Intracellular receptor complexes

Key Words Adrenergic receptors cellular localization or receptors cellular trafficking of receptors receptor subtypes signaling from intracellular receptor complexes. 

Fig. 1.—Diagrammatic Representation of the Three Steps in the Taste-cell Transduction. Step 1, interaction of stimulus (S) with membrane-bound receptor (R) to form stimulus-receptor complex (SR) step 2, conformational change (SR) to (SR), brought about by interaction of S with R (this change initiates a change in plasma-membrane conformation of taste cells, probably below the level of the tight junction) and step 3, conformational changes of the membrane result in lowered membrane resistance, and the consequential influx on intracellular ionic species, probably Na. This influx generates the receptor potential which induces synaptic vesicular release to the innervating, sensory nerve, leading to the generator potential.

FIGURE 52-2 There are two modes of hormonal action. (A) Activation of cell-surface receptors and coupled second-messenger systems, with a variety of intracellular consequences. (B) Entry of hormone into the target cell, binding to and activation of an intracellular receptor and binding of the receptor-hormone complex to specific DNA sequences to activate or repress gene expression. DAG, diacylglycerol HRE, hormone-response element. 

Ebner, R., and R. Derynck. Epidermal growth factor and transforming growth factor-cc differential intracellular routing and processing of ligand-receptor complexes. Cell Regulation. 2 599-612.1991. 

Mediation of the anxiolytic-like action of benzodiazepines through the enhancement of GABAergic transmission at the GABA-A/benzodiazepine receptor complex is well known. Benzodiazepines act at the y-subunit of the GABA receptor complex to enhance chloride influx and thereby cause hyperpolarization of neurons. However, there is much less known about the role of intracellular signal transduction in the actions of benzodiazepines. 

The switch for the activation of an intracellular signaling pathway is in most cases an increase in the concentration of the freely circulating hormone. This leads to an increase in the concentration of the hormone-receptor complex, which results in an increased activation of subsequent reactions in the cell. The concentration of the circulating hormone is thus the main regulatory parameter in cellular conummication. The relation between hormone concentration, binding of the hormone to the receptor, and subsequent reaction in the cell is illustrated in fig. 3.7 for the case of adrenaline and the P-adrenergic receptor. 

Figure 12.6 Mechanism of action of mineralocortjcoid receptor antagonists in the collecting tubule. Aldosterone enters the tubular cell by the basolateral surface and binds to a specific mineralocorticoid receptor (MNR) in the cytoplasm. The hormone receptor complex triggers the production of an aldosterone-induced protein (AlP) by the cell nucleus (NUC). The AIP acts on the sodium ion channel (ic) to augment the transport of Na+across the basolateral membrane and in to the cell. An increase in AIP activity leads to the recruitment of dormant sodium ion channels and Na pumps (P) in the cell membrane. AIP also leads to the synthesis of new channels and pumps within the cell. The increase in Na+conductance causes electrical changes in the luminal membrane that favour the excretion of intracellular cations, such as K+and H-h. Spironolactone competes with aldosterone for the binding site on the MNR and forms a complex which does not excite the production of AIP by the nucleus.

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