Adenylate cyclase activity stimulatory effects

When tested on osmotically-lysed bovine parathyroid cells, dopamine enhances the activity of adenylate cyclase, the enzyme converting ATP to cAKP (111. In comparison with its effect on cAMP accumulation, the effect of dopamine on adenylate cyclase activity is relatively modest, only a 2-fola increase in enzyme activity (Figure 7). Guanosine 5 -triphosphate (GTP) increases the stimulatory effect of dopamine in the presence of GTP, there is 3 to i<-fold stimulation of enzyme activity (11). 

The role of cyclic AMP as modulator of prolactin secretion was first suggested by the finding of a stimulatory effect of cyclic AMP derivatives (17-22) and inhibitors of cyclic nucleotide phosphodiesterase activity such as theophylline and IBMX (22-26) on the secretion of this hormone. More convincing evidence supporting a role of cyclic AMP in the action of dopamine on prolactin secretion had to be obtained, however, by measurement of adenohypophysial adenylate cyclase activity or cyclic AMP accumulation under the influence of the catecholamine. As illustrated in Fig. 1, addition of 100 nM dopamine to male rat hemipituitaries led to a rapid inhibition of cyclic AMP accumulation, a maximal effect (30% inhibition) being already obtained 5 min after addition of the catecholamine. Thus, while dopamine is well known to stimulate adenylate cyclase activity in the striatum (27, 28), its effect at the adenohypophysial level in intact cells is inhibitory. Dopamine has also been found to exert parallel inhibitory effects on cyclic AMP levels and prolactin release in ovine adenohypophysial cells in culture (29) and purified rat mammotrophs (30). Using paired hemipituitaries obtained from female rats, Ray and Wallis (22) have found a rapid inhibitory effect of dopamine on cyclic AMP accumulation to approximately 75% of control. 

As Illustrated in Fig. 7, 3 yM CRF and 1 yM (-)Isoproterenol cause a 190 and 110% stimulation of adenylate cyclase activity In rat pars intermedia particulate fraction, respectively. An additive effect Is observed when both stimulatory agents are present. Dopamine (30 yM), on the other hand, has no significant effect alone. However, In the presence of GXP, the catecholamine causes a 40 to 60% Inhibition of adenylate cyclase activity stimulated by CRF, ISO or CRF + ISO. It can also be seen that while 0.3 mM GXP alone causes a 100% increase In basal adenylate cyclase activity, it leads to a marked potentiation of the effect of ISO and CRF on [ 2P] cyclic AMP accumulation. It should be noticed that In the absence of the guanyl nucleotide, dopamine has no Inhibitory effect on adenylate cyclase activity In any of the groups studied. 

As mentioned earlier, guanyl nucleotides have been found to play an important role In the activation of adenylate cyclase activity by many hormones (90, 91). The present observations show that in pars Intermedia tissue, GXP causes an almost doubling of the stimulatory effect of CRF while that of the B-adrenerglc ago-... 

Bld-HrTH administration to B. discoidalis in vivo or to isolated fat body fails to stimulate either fat body cAMP levels or adenylate cyclase activity and supports the previous findings (25). Nevertheless, for B. discoidalis, fat body phosphorylase activity is elevated and trehalose levels increase both in vivo and in vitro, and calcium is essential in vitro in addition to Bld-HrTH. No stimulation of trehalose synthesis is noted with agents that elevate adenylate cyclase, such as forskolin, or by inhibitors of phosphodiesterase such as theophylline or isobutylmethylxanthine (IBMX). Additions of cAMP, dibutyryl cAMP or 8-bromo-cAMP are not stimulatory to trehalose synthesis either in vivo or in vitro. This same result was observed for P. americana in that neither cAMP nor dbcAMP stimulated trehalose production by fat body in vitro, and xanthine inhibitors of phosphodiesterase that should cause accumulation of intracellular cAMP were inhibitory, except for isobutylmethylxanthine (IBMX) which was stimulatory for unknown reasons (26). We have not observed a stimulatory effect by IBMX with B. discoidalis fat body in vitro. 

