activation of a surface receptor that interacts with inositol phosphates, diacylglycerol, calcium and calmodulin signalling pathways. ER, endoplasmic reticulum; PI, phosphatidyl inositol; DAG, diacylglycerol; PKC, protein kinase C; PLC, phospholipase C.

Figure 12. Key events in the actions of those hormones that interact directly with the genome. Free hormone enters the cell to become associated with a cytoplasmic receptor or binding protein and on its entering the nucleus a well-ordered sequence of events leads to the production of a new protein to give the biological response. HBP, hormone-binding protein.

Figure 13. Global view of regulatory levels in the endocrine system. Level A represents the molecular and cellular components and may be represented as single cells or whole endocrine glands. The biosynthetic machinery comes under the control of trophic or inhibitory agents and will include enzyme–substrate interactions and posttranslational processing. Intracrine factors can be key regulators at this level, while the release may be inhibited for some hormones to be stored for autocrine, paracrine and endocrine purposes. Many factors may signal directions at this level. Level B involves transport to the sites of action where molecular interactions take place to elicit the biological response. At this level, modulation of receptor activities is key, with hormones sometimes autoregulating their own receptor affinities and numbers. Level C represents the final biological response, which may range from apoptosis to a behavioural response in the whole animal. The arrows on the right represent feedback control within levels A and B, while the vertical arrow on the left indicates the possibility that there may be feedback regulation from the final response across levels action from changing the intracellular cascades to activation or inhibition of the biosynthesis of the hormone. Horizontal arrows (left) indicate controls at the interfaces between the various levels (secretion and transport; molecular interactions on or within the cell).