Modulation of cyclic AMP content of the rat myometrium: desensitization to isoproterenol, PGE2 and prostacyclin

Regulation of connexin 43 by nitric oxide in primary uterine myocytes from term pregnant women

OBJECTIVE

Our purpose was to test the hypothesis that nitric oxide signals regulate the expression of the gap-junction protein connexin 43 in primary uterine myocytes from pregnant women at term.

STUDY DESIGN

Northern analysis and immunoblotting were used to determine the expression of connexin 43 in myocytes cultured in the presence of the nitric oxide donors S -nitroso-N -acetyl-penicillamine (SNAP) (100 micromol/L) and (Z)-1-[2-(2-aminoethyl)-N -(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate diethylenetriamine (NOC-18) (100 micromol/L). We also tested the effect of the NO stimulants 8-bromo-cyclic adenosine 3',5'-monophosphate (cAMP) (100 micromol/L) and 8-bromo-cyclic guanosine 3',5'-monophosphate (cGMP) (200 micromol/L), and the nitric oxide synthase inhibitors N -nitro-L-arginine methyl ester (NAME) (100 micromol/L) and L -N (1-iminoethyl)lysine (NIL) (50 micromol/L).

RESULTS

Nitric oxide and 8-bromo-cAMP reduced the level of connexin 43 expression, and 8-bromo-cGMP had no effect. In contrast, NIL, but not NAME, increased the levels of connexin 43 protein without affecting the level of connexin 43 messenger RNA. With immunoblotting, expression of inducible nitric oxide synthase was not detected in these cells.

CONCLUSION

Nitric oxide down-regulates the expression of connexin 43 in cultured human myocytes. We speculate that this effect may decrease the efficacy of intermyocyte signaling and thus contribute to uterine quiescence during human pregnancy.

Is the attenuation of beta-adrenergic agonist efficacy during labor caused by elevated prostaglandin E levels?

OBJECTIVE

Our purpose was to investigate whether attenuation of beta-adrenergic agonist efficacy during labor is due to elevated prostaglandin E levels.

STUDY DESIGN

beta-Receptor function (response to beta-agonist or forskolin) was tested in mononuclear leukocytes collected from women at term before or during labor. beta-Receptor function was also tested in mononuclear leukocytes of nonlaboring patients after in vitro incubation with oxytocin (1 micromol/L), prostaglandin E2 (10 micromol/L), prostaglandin F2alpha (10 micromol/L), or buffer alone.

RESULTS

Mononuclear leukocytes from women in active labor exhibited a significant attenuation of beta-adrenergic receptor function as a result of reduced adenylyl cyclase activity. This effect could be induced in mononuclear leukocytes from nonlaboring women by preincubation with prostaglandin E but not oxytocin or prostaglandin F.

CONCLUSIONS

Prostaglandin E induces heterologous desensitization of the beta-adrenergic receptor system in mononuclear leukocytes, comparable to that seen during labor. We speculate that during labor the elevated levels of prostaglandin E may induce similar effects on the myometrium, thereby decreasing the efficacy of beta-agonists as tocolytics.

Role of prostaglandin E2 in alterations of the beta-adrenergic system from rat eclamptic uterus

The inotropic effect of isoproterenol, as well as the beta-adrenoceptor population, was measured in pregnant uterine tissue from female spontaneous hypertensive rats (SHR) (control group: C) and female SHR that were grafted with skin from Holtzman male rats (eclamptic group: E). The Kd value of the concentration-response curve of isoproterenol was higher for uteri from E rats than C rats. This phenomenon was not accompanied by a modification in the expression of beta-adrenoceptors. Inhibition of the synthesis of prostaglandins prevented the hyporeactivity to isoproterenol during eclampsia. Moreover, uteri from E rats generated and released greater amounts of prostaglandin E2 (PGE2) than uteri from C rats, even in the presence or absence of isoproterenol. In addition, whereas isoproterenol administered alone increased basal cyclic AMP (cAMP) production from C uteri, PGE2 administered alone enhanced cAMP production in E uterine tissue. These results suggest that the decrease in beta-adrenergic response to the agonist in E rats is ascribed to PGE2 production. The abnormal reactivity to the beta-agonist could be associated with a heterologous desensitization of uterine beta-adrenoceptors exerted by PGE2 overload in uteri from E rats. These results bear directly on the regulation of uterine motility during pregnancy, since an impaired response to beta-adrenergic innervation could lead to increased uterine motility, impairing the maintenance of pregnancy.

