CAMP accumulation inhibition

Methylxanthines have relaxing and anti-inflammatory effects. Accumulation of intracellular cAMP by inhibition of PDE3 (phosphodiesterase-3) relaxes airway... 

The multiplicity of G proteins coupled to opiate receptors may explain how different opiates can bind to the same receptor yet induce different cellular responses. For example, morphine binds to the cloned rat fi receptor expressed in HEK 293, CHO and COS-7 cells and inhibits cAMP accumulation [80-82]. Morphine can be continuously applied to the cells for up to 16 h, and the potency and magnitude of morphine inhibition of adenylyl cyclase does not diminish [80, 81]. In contrast, the opiate sufentanil can bind to the same cloned fi receptor in HEK 293 cells to inhibit cAMP accumulation. However, sufentanil s actions rapidly desensitize [83]. Since both compounds bind to the same receptor, and the fi receptor is the only receptor these drugs can interact with in these cells, the ability of these two full agonists to differentially regulate the fi receptor must be due to their abilities to affect separate adaptive processes in these cells. 

Mutagenesis studies have shown that morphine and sufentanil bind differently to the jj, receptor [83, 85]. Mutation of an aspartic acid at residue 114 of the // receptor to an asparagine resulted in a mutant that did not bind morphine and morphine was ineffective in inhibiting adenylyl cyclase via that receptor. In contrast, sufentanil bound to the mutant and wild-type receptors equally well and it effectively inhibited cAMP accumulation via the mutant receptor. These findings demonstrate that morphine and sufentanil have different requirements for binding to the // receptor. By binding differentially, these two agonists may induce the ft receptor to interact with different G proteins to induce distinct cellular effects. 

While chronic morphine treatment uncouples the // receptor from K+ channels, it did not affect the coupling of ft receptors to adenylyl cyclase. Pretreatment of the cloned ft receptor expressed in HEK 293, AtT-20, CHO and COS cells with morphine or DAMGO for up to 16h did not alter the subsequent ability of fi agonists to inhibit cAMP accumulation [25, 65, 80-82]. These findings suggest that morphine treatment induces a selective desensitization of the coupling of the fi receptor to K+ channels. 

The effects of Li+ upon this system have been reviewed in depth by Mork [131]. Animal studies originally demonstrated that Li+ inhibits cAMP formation catalyzed by adenylate cyclase in a dose-dependent manner [132]. The level of cAMP in the urine of manic-depressive patients changes with mental state, being abnormally elevated during the switch period between depression and mania it is proposed that Li+ s inhibitory effect upon adenylate cyclase activity may correct this abnormality. Subsequent research, in accord with the initial experiments, have shown that Li+ s interference with this second messenger system involves more than one inhibitory action. At therapeutic levels, Li+ inhibits cAMP accumulation induced by many neurotransmitters and hormones, both in... 

Li+, at therapeutically relevant concentrations, is a potent inhibitor of norepinephrine-stimulated adenylate cyclase activity ex vivo in both rat [133] and human brain [134], and it inhibits norepinephrine-stimulated cAMP accumulation in Li+-treated patients. Li+ also inhibits dopamine-stimulated cAMP accumulation in rat brain [135]. These inhibitory effects of Li+ have been shown to be region specific within rat brain, a fact that has obvious significance for a therapeutic mechanism of action. It is interesting that other antimanic drugs may also have dampening effects on dopaminergic neurotransmission. 

The pupils become dilated and there are associated signs of hyperactivity of the sympathetic nervous system, such as hypertension and pilomotor stimulation. The mechanism(s) underlying tolerance and dependence are poorly understood. While acute activation of Gi/o-coupled receptors leads to inhibition of adenylyl cyclase, chronic activation of such receptors produces an increase in cAMP accumulation, particularly evident upon withdrawal of the inhibitory agonist. This phenomenon, referred to as adenylyl cyclase superactivation, is believed to play an important role in opioid addiction. 

The Di receptor is typically associated with the stimulation of adenylyl cyclase (Table 9-1) for example, Di-receptor-induced smooth muscle relaxation is presumably due to cAMP accumulation in the smooth muscle of those vascular beds in which dopamine is a vasodilator. D2 receptors have been found to inhibit adenylyl cyclase activity, open potassium channels, and decrease calcium influx. 

Receptor desensitization may also be mediated by second-messenger feedback. For example, adrenoceptors stimulate cAMP accumulation, which leads to activation of protein kinase A protein kinase A can phosphorylate residues on receptors, resulting in inhibition of receptor function. For the B2 receptor, phosphorylation occurs on serine residues both in the third cytoplasmic loop and in the carboxyl terminal tail of the receptor. Similarly, activation of protein kinase C by Gq-coupled receptors may lead to phosphorylation of this class of G protein-coupled receptors. This second-messenger feedback mechanism has been termed heterologous desensitization because activated protein kinase A or protein kinase C may phosphorylate any structurally similar receptor with the appropriate consensus sites for phosphorylation by these enzymes. 

In other cell types, guanine nucleotides interact with a guanine nucleotide subunit (G- or Ng-subunit) to translate receptor stimulation into increased adenylate cyclase activity (12.) Cholera toxin inhibits a specific GTPase on this guanine nucleotide subunit and thereby increases adenylate cyclase activity (13.). In dispersed cells from the bovine parathyroid gland, cholera toxin markedly increases cAMP formation and causes a 3 to 10-fold increase in the apparent affinity cf dopamine for its receptor (as determined by cAMP accumulation or IR-PTH secretion (J y.). The effects of guanine nucleotides and cholera toxin on cAMP accumulation in parathyroid cells result from interactions with the guanine nucleotide subunit in this cell. 

Figure 6. Inhibition of cAMP accumulation stimulated by 1 pM dopamine by lisuride (O), a-flupenthixol ( ,), fl-flupenthixol (M)> lergotrile (A), a-bromoergo-cryptine (%), or fluphenazine (A).

For each response examined, inhibition of isoproterenol-stimulated cAMP accumulation by intact cells ( ) inhibition of basal release of IR-aMSH by intact cells ( J occupancy of specific [3H]-spiroperidol binding sites in a cell-free homogenate (Q) and inhibition of isoproterenol-stimulated adenylate cyclase activity in a cell-free homogenate (M), the effect achieved with the indicated concentration of apomorphine is expressed as a percentage of the maximal effect of apomorphine (33). 

Heindel JJ, Chapin RE. 1989. Inhibition of FSH-stimulated cAMP accumulation by mono(2-ethylhexyl) phthalate in primary rat Sertoli cell cultures. Toxicol Appl Pharmacol 97 377-385. 

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