The adenylate cyclase-cAMP system

Gershengorn et al. [9] found in thyrotropic tumor cells in culture that cAMP analogues, phosphodiesterase inhibitors and cholera toxin caused an increase in TSH release that was additive to that of TRH, that there was no correlation between TSH release and cAMP levels and that there was no change in binding of cAMP to protein kinase, suggesting that cAMP is not the physiological mediator of TRH-in-duced TSH release. 

Although earlier studies failed to find consistent effects of DA on cAMP levels or adenylate cyclase activity in anterior pituitary cells [13], intact pituitary gland [14,15] or homogenates [13,16], a functional connection between the two is now supported by many experimental approaches. DA and DA agonists inhibit cAMP levels in cultured rat pituitary cells at concentrations in the nanomolar range, comparable to those which inhibit PRL release [17-21], DA also inhibits cAMP accumulation stimulated by VIP or TRH [20]. Inhibition is also seen in human prolactinoma cells [22]. 

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At micromolar concentrations opioids cause an increase in the cell membrane threshold, shortened action potentials, and inhibition of neurotransmitter release. At nanomolar concentrations opioid agonists are excitatory and prolong the action potential via the stimulatory G proteins, which act on the adenylate cyclase/cAMP system and on protein kinase A-dependent ion channels. Tolerance is proposed to be the result of an increase in the association of opioid receptors to stimulatory G proteins, to an activation of A-methyl-o-aspartate receptors via protein kinase C, and calmodulin-dependent increases in cytosolic calcium, resulting in cellular hyperexcitability. 

Many hormones, such as the hormonal amines and all pep-tidic hormones, are unable to penetrate the lipid matrix of the cell membrane, and thus depend on the presence of receptor sites at the surface of target cells. As listed in Table 30-4, there are several types of cell membrane receptors for these hormones, each of which is coupled to a distinct set of intracellular postreceptor pathways. The surface receptors all initiate postreceptor events that involve the phosphorylation of one or more intracellular proteins, some of which are enzymes whose activities depend on the state of phosphorylation. In two of these cases, an intracellular second messenger is utilized to implement the hormonal action and involves G-protein-coupled receptors. One is coupled to the adenylate cyclase-cAMP system and the other is associated with the phosphatidylinositol-Ca + pathway (IP3 pathway). 

These criteria have been fulfilled for many polypeptide hormones, catecholamines, and certain neurotransmitters. Thus, it is clear that the mechanism of action of a large number of extracellular signal molecules occurs via the adenylate cyclase-cAMP system, by first binding to a membrane receptor. As is pointed out by Sutherland (Robison et al., 1971), the major problems remaining are to determine the molecular events whereby a specific hormone activates adenylate cyclase to generate cAMP, and to determine how cAMP mediates the various physiological functions attributed to the specific hormones. 

The eicosanoids have a wide variety of physiologic effects, which are generally initiated through an interaction of the eicosanoid with a specific receptor on the plasma membrane of a target cell (Table 35.4). This eicosanoid-receptor binding either activates the adenylate cyclase-cAMP-protein kinase A system (PGE, PGD,... 

An example of a hormone that exerts its effects through a surface receptor-second messenger system is ACTH.36 ACTH is a polypeptide that binds to a surface receptor on adrenal cortex cells. The surface receptor then stimulates the adenylate cyclase enzyme to increase production of cAMP, which acts as a second messenger (the hormone was the first messenger), and increases the activity of other enzymes within the cell to synthesize adrenal steroids such as cortisol. For a more detailed description of surface receptor-second messenger systems, see Chapter 4. 

Glucagon appears to exert its effects on liver cells by a classic adenyl cyclase-cyclic adenosine monophosphate (cAMP) second messenger system (see Chapter 4).93 Glucagon binds to a specific receptor located on the hepatic cell membrane. This stimulates the activity of the adenyl cyclase enzyme that transforms adeno-... 

Glucagon stimulates the adenylate cyclase system in the liver and thereby the formation of cAMP, which gives rise to important metabolic changes, (s. tab. 3.10) Furthermore, there is a consequent decline in cholesterol synthesis, improvement in alanine membrane transport, activation of the enzymes of the urea cycle and stimulation of amino acid degradation. 