Beta-adrenoceptor desensitization by/clenbuterol in rat uterus

1. The relaxant response and cAMP production mediated by stimulation of isoproterenol is reduced in uterine rings from clenbuterol treated rats (0.25 mg kg-1 s.c. 24 hr before experiments) precontracted with 50 mM KCl. 2. Forskolin, in contrast, showed similar relaxant responses in untreated or clenbuterol treated rats. 3. Isoproterenol produces a biphasic response that is composed of a rapid relaxation followed by a slower regaining of tension, which is considered as desensitization. 4. The kinetic study demonstrates marked changes in the desensitization process of beta-adrenoceptors after clenbuterol administration.

Synaptic block of a transmitter-induced potassium conductance in Aplysia neurones

A voltage-clamp study was made of a slow excitatory post-synaptic potential (slow e.p.s.p.) that can be elicited in the medial cells of the left pleural ganglion of Aplysia californica by the firing of at least three different presynaptic neurones (labelled I, II and III). Each of these three neurones elicits other permeability changes in addition to the slow e.p.s.p., and all elements of these synaptic responses were shown to be mediated monosynaptically. The slow e.p.s.p., associated with an increase in membrane resistance, was shown to be due to a decrease in K permeability. When the slow e.p.s.p. was present spontaneously, it could be blocked by three compounds (tetraethylammonium (TEA), phenyltrimethylammonium (PTMA), or methylxylocholine (beta-TM 10], all previously shown to block the cholinergic receptor that mediates an increase in K conductance in the medial cells (see Kehoe, 1972b). Furthermore, in ganglia in which no slow e.p.s.p. was seen in response to firing of the neurones I, II, and III, such a response became manifest when agonists capable of activating the cholinergic receptor were applied (e.g. acetylcholine (ACh), carbachol, arecoline, or F2268). The slow e.p.s.p. thus appears to result from the reduction, induced by any one of three 'blocking neurones', of a cholinergically controlled K conductance. Finally, when presynaptic neurone I (the only neurone tested) was fired shortly before or during the activation of presynaptic neurone IV, previously shown to be cholinergic (Kehoe, 1972b), the K component of the cholinergic post-synaptic inhibitory potential was markedly reduced. The concentration at which a given agonist caused the manifestation of the synaptic diminution in K conductance (i.e. the slow e.p.s.p.) was found to be the same as that at which it caused a reduction in the synaptically activated, cholinergic, K-dependent conductance elicited by presynaptic neurone IV. Intracellularly injected adenosine 3',5'-cyclic monophosphate (cyclic AMP) imitated the effect of the 'blocking neurones' on the K conductance activated by bath-applied cholinomimetics. This effect was superimposed on a cyclic-AMP-induced, voltage-dependent inward current that disappeared when the cell was bathed in Na-free sea water, or when the extracellular Ca concentration was increased to 60 mM. The effect of cyclic AMP on the cholinergic K conductance remained even after this cyclic-AMP-activated inward current was eliminated.(ABSTRACT TRUNCATED AT 400 WORDS)

Arachidonic acid metabolites induce beta-adrenoceptor desensitization in rat lung in vitro

The possible involvement of arachidonic acid (AA) or its metabolites in beta-adrenoceptor desensitization has been studied in rat lung parenchyma both from a functional and a biochemical point of view. In vitro perfusion of rat lungs with AA (3 X 10(-5)M for 20 min) reduced the relaxant effect of isoproterenol (ISO) on lung parenchymal strips, shown by a shift to the right of ISO dose-response curve, similar to that obtained using desensitizing concentration of specific beta-agonist. Moreover, AA treatment reduced the capacity of ISO to stimulate adenylate-cyclase activity, whereas the number of beta-receptor binding sites was not significantly modified. Inhibition of cyclo-oxygenase pathway by indomethacin (INDO) (1.5 X 10(-5)M) prevented both the loss of ISO-relaxing capacity and the decrease of adenylate-cyclase activity induced by AA treatment. In order to support the role of eicosanoids in beta-adrenoceptor desensitization, changes of endogenous free AA levels have also been studied in lung homogenates. Perfusion of rat lung with ISO (10(-6)M for 20 min) decreased by about 50% the levels of free AA and the pretreatment with BW755C (9 X 10(-5)M), a lipo- and cyclo-oxygenase inhibitor, prevented this phenomenon. On the basis of these results, we suggest that the activation of AA cascade is actually involved in beta-adrenoceptor desensitization in lung tissues with a possible interference at the site beyond the drug-receptor interaction.

