Relaxation of the guinea-pig tracheal chain preparation by N6,2'-O-dibutyryl 3',5'-cyclic adenosine monophosphate

Beta-adrenergic actions on membrane electrical properties of dissociated smooth muscle cells

Studies were carried out to determine the effects of the beta-adrenergic agent, isoproterenol (ISO), on membrane electrical properties in single smooth muscle cells enzymatically dispersed from toad stomach. In cells bathed in buffer of physiological composition, the average resting potential was -56.4 +/- 1.4 mV (mean +/- SE, n = 35). The dominant effect of exposure to ISO was hyperpolarization. The hyperpolarization was apparent in all cells studied and averaged 11.6 +/- 1.2 mV (n = 27). In the majority of the cells, hyperpolarization was accompanied by a decreased input resistance (Rin). Often the change in resistance appeared to lag behind the change in membrane potential. The lack of coincident changes in membrane potential and resistance may reflect a superposition of the outward rectification properties of the membrane on beta-adrenergic-induced increases in ionic conductance. In about half of the cells, an initial small depolarization (3.1 +/- 0.3 mV, n = 14) was accompanied by a small but distinct increase in Rin (12 +/- 2.5%). When membrane potential was made more negative than the estimated equilibrium potential for K+ (EK) by injection of current, ISO also produced biphasic effects, an initial hyperpolarization which reversed to a sustained depolarization to a value (-90 mV) near the estimated EK. The hyperpolarization by ISO could be diminished in a time-dependent manner by previous exposure to ouabain. The inhibition by ouabain, however, appeared to be a fortuitous result of glycoside-induced positive shifts in EK. These observations indicate that the dominant electrophysiological effect of beta-adrenergic stimuli is to hyperpolarize the cell membrane.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical regulation of airway smooth muscle tone: current knowledge and therapeutic implications

Evidence collected during the last decade indicates that the molecular processes responsible for smooth muscle contraction are fundamentally different from those responsible for skeletal muscle contraction. Furthermore, because of the diverse functional roles of various smooth muscles, it would not be surprising if significant differences in regulatory processes also exist among different smooth muscles. Such diversity may already be exemplified by differences in cross-bridge kinetics and sources of activator Ca2+. Additional unique regulatory features of various smooth muscle types will undoubtedly be uncovered by further research. A convincing body of data suggests that activation of the adenylate cyclase/protein kinase cascade is responsible for the bronchodilation produced by beta-adrenoceptor agonists. Although the exact mechanism by which the activation of cAMP-dependent protein kinase induces relaxation is not clear, the phosphorylation of multiple substrates may be involved. Phosphorylation of these substrates can promote relaxation by decreasing the myoplasmic Ca2+ concentration, decreasing the Ca2+ sensitivity of the contractile apparatus, or both. Thus, because beta-adrenoceptor agonists act as physiologic antagonists of broncho-constriction, they should relax airway smooth muscle regardless of the mediator(s) responsible for the bronchospasm. Perhaps this is the major reason that the beta-adrenoceptor agonists have become the premier class of drugs used in the treatment of bronchial asthma. As useful as the sympathomimetic bronchodilators have been, they are not without liabilities. These liabilities include: cardiovascular and skeletal muscle side effects, an inherent subsensitivity of the asthmatic patient population to beta-adrenoceptor agonists, the development of tolerance and a loss of efficacy during severe asthmatic episodes. The fact that these drawbacks are probably shared by all sympathomimetic bronchodilators suggests that little therapeutic advantage will be gained by developing new beta-adrenoceptor agonists. The task of developing novel bronchodilators will be facilitated by an understanding at the molecular level of the diversity among smooth muscles and the processes that regulate smooth muscle tone. Hopefully, such knowledge will lead to a new generation of highly effective, tissue-selective bronchodilators with significant therapeutic advantages over those currently available.

Induction of cells with a myoepithelial cell phenotype by treatment with dibutyryl cyclic AMP in human salivary adenocarcinoma cells grown in athymic nude mice

The adenocarcinoma produced by transplantation into nude mice of a neoplastic human salivary intercalated duct cell line was treated with 0.1 ml of minimal essential medium (MEM) containing dibutyryl cyclic AMP (dB-cAMP) at a final concentration of 1 mM daily for 28 days and examined morphologically and immunohistochemically. The dB-cAMP treatment resulted in a marked suppression of tumor growth. In addition, tumor nests with a myoepithelial cell phenotype characterized by the presence of microfilament systems reactive with antimyosin and anti-S-100 protein sera were often observed in the treated tumors, but not in untreated controls. These findings lead us to suggest that neoplastic intercalated duct cells can be induced to differentiate into myoepithelial cells and that levels of cAMP within the cells may regulate this cytodifferentiation.

