Cyclic nucleotides, second messenger systems

Calmodulin and cyclic nucleotide-dependent second messenger systems... 

The second messenger molecules Ca2+ and cyclic AMP (cAMP) provide major routes for controlling cellular functions. In many instances, calcium (Ca2+) achieves its intracellular effects by binding to the receptor protein calmodulin. Calmodulin has the ability to associate with and modulate different proteins in a Ca2+-dependent and reversible manner. Calmodulin-dependent cyclic nucleotide phosphodiesterase (CaMPDE, EC 3.1.4.17) is one of the key enzymes involved in the complex interactions that occur between the cyclic-nucleotide and Ca2+ second messenger systems (see Figure 13.2). CaMPDE exists in different isozymic forms, which exhibit distinct molecular and catalytic properties. The differential expression and regulation of individual phosphodiesterase (PDE) isoenzymes in different tissues relates to their function in the body. 

Cyclic AMP, the intracellular second messenger in this system, is short-lived. It is quickly degraded by cyclic nucleotide phosphodiesterase to 5 -AMP (Fig. 12-12, step (7)), which is not active as a second messenger ... 

Tlie second messengers that we have seen recur in many additional signal transduction pathways. For exampile, in a consideration of the sensory systems in Chapter 32, we will see how Ca -based signaling and cyclic nucleotide-based signaling play key roles in vision and olfaction. 

Prostaglandins have been associated with neurotransmitter release and synaptic regulation at specific nerve synapses. They have also been reported to influence cyclic nucleotide turnover and the release of anterior pituitary hormones. They may also be involved with fever and temperature regulation. Their participation in many other central nervous system functions as second messengers has also been documented, but in deference to brevity only the topics listed above will be briefly discussed. 

Cyclic nucleotides, 3, 5 -cyclic adenosine monophosphate (cAMP) and 3, 5 -cyclic guanosine monophosphate (cGMP), function to regulate cell-to-cell communication processes (33). Cellular communication follows primarily three pathways. The first involves the transmission of electrical impulses via the nervous system. The second involves chemical messengers or hormonal secretions. The third involves de novo protein synthesis. All three processes are usually in response to some demand or stimulus and involve, at least to some extent, regulation by cyclic nucleotides ... 

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