Forskolin, or other elevators of intracellular cAMP such as rolipram (a cAMP phosphodiesterase inhibitor) can induce PP2A activation [111, 143, 144]. Although most of their cellular effects depend on augmented PKA and adenylate cyclase activities, the mechanism by which forskolin activates PP2A does not require an increase in PKA activity [ 143]. This is best proven by the persisting PP2A stimulatory effect of 1,9-dideoxy-forskolin [ 111 ], a forskolin derivative that does not result in adenylate cyclase activation and increased cAMP production. In cellular [111, 113, 144] and in vivo [111] models of CLL, CML, and AML, rolipram, forskolin. 

Further tissue-specific effects were observed in membrane preparations obtained from capsaicin-desensitized rats. The basal adenylate cyclase activity was dramatically increased in the preoptic hypothalamus of these animals but unchanged in the cerebellum or cortex. Moreover, in the cerebellum, normal stimulatory responses were obtained, whereas in the preoptic region not only did capsaicin desensitization cause a failure in the ability of capsaicin to stimulate adenylate cyclase in vitro, but it also produced an inhibition of the augmented basal enzyme activity (Jancso and Wollemann, 1977). Prior exposure of cortical membranes to capsaicin in vitro was also found to increase basal adenylate cyclase activity, and this effect was found to be dependent on calcium ions (Horvath et ai, 1979). 

Adenylate Cyclase. Another secondary cellular effect of PT is the enhancement of adenylate cyclase activity in S49 lymphoma cell membranes (41). The addition of PT to isolated cell membranes resulted in an increase in the forskolin-induced cAMP production in both wild type and eye" variant cell membranes. The stimulatory G protein (Gs) was not involved in the action of PT on adenylate cyclase, but PT did appear to reduce the effect of Gi on the enzyme. CTX also enhanced the activity of the enzyme, and both toxins appear to exert their effect by acting on the membrane, followed by a secondary activation of adenylate cyclase in the membrane. 

G proteins are divided into several types, depending on their effects. Stimulatory G proteins (Cs) are widespread. They activate adenylate cyclases (see below) or influence ion channels. Inhibitory G proteins (Cj) inhibit adenylate cyclase. G proteins in the Gq family activate another effector enzyme—phospholipase c (see p. 386). 

Apart from forskolin, a number of other manoyl oxides have been shown to interact with the AC enzyme system. Biotransformation of certain ent- 3-epi-manoy oxides by Curvularia lunata resulted in compounds functionalized in C-3 or in C-3 and C-12, which exhibited an AC stimulatory effect, although milder than that of forskolin (about 30 times less) [173]. The same activity was also ascribed to some synthetic derivatives of en/-8a-hydroxy-13 (16), 14 dien-18-oic acid methyl ester [178,179], The biotransformation of ent-manoyl oxide-16-hydroxy 18-oic acid methyl ester with Rhizopus nigricans, however, resulted in carbomanoyl oxide which showed a selective inhibitory action on the activity of adenylate cyclase depending on the material initially used to stimulate the enzyme. This manoyl oxide inhibited the activity of the enzyme previously stimulated by forskolin but not by glucagon. A manoyl oxide ent-3fi, 6/ -dihydroxy-13-e/ z-manoyl oxide) which also inhibited the activity of AC, was produced from the biotransformation of... 

In some systems, the eicosanoids appear to modulate the degree of activation of adenylate cyclase in response to other stimuli. In these instances, the eicosanoid may bind to a regulatory subunit of the GTP-binding proteins (G proteins) within the plasma membrane of the target cell (see Chapter 11). If the eicosanoid binds to the stimulatory subunit, the effect of the stimulus is amplified. Conversely, if the eicosanoid binds to the inhibitory subunit, the cellular response to the stimulus is reduced. Through these influences on the activation of adenylate cyclase, eicosanoids contribute to the regulation of cell function. 

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