The effect of histamine on cyclic AMP levels in guinea-pig tracheal smooth muscle

Histamine (10(-5)--3 x 10(-4) M) increased the cylic AMP content of guinea-pig tracheal smooth muscle, the maximal effect being a 3-fold increase after 2-min incubation with 10(-4) histamine. Histamine-induced accumulation of cyclic AMP was not affected by propranolol or atropine, but was reduced by mepyramine. Aspirin and indomethacin abolished the cyclic AMP response to histamine and potentiated histamine-induced contractions of the smooth muscle. These results suggest that the elevation of cyclic AMP levels in response to histamine is mediated by prostaglandins, and represents an important negative feedback regulatory mechanism which modulates the contractile response of guinea-pig tracheal smooth muscle to histamine.

On the mechanism of relaxation of tracheal muscle by theophylline and other cyclic nucleotide phosphodiesterase inhibitors

The mechamism of action of theophylline was studied by investigating the relationship between relaxant effect and inhibition of cyclic nucleotide phosphodiesterase (PDE) and by studying interactions with adenosine actions. Guinea pig tracheal smooth muscle cyclic AMP PDE had two apparent KmS': 0.4 and 70 microM for cyclic AMP. Theophylline and papaverine competetively inhibited the low Km form. Hydrolysis of 2.0 microM cyclic AMP and cyclic GMP was inhibited by several drugs. Some agents (e.g. ZK 62 711, ICI 63,197, Ro 20--1724, dipyridamol) were considerably more potent as inhibitors of cyclic AMP than of cyclic GMP hydrolysis, while other agents (M & B 22.948 and dilazep) selectively inhibited cyclic GMP breakdown, and some (theophylline, papaverine, IBMX and SQ 20,006) showed little selectivity. There was a weak but significant correlation between inhibition of cyclic AMP phosphodiesterase and relaxation of tracheal smooth muscle in vitro. There was also a correlation between the ratio of IC25 cyclic AMP/IC25 cyclic GMP and the smooth muscle relaxation, indicating that inhibition of cyclic AMP rather than cyclic GMP hydrolysis determined relaxation. However, there was a marked tachyphylaxis to the relaxant effect of the cyclic AMP selective PDE-inhibitors, while the nonselective methylxanthines did not show tachyphylaxis. The effect of theophylline was antagonized by low concentrations of adenosine, which by itself caused a weak tracheal contraction. The effect of PDI-inhibitors can be partly explained by decreased cyclic AMP breakdown but other mechanisms, such as antagonism of endogenous adenosine, may contribute to the observed relaxant action.

Beta-adrenoceptor stimulation and cyclic AMP levels in bovine tracheal muscle of old and young animals

The relationship between cyclic AMP levels and mechanical activity after exposure to isoprenaline was studied in tracheal smooth muscle. It was found that the basal cyclic AMP content decreased with age. The relaxing effects of isoprenaline, theophylline and papaverine were tested on muscles contracted by histamine, acetylcholine or carbacholine. Isoprenaline completely relaxed histamine contracted tracheas, but not those contracted by acetylcholine or carbacholine. Theophylline and papaverine completely relaxed the tracheas irrespective of whether the contracting agent was histamine, acetylcholine or carbacholine. Isoprenaline increased the cyclic AMP content of bovine trachea; this effect was stronger in muscles with spontaneous tension than in histamine contracted muscles. In muscles contracted by carbachol, isoprenaline increased the cyclic AMP level after 5 min. The correlations between the changes in the cyclic AMP levels and the tension in tracheal smooth muscle support the hypothesis that this nucleotide plays a role in the relaxation process.

Cyclic AMP

Cyclic AMP mediates the effects of many hormones on their target cells. Some of the mechanisms by which this occurs are discussed in this review. The role of cyclic AMP in regulating glycogen metabolism in liver and skeletal muscle is firmly established. Data suggesting that cyclic AMP mediates hormonally stimulated salivary gland amylase secretion, cardiac muscle contractility, smooth muscle relaxation, and platelet aggregation are summarized.