Receptor-specific mechanisms of desensitization of beta-adrenergic receptor function

beta-Adrenergic receptor (beta AR)-specific, agonist-induced desensitization of adenylate cyclase can be shown in most mammalian cells examined to involve at least three reactions. An initial 'uncoupling' reaction leads to a 40-60% loss of catecholamine-stimulated adenylate cyclase activity at a time when no detectable loss of beta AR has occurred. This process precedes by 45-90 sec the appearance of beta AR in cytoplasmic vesicles. Such beta AR exhibit ligand binding properties consistent with their existence on the inside of membrane vesicles; thus, they appear to be formed by a process of agonist-induced beta AR internalization (endocytosis). A third process results in the loss of beta AR, at least in some cases due to receptor degradation. In general, agonist-induced desensitization or down-regulation reactions do not require protein synthesis. Recovery from the desensitized states does not require protein synthesis, whereas recovery from beta AR down-regulation (degraded receptors) requires new receptor synthesis. Agonist-induced beta AR desensitization and down-regulation reactions appear to have much in common with the process of polypeptide hormone-induced receptor down-regulation. The availability of a large number of ligands (agonists, partial agonists and antagonists) for the beta AR should allow the use of this receptor system to gain unique insights into the general processes of ligand-induced, cell surface receptor endocytosis.

Starvation-induced changes of palmitate metabolism and insulin secretion in isolated rat islets stimulated by glucose

The influence of 48 h starvation on glucose-induced changes of palmitate metabolism and insulin release in isolated rat islets was investigated. (1) Islet insulin response to 20 mM-glucose was abolished after 48 h starvation, and it was restored by 0.25 mM-2-bromostearate, an inhibitor of fatty acid oxidation. (2) The increase in glucose concentration from 3 to 20 mM was accompanied by a 50% decrease in the oxidation rate of 0.5 mM-[U-14C]palmitate in control (fed) islets, and a concomitant increase (100%) in its incorporation into triacylglycerol and phospholipid fractions. (3) Starvation induced a higher basal (3 mM-glucose) rate of palmitate oxidation, which was resistant to inhibition by 20 mM-glucose. The latter also failed to increase palmitate incorporation into islet triacylglycerols and phospholipids. (4) 2-Bromostearate (0.25 mM) strongly inhibited the high oxidation rate of palmitate in islets of starved rats, and allowed a normal stimulation of its incorporation rate into islet lipids by 20mM-glucose. (5) The results suggest that starvation restricts islet esterification of fatty acids by inducing a higher rate of their oxidative degradation that is insensitive to regulation by glucose.

Effect of ritodrine hydrochloride and dibutyryl cyclic AMP on contractile activity and prostanoid production of uteri from pregnant rats in vitro

Ritodrine hydrochloride is a beta-mimetic amine which is used to inhibit premature labor. While the mechanism of action of beta-mimetic drugs is believed to be a function of its action on the adenylate cyclase system, the drug may also act via other mechanisms. We examined the effect of this drug on both contractile activity and prostanoid production using an in vitro preparation of a uterus from a 21-day pregnant rat. Two uterine segments were simultaneously studied in separate incubation chambers. Ritodrine (2.0 mg/ml) was added to one tissue chamber while the other tissue served as a control. Frequency and contractile force were monitored polygraphically for 45 min. Incubation medium was then removed for analysis of prostaglandin E2 (PGE2), PGF2 alpha, 6-keto-PGF1 alpha, and thromboxane B2 (TXB2) by radioimmunoassay. Ritodrine-treated uteri demonstrated a contractile force which was 16.8% of that of the control, a significant decrease. Ritodrine-treated uteri also produced less prostanoids. The greatest effect was on PGE production (27.3% of the control, p less than 0.001). The effect of ritodrine on the other prostanoids was less pronounced (PGF2 alpha, 71%; 6-keto-PGF1 alpha, 84%; TXB2, 67% of the control). The presence of 0.2 mM dibutyryl cAMP in the incubation media also suppressed contractile force; however, prostanoids were not reduced and in some cases were elevated. It is concluded that one effect of ritodrine is a reduction in prostanoid production, predominantly PGE2 and this in part may play a role in the drug's efficacy. The reduction does not appear to be mediated through the adenylate cyclase system.

