CAMP, intracellular concentration

G proteins regulate intracellular concentrations of second messengers. G proteins control intracellular cAMP concentrations by mediating the ability of neurotransmitters to activate or inhibit adenylyl cyclase. The mechanism by which neurotransmitters stimulate adenylyl cyclase is well known. Activation of those neurotransmitter receptors that couple to Gs results in the generation of free G(IS subunits, which bind to and thus directly activate adenylyl cyclase. In addition, free Py-subunit complexes activate certain subtypes of adenylyl cyclase (see Ch. 21). A similar mechanism appears to be the case for G(IO f, a type of G protein structurally related to G that is enriched in olfactory epithelium and striatum (Ch. 50). 

This can be illustrated by known interactions between the cAMP and Ca2+ pathways. A first messenger that initially activates the cAMP pathway would be expected to exert secondary effects on the Ca2+ pathway at many levels via phosphorylation by PKA. First, Ca2+ channels and the inositol trisphosphate (IP3) receptor will be phosphorylated by PKA to modulate intracellular concentrations of Ca2+. Second, phospholipase C (PLC) is a substrate for PKA, and its phosphorylation modulates intracellular calcium concentrations, via the generation of IP3) as well as the activity of PKC, via the generation of DAG, and several types of CAMK. Similarly, the Ca2+ pathway exerts potent effects on the cAMP pathway, for example, by activating or inhibiting the various forms of adenylyl cyclase expressed in mammalian tissues (see Ch. 21). 

Other protein kinases may indirectly influence the activation of NF-kappap. For example, in contrast to the pro-inflammatory effects typically observed with activation of kinases, the elevation ofcAMP activates PKA and blocks transcription of iNOS mRNA [51,178, 229, 230]. Astrocytes contain a variety of NT receptors that are coupled to Gs-adenylate cyclase [231] and, either activation of P-adrenergic/dopamine receptors or employing agents that increase cAMP, such as forskolin (adenylate cyclase activator), PDE inhibitors [i.e. pentoxifylline], dibutyrl cAMP, or 8-bromo cAMP can attenuate lipopolysaccharide (LPS)/cytokine activated iNOS mRNA in microglia, astrocytes and a number of other cell types [51,176,177,178, 232-237]. In contrast, agents that suppress the intracellular concentration of cAM P such as H-89 and Rp-cAM P are pro-... 

The induction of this operon responds to the intracellular concentration of cAMP, which is determined by the carbon source available to the cell. When cells are grown on cellobiose or cellulose that do not inhibit adenylate cyclase, cAMP is made in sufficient quantities for induction of cellulase. On the contrary, when cells are grown on glucose or other readily metabolized carbohydrates that do inhibit adenylate cyclase. 

Phosphodiesterases are a group of enzymes that, among other actions, hydrolyse cAMP. Phosphodiesterase inhibitors are selective for phosphodiesterase III (PDE-III) isoenzyme present in the heart. They prevent the degradation of cAMP, thereby increasing its intracellular concentration (Figure 8.4). This leads to an increase in the intracellular concentration of Ca2+ and an increased contractility and heart rate. PDE-III inhibitors have no adrenoceptor agonistic activity and therefore can be used in combination with other sympathomimetic drugs. They also increase cAMP levels in vascular smooth muscle, but this results in lower intracellular Ca2+ concentrations and thus vasodilatation. 

Evidence has been presented that the concentration of cAMP is transiently increased in PMNs stimulated to form O2 by FMLP or Csa . Paradoxically, agents which increase the intracellular concentration of cAMP caused a dose dependent fall in the formation of O by PMNs which was elicited by FMLP. A rise in the concentration of cAMP within PMNs was also observed by Smolen after stimulation with several agents. The increase in the concentration of cAMP preceded the release of lysosomal enzymes and the formation of O but did not occur before the change in membrane potential. The increase in the concentration of cAMP was judged not to be sufficient for the elaboration of O2 or the secretion of enzymes from lysosomes because three maneuvers produced changes in cAMP but no subsequent response. However, whether the concentration of cAMP must rise in the normal chain of events leading from stimulus to formation of O is not clear. 

Dopamine activates adenylate cyclase and phospholipase C (PLC) via a D, receptor and inhibits through a D2 receptor, thereby regulating the production of intracellular second messengers, cAMP, Ca2+, and 1,2-diacylglycerol. D, and D2 receptors are decreased in the striatum of patients with dementia. There is considerable evidence to suggest that intracellular levels of cAMP have a protective role for dopaminergic neurons. Intracellular concentrations of cyclic nucleotides are regulated by cyclic nucleotide phosphodiesterases and CaMPDE, one of the most intensely studied and best-characterized phosphodiesterases. 

Many extracellular ligands act by increasing the intracellular concentrations of second messengers such as cyclic adenosine-3, 5 -monophosphate (cAMP), calcium ion, or the phosphoinositides... 

Amrinone [AM ri none] and milrinone [MIL ri none] are phosphodiesterase inhibitors that increase the intracellular concentration of cAMP (Figure 16.11). This results in an increase in intracellular calcium, and therefore cardiac contractility, as discussed above for the P-adrenergic agonists. [Note Recent clinical trials have shown that amiodarone did not reduce the incidence of sudden death or prolong survival in patients with CHF (see p. 172). Milrinone showed increased mortality and no beneficial effects.]... 

Initial indications for direct receptor-G protein-K+ channel coupling came from experiments which examined atrial muscarinic receptor regulation of K+ channels present in a cell-attached patch [161]. It was found that K+ channels in the patch are insensitive to acetylcholine (ACh) added to the bath, i.e., to the cell membrane outside the physically and electrically isolated patch. However, application of acetylcholine directly to the patch, using a specially constructed pipette opened the K+ channels. If the effect of acetylcholine on the heart atrial K+ channels were mediated by a change in the intracellular concentration of a second messenger, such as Ca2+ or cAMP, application of acetylcholine outside of the patch should have elicited a response. Moreover, the coupling mechanism was not addressed by this experiment. 

Elevated intracellular concentrations of cAMP in crypt cells activate the CFTR, resulting in secretion of chloride ions into the lumen. 

Some PHOSPHODIESTERASE INHIBITORS (e.g. cnoximone and milrinone) are valuable, and some exert most of their effect on the myocardium (those acting at a heart-specific subtype of this enzyme (type III phosphodiesterase) to raise the intracellular concentration of cAMP) and may be used as positive INOTROPIC AGENTS in the short-term treatment of severe congestive heart failure. 

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