Dibutyryl cyclic adenosine monophosphate effects in the ischemic-hypoxic cat

The effects of dibutyryl cyclic adenosine monophosphate (dB-cAMP) were studied in fifty cats, twenty anesthetized with pentobarbital and thirty with halothane. Nasopharyngeal temperature and Paco2 were maintained at normal values. Somatosensory evoked response was monitored and used as an indicator of cerebral cortical function. Ischemic hypoxic injury was produced by an orthopedic tourniquet snugly applied around the animal's neck and inflated for a period of fifteen minutes. This method produces a reliable and reproducible injury. Times for recovery of the evoked response to 10% of control value, as well as immediate and long-term animal survival, were noted. The dBcAMP was administered at the end of the hypoxic insult. Treated animals recovered the evoked response earlier than the untreated controls and had better immediate and long-term survival rates.

Cyclic 3',5'-adenosine monophosphate and bronchial tone

The present studies demonstrate that adenylate cyclase and cyclic 3',5'-adenosine monophosphate (cAMP)-phosphodiesterase activities in dog bronchus are comparable to those found in other smooth muscle preparations. Catecholamines, in the order isoproterenol greater than epinephrine greater than norepinephrine, increase the rate of cAMP formation. This effect can be competitively inhibited by propranolol and potentiated by a cAMP-phosphodiesterase inhibitor. The kinetic study of bronchial cAMP-phosphodiesterase showed two different rates of cAMP hydrolysis, with apparent Km values of 1.4 and 48.0 muM. The high affinity cAMP-phosphodiesterase was inhibited competitively by theophylline and papaverine, the latter being about 20 times more potent than the former. The potency of each compound to inhibit the enzyme and to relax the bronchial strip was comparable. These results, the similar order of potency of the catecholamines to relax the bronchus and to increase the rate of cAMP formation, the competitive inhibition of both effects by propranolol, and the relaxing effect of dibutyryl cAMP on bronchial strip, are compatible with the assumption that the cAMP system is one of the biochemical mechanisms mediating bronchial smooth muscle relaxation.

Influences of aminophylline and reduction in external Na on the antispasmodic action of isoproterenol in the isolated rat rectum

Effects of aminophylline and the reduction in external Na on the antispasmodic action of isoproterenol were investigated in relation to the mobilization of Ca in the isolated rat rectum. The antispasmodic action of isoproterenol on the phasic contractions by acetylcholine and K both in Ca-free and in Ca-free and Na-poor media was potentiated by treatment with aminophylline, however the antispasmodic action was attenuated by reducing Na in the Ca-free medium. Dibutyryl cyclic AMP inhibited media and the inhibitory action was also potentiated by treatment with aminophylline, while the inhibitory action of dibutyryl cyclic AMP was attenuated by reducing Na in the Ca-free medium. From these findings, it appears that isoproterenol inhibits the release of Ca from storage sites induced by acetylcholine and K via the increase of intracellular cyclic AMP content and that the external Na may play an important role in the Ca release-inhibiting effect of cyclic AMP.

Imitation of glucagon effects on splanchnic hemodynamics and liver function by N6,2'-O-dibutyryl 3',5'-cyclic AMP (DBcAMP) in cats

Fasting cats anesthetized with chloralose were used for the experiments. DBcAMP infused at a rate of 340 nmol/kg/min increased the gastrointestinal and intrahepatic portal conductances whereas the hepatic arterial conductance was decreased. The hemodynamic responses to portal and systemic venous administration of DBcAMP were identical. In half of the experiments DBcAMP increased the splanchnic ethanol elimination rate and oxygen consumption and in all experiments there was a decrease in the plasma clearance and extraction ratio of Indocyanine Green. No change in bile flow was observed. DBcAMP infused at a rate of 85 nmol/kg/min was without significant effects on either splanchnic hemodynamics or liver metabolism. DBcAMP infused at a rate of 850 nmol/kg/min accentuated the decrease in hepatic arterial conductanc- but was found to decrease the splanchnic ethanol elimination rate and oxygen cownsumption. Infusion of cAMP, AMP and adenosine at a rate of 340 nmol/kg/min were without measurable effects. Based on these results it is concluded that like the metabolic effects also the vascular effects of glucagon are caused by stimulation of specific glucagon receptors which results in an intracellular release of cAMP.