Differential role of microtubules in the control of prostaglandin E2 and beta-adrenergic stimulation of cyclic AMP accumulation in the rat myometrium

A possible modulatory role of microtubules was investigated for the beta-adrenergic and prostaglandin E2 (PGE2)-induced cyclic AMP accumulation in the estrogen-treated rat myometrium. Colchicine, vinblastine and nocodazole, three different antitubulin drugs, enhanced cyclic AMP accumulation caused by PGE2. The effect of inhibitors was dose-(0.1-5 microM) and time-dependent, increased maximal responses without changing EC50 for PGE2, did not occur with trimethyl-colchicinic acid, which does not bind to tubulin, and was totally prevented in tissues pretreated with taxol, an agent that enhances polymerization and stabilization of microtubules. Concomitantly, colchicine reduced the rate and extent of PGE2-induced refractoriness in terms of cyclic AMP. In contrast, the antitubulin drugs failed to affect the rise in cyclic AMP evoked by isoproterenol and cholera toxin but enhanced the response to prostacyclin (PGI2), which is assumed to share common receptors with PGE2. Colchicine and vinblastine also failed to alter the ability of the beta-adrenergic agonist to provoke a cyclic AMP refractory state. Stimulations induced by all effectors were totally insensitive to cytochalasin B. The data suggest a relation between the constraints associated with the microtubules and/or membrane tubulin of the myometrium and the efficacy of PGE2 and PGI2 (but not the beta-adrenergic agonist or cholera toxin) to interact with the cyclic AMP forming system.

Desensitization of beta-adrenoreceptors in guinea-pig trachea: a prostaglandin mediated phenomenon

1 The formation and release of PGE2-like material (PGE2-lm) from guinea-pig isolated trachea and human bronchi following relaxation with isoprenaline (I) was investigated. 2 When airway smooth muscle precontracted with pilocarpine is relaxed by I, PGE2-lm is released in the bathing fluid. Under conditions of beta-adrenoreceptor desensitization, a greater amount of PGE2-lm is formed and the responsiveness of guinea-pig tracheal spirals to cumulative doses of I is diminished. 3 Inhibition of cyclo-oxygenase with indomethacin does not modify the relaxation induced by I but prevents the occurrence of refractoriness to I as well as formation and release of PGE2-lm. Addition of exogenous PGE2 to guinea-pig tracheas which were treated with indomethacin is able to restore the capacity of I to cause tachyphylaxis. 4 It is concluded that PGE2 which does not mediate the relaxation induced by I in airway smooth muscle, is responsible for the onset of desensitization of the beta-adrenoreceptor.

Lymphocyte beta-adrenergic refractoriness induced by theophylline or metaproterenol in healthy and asthmatic subjects

Beta-adrenergic refractoriness was assessed in human lymphocytes following in vivo administration of the beta-adrenergic agonist, metaproterenol, the phosphodiesterase inhibitor, theophylline, or both concomitantly, to normal and asthmatic subjects. In normal subjects both beta-adrenergic receptor number and isoproterenol stimulated cAMP response decreases during therapy with metaproterenol (59 +/- 3; 51 +/- 16% of control, respectively), theophylline (76 +/- 6; 78 +/- 16), or concomitant metaproterenol and theophylline (47 +/- 4; 69 +/- 13). The asthmatic subjects were of two types; one type responding to metaproterenol or theophylline therapy by down regulation of receptor number to zero or near zero values, and a second group of asthmatics insensitive to down regulation of receptor number. The results suggest that the induction of the refractory state is different between asthmatics and non-asthmatics, and that there may be a role for cAMP in the development of beta-adrenergic refractoriness, in vivo.

Interactions between three slow potassium responses controlled by three distinct receptors in Aplysia neurones

1. A voltage clamp study was made of the K(+) permeability increases produced in certain identifiable neurones of the cerebral ganglion of Aplysia by any one of three distinct agonists (carbachol, histamine and dopamine).2. The three K(+) responses involve three distinct receptors, as shown by the selective effects of reversible antagonists (Gruol & Weinreich, 1979a) as well as by the differential, irreversible effects of trimethyloxonium (TMO) ions.3. Prolonged exposure of the neurones to one of the agonists reduces the response to the same agonist (desensitization) and, over the low concentration range, doubling the concentration of an agonist leads to supra-linear summation (potentiation).4. Prolonged exposure of the neurones to one of the agonists also reduces the response to the other agonists (cross-desensitization) and combined application of two agonists reveals cross-potentiation.5. The time course of desensitization (onset and decay) was the same for the histamine and carbachol responses and, except at very high concentration, was indistinguishable from that of cross-desensitization. Likewise, potentiation was similar in the two agonist systems and did not differ significantly from cross-potentiation.6. The results can be interpreted by assuming that the responses to the three agonists involve specific steps followed by common reaction steps, and that some of the common reaction steps control both potentiation and desensitization.7. The responses to carbachol and histamine differ in their voltage sensitivity. This suggests that one or more of the specific steps are voltage-sensitive.8. Although an increase of the intracellular Ca(2+) concentration can itself open K(+) channels, and also inhibit the responses to the three agonists, an increase of internal Ca(2+) does not appear to play an important role either in the development of the response or in the desensitization process.