Intracellular effectors

Two AR subtypes, Ax and A3, couple through G to inhibit adenylate cyclase, while the other two subtypes, A2a and A2B, stimulate adenylate cyclase through Gs or G0if (for A2a). The A2BAR is also coupled to the activation of PLC through Gq. Furthermore, each of these receptors may couple through the (3,y subunits of the G proteins to other effector systems, including ion channels and phospholipases. Levels of intracellular... 

CC, and one CX3C and XC chemokine receptors have been cloned so far [2]. Receptor binding initiates a cascade of intracellular events mediated by the receptor-associated heterotrimeric G-proteins. These G-protein subunits trigger various effector enzymes that lead to the activation not only of chemotaxis but also to a wide range of fimctions in different leukocytes such as an increase in the respiratory burst, degranulation, phagocytosis, and lipid mediator synthesis. 

Receptors are proteins that bind specific hormones and generate an intracellular signal (receptor-effector coupling). 

Fig. 5.12 (a) Synaptic types along dendritic spines of M/T and GC units uni-, and bi-directional junctions, (b) Transmitter systems at a reciprocal synapse, Mitral-Granule cell junction. [Glu, glutamate (R, receptor) GABA, y-aminobutyric acid (R, receptor) E, intracellular effector and aAR, alpha-adrenergic receptor.]. (From Hayashi et al., 1993.)... 

The neurotransmitters of the ANS and the circulating catecholamines bind to specific receptors on the cell membranes of effector tissue. Each receptor is coupled to a G protein also embedded within the plasma membrane. Receptor stimulation causes activation of the G protein and formation of an intracellular chemical, the second messenger. (The neurotransmitter molecule, which cannot enter the cell, is the first messenger.) The function of intracellular second messenger molecules is to elicit tissue-specific biochemical events within the cell that alter the cell s activity. In this way, a given neurotransmitter may stimulate the same type of receptor on two different types of tissue and cause two different responses due to the presence of different biochemical pathways within each tissue. 

Mutagenesis studies have established that the C-terminal region of the // and S receptors is not essential for the receptors to couple to adenylyl cyclase [131, 132]. The remaining intracellular domains of the opiate receptors have almost identical amino acid sequences. As a consequence, it is likely that the intracellular loops are the main regions of the opiate receptor involved in G protein coupling and effector system regulation. 

---