Effect of inosine on guinea-pig bronchial muscle

Inosine, like other purine derivatives, shows antibronchospastic effects both in vivo and in vitro. Its activity remains in propranololor α-methyl-p-tyrosine-pretreated guinea-pigs, but is prevented by theophylline. Moreover, the interaction between 3′5AMP and inosine suggests potentiation. In view of recent findings showing that inosine inhibits phosphodiesterases, the conclusion is drawn that the antibronchospastic effect of this nucleoside depends on the increase of 3′5′AMP intracellular levels in the bronchial muscle.

The effects of some phosphodiesterase inhibitors on the conductance of the perfused vascular beds of the chloralosed cat

1. The vasoactivity of a number of phosphodiesterase inhibitors in the auto-perfused splanchnic region and hindquarters of the cat have been investigated.2. Theophylline, dipyridamole and 4-(3,4-dimethoxybenzyl)-2-imidazolidinone (Ro 7-2956) and its analogues were more potent vasodilators in the splanchnic region than in the hindquarters, whereas papaverine was a potent vasodilator in both these vascular beds.3. The vasodilator response to the phosphodiesterase inhibitors in the splanchnic region and hindquarters was not susceptible to beta-adrenoceptor blockade.4. Infusion of dibutyryl cyclic AMP into the superior mesenteric artery increased vascular conductance of the splanchnic region. This effect was enhanced by Ro 20-1724 in doses below those in which this compound affected conductance.5. These results are consistent with the hypothesis that accumulation of cyclic AMP in vascular smooth muscle mediates the vasodilator response to phosphodiesterase inhibitors.

Effect of adenosine 3'-5'-cyclic monophosphate on cell proliferation

Secondary cultures of human diploid fibroblasts, which demonstrate density-dependent inhibition of cell growth, were used to study the effect of adenosine 3'-5'-cyclic monophosphate (cAMP) on cell proliferation. DNA synthesis in nonconfluent cultures and in contact-inhibited cultures stimulated to grow by refeeding with fresh medium was found to be inhibited by exogenous cAMP. The properties of this inhibition of DNA synthesis, together with the alterations in cAMP metabolism observed in confluent cultures of cells stimulated with fresh medium to resume growth, strongly suggest that cAMP is involved in contact-inhibition of cell proliferation.

Some bronchoconstricting and bronchodilating responses of human isolated bronchi: evidence for the existence of -adrenoceptors

Isolated strips of human bronchi obtained during thoracic surgery exhibited pharmacological responses very similar to those of other species (e.g. guinea-pig). Analysis of the action of some sympathomimetic amines indicated that the human bronchi also possess a sparse population of α-adrenoceptors. Propranolol (>1 μg/ml) had a direct bronchoconstricting action, whereas another β-adrenoceptor blocking agent, alprenolol, produced bronchodilatation. Phentolamine (>4 μg/ml) also produced a bronchodilatation of its own. Theophylline and dibutyryl cyclic AMP produced dose-dependent relaxations. The strips were effectively contracted by acetylcholine, histamine, prostaglandin F2α (PGF2α), slow reacting substance (SRS) and bradykinin. Bradykinin, regardless of the dose, produced bronchoconstriction in some preparations and dilatation in others. 5-Hydroxytryptamine produced bronchodilatation but at very high concentrations a constriction was obtained.

Pharmacological properties of centrally administered ouabain and their modification by other drugs

1. Ouabain given by intracerebroventricular injection to mice in small doses (0.1-0.4 mug) produced a dose related depression of central nervous activity, characterized by a reduction in spontaneous locomotor activity, hypothermia, catalepsy and ptosis, lowered body posture and lack of response to external stimuli. Doses above 0.4 mug were excitatory, convulsant and lethal.2. The depressant effects could be antagonized by (+)-amphetamine, desmethylimipramine, dibutyryl cyclic 3'5'-adenosine monophosphate and caffeine.3. The MAO inhibitor nialamide produced only a small antagonism of ouabain, resulting in a greater rate of recovery from the depressant effects of ouabain.4. The depressant effects were associated with a marked elevation of whole-brain dopamine levels with little change in noradrenaline or 5-hydroxytryptamine.5. The dopamine-beta-hydroxylase inhibitor sodium diethyldithiocarbamate, administered by intracerebroventricular injection, produced effects qualitatively similar to those seen after ouabain.6. An interference with central transmitter function is postulated as a possible mode of action of intracerebroventricularly injected ouabain.