GIP stimulates the adenylate cyclase system in pancreatic B-cells (Szecowka et al., 1982). This suggests that GIP potentiates insulin release by promoting formation of cAMP. Recent data (Wahl et al., 1992) suggest that amplification of insulin release by GIP is based on enhancement of Ca2+ uptake. 

As discussed above (see chapter 6, section 4.1), initiation of insulin secretion via depolarization can be modulated by compounds that affect the adenylate cyclase system. It is therefore not surprising that glucagon and db-cAMP potentiate tolbutamide-induced insulin secretion (Ammon, 1975). This also holds for methylxanthines which, at the concentrations used in vitro, inhibit phosphodiesterase and thus cAMP (Lambert et al., 1971 Ammon, 1975). 

The precise intracellular events that lead to acid secretion are not clear, but the second messengers in this process appear to be cAMP and Ca +. The H2 receptor is coupled to the adenylate cyclase system (Chapter 30), and its activation results in the intracellular elevation of cAMP concentration. Stimulation of cholinergic receptor systems is coupled to increased Ca + permeability. 

Response of the adenylate cyclase system to a hormone is determined by the types and amounts of various constituent proteins. Cyclic AMP production is limited by the amount of adenylate cyclase present. When all the adenylate cyclase is fully stimulated, further hormone binding to Rs s cannot increase the rate of cAMP synthesis. In cells having many different Rs s (adipocytes have them for epinephrine, ACTH, TSH, glucagon, MSH, and vasopressin), maximal occupancy of the receptors may not... 

Fig. 6.25. Origin of complex oscillations in the cAMP signalling system of D. discoideum. Complex behaviour (birhythmidty, bursting and chaos) originates from the interaction of two endogenous oscillatory mechanisms that are coupled in parallel. The two mechanisms share the same feedback loop of selfamplification by extracellular cAMP, via the binding of the latter to the receptor, and differ by the process responsible for limiting autocatalysis (dotted area) the first limiting process is based on receptor desensitization, and the second on substrate availability at the adenylate cyclase reaction site (Goldbeter Martiel, 1987).

The adenylate cyclase system is a complex of proteins which responds to several stimuli with a single, integrated response. A hormonal signal is translated by multiple systems, including the stimulation of cAMP, to the nucleus [1,2]. Regulation of cAMP synthesis in response to a hormone requires GTP as a cofactor for hormonal stimulation of the adenylate cyclase system [3,4]. GTP appears critical for the action of various toxins, such as cholera and pertussis toxin, as well as for hormonal activity [5,6]. 

Choleragen (cholera toxin) exerts its effects on animal cells by activating adenylate cyclase, thereby increasing intracellular cAMP content (1). The Ai protein of choleragen, released from the holotoxin by reduction of a single disulfide bond linking the Ai and A2 proteins, catalyzes the mono-ADP-ribosylation of Gso, a regulatory component of the adenylate cyclase system that is responsible for the GTP-dependent activation of the cyclase catalytic unit. ADP-ribosylation of Gsa apparently increases its sensitivity to GTP and its dissociation from the inhibitory Gpy complex (1). 

Whereas Hi receptors are involved with positive effects, H2 receptors appear to mainly mediate suppressive activities of histamine including gastric acid secretion, heart contraction, cell proliferation, differentiation, and some effects on the immune response. H2 receptors are coupled to the adenylate cyclase as well as the phosphoinositide second messenger systems via separate GTP-dependent mechanisms, but H2-dependent effects, particularly those of the central nervous system, are predominantly mediated through cAMP. It has been shown that receptor binding stimulates activation of c-Fos, c-Jun, PKC, and P70S6 kinase. Alternative signaling pathways have been reported (Fig. 3.7). These include a receptor-mediated increase in intracellular Ca and/or IP3 levels in HL-60 human promyelocytic leukemia cells and an increase in cAMP and inhibition of release of arachidonic acid in Chinese hamster ovary (CHO) cells transfected with rat cDNA and induced by calcium ionophore. 

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