First messengers
Fig 2.4 Spare receptors.Three saturation binding curves are shown for three different situations. In the upper |
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curve the maximum extent of binding (Rtot) exceeds the level required for optimal stimulation (5 pmol L 1, |
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indicated by the dotted line) by 6-fold. Maximal activation is therefore achieved at the hormone concentration |
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corresponding to the intersection of the line and the curve. If the receptor reserve is reduced (middle curve) so |
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that Rtot is only three times greater than the required occupancy level, the intersection moves to the right and if |
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the reserve approaches zero (Rtot 5 pmol L 1), the point of intersection moves towards infinity (lower curve). |
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Under this condition it is never possible to achieve saturation of the receptors. |
Fig 2.5 Action of theophylline. |
elevation of cAMP induced by receptor activation. Therefore the steady state concentration of cAMP due to stimulation of receptors can be induced by much lower concentrations of hormone.
Down-regulation of receptors
Cells with receptors that are subjected to regular or persistent activation, for example by drugs that are resistant to metabolic breakdown, may become less amenable to stimulation. This down-regulation may be due to depletion of the number of exposed receptors, a reduction of their binding affinity
or both. Either of these effects will increase the EC50 by reducing Rtot or increasing KD. There is a familiar example. The instructions for use of nasal decongestants based on xylometazoline give a warning not to continue usage beyond 7 days. After this time there is a tendency for the drug to cause ‘rebound congestion’ and further use then exacerbates instead of relieves the condition. Xylometazoline binds and activates 2-adrenergic receptors that suppress production of cAMP. However, the specificity though good, is not perfect and the drug has a low but nonetheless significant affinity for1-receptors. The result is that when the preferred targets, the 2-receptors, have eventually been desensitized and removed from the epithelial surface,
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Signal Transduction
Earl W. Sutherland (1915–1974), awarded the Nobel Prize in 1971 ‘for his discoveries concerning the mechanisms of action of hormones’.
action is transduced by the 1-receptors which now predominate. The consequence is nasal congestion, most probably mediated, not by cAMP, but through the activation of phospholipase C, Ca2 , and protein kinase C.
An important mechanism of receptor down-regulation, to which we return in Chapter 4, is phosphorylation of the intracellular chains of receptor proteins by enzymes such as the protein kinases A and C and specific receptor kinases. This marks them as targets for removal from the cell surface by endocytosis.13 It also allows the redirection of signals into different pathways.14
All in all, one may conclude that receptors are not static components of cells; they are in a dynamic state that is influenced by both exogenous and endogenous factors.
Discovery of the first second messenger, cAMP
Although experiments with radioactively labelled hormones and related reagents enabled the quantitative measurement of binding parameters of receptors, they told nothing of what receptors are, or what they do. One critical advance was made in 1957 by Earl W. Sutherland. With his colleagues, he showed that the activation of glycogen breakdown in liver stimulated by adrenaline or glucagon occurs in two distinct stages. The first of these is the generation of a heat-stable and dialysable factor.15 When this was applied together with ATP to the supernatant fraction of liver homogenate, inactive glycogen phosphorylase (‘dephospho liver phosphorylase’, also called phosphorylase b) was phosphorylated to its active form (phosphorylase a). In a footnote they point out that
The active factor recently has been purified to apparent homogeneity. From ultraviolet spectrum, the orcinol reaction, and total phosphate determination, the active factor appeared to contain adenine, ribose and phosphate in a ratio of 1:1:1. Neither inorganic phosphate formation nor diminution of activity resulted when the factor was incubated with various phosphatase preparations … However, the activity of the factor was rapidly lost upon incubation with extracts from dog heart, liver and brain.
With the identification of the second messenger as cyclic AMP (cAMP), it became possible to link activation of specific classes of receptors with specific biochemical responses. Of course, elevation of cAMP is not the exclusive second-messenger response to hormones. Nor is it the exclusive response to adrenaline and other closely related catecholamines. These bind, depending on the cell type or tissue, to a family of catecholamine or ‘adrenergic’ receptors, 1, 2, 1, 2, 3, each of which has several distinct functions. These include the synthesis of cAMP (generally ), suppression of synthesis of cAMP (generally 2) and elevation of Ca2 ( 1).
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