Beta-adrenergic effects on rat myometrium: role of cyclic AMP

Tocolytic activity of formoterol against premature delivery in mice

The tocolytic activity of formoterol (eformoterol), a long-acting potent beta(2)-adrenoceptor agonist, was assessed in pregnant mice, with determination of uterine effects on the 15th and 16th days of gestation. For examination in the lipopolysaccharide-induced premature delivery model, osmotic pumps filled with formoterol or saline solution were implanted subcutaneously under the back skin. The mice were sacrificed 18-20 h thereafter, and the numbers of fetuses in the uteri and the newborn were counted. The uteri, amniotic membranes and placenta were also rapidly removed for determination of IL-6 concentrations. Furthermore, the effect of formoterol on IL-6 secretion from mouse amnion cells was determined. Formoterol and ritodrine inhibited contraction responses of isolated mouse uteri and their intravenous administration resulted in lowered uterine motility. Lipopolysaccharide (30 microg mL(-1)/mouse) induced premature delivery, attributable to increased IL-6 secretion, and formoterol suppressed this. Doses of 5-500 microg/mouse thus reduced the number of prematurely delivered newborn, and 50 microg/mouse also depressed IL-6 secretion. On histopathologic analysis, the marked oedema and slight haemorrhage in the mouse cervix induced by lipopolysaccharide were reduced by administration of the beta(2)-adrenoceptor agonist. Neither formoterol (10(-7)-10(-5) M) nor ritodrine (10(-7)-10(-5) M) influenced spontaneous secretion of IL-6 in amnion cells. However, at 10(-7) and 10(-5) M, and 10(-6) and 10(-5) M, respectively, they inhibited lipopolysaccharide-induced IL-6 secretion and this inhibitory effect was competitively reversed by addition of ICI-118,551 (beta(2)-adrenoceptor antagonist), but not atenolol (beta(1)-adrenoceptor antagonist). These findings strongly suggest that formoterol can suppress premature delivery mediated by its actions on IL-6 secretion.

Tocolytic effects of a long-acting beta2-adrenoceptor agonist, formoterol, in rats

We have assessed the tocolytic activity of formoterol, a novel long-acting and potent beta2-adrenoceptor agonist, through its production of cyclic adenosine monophosphate, in comparison with ritodrine, a beta2-adrenoceptor agonist used clinically to counter premature delivery. Formoterol and ritodrine inhibited the amplitude and frequency of rat uterine contraction, with IC50 values of 3.8 x 10(-10) and 4.7 x 10(-7) M, respectively. Intravenous administration of formoterol or ritodrine caused inhibition of uterine motility and increased heart rate in a dose-dependent manner. Inhibition of uterine motility by oral administration of formoterol (0.3 and 1 mg kg(-1)) continued for at least 60 min, whereas that with ritodrine (100 mg kg(-1)) persisted for 15 min with rapid recovery thereafter in pregnant rats. The beta-adrenoceptor binding of [125I]iodopindolol to the myometrium of pregnant rats was competitive with formoterol and ritodrine, with Ki values of 0.04 and 6.10 nM, respectively. Formoterol (10(-6)-10(-4) M) and ritodrine (10(-6)-10(-4) M) increased the level of cyclic adenosine monophosphate in lymphocytes in a dose-dependent manner. The results suggested that formoterol caused relaxation of uterine motility through production of cyclic adenosine monophosphate. Thus, formoterol may be useful as a treatment to counter premature delivery.

Potent inhibition of spontaneous rhythmic contraction by a novel beta 2-adrenoceptor agonist, HSR-81, in pregnant rat uterus

We examined the effect of HSR-81 ((-)-(R)-alpha-[(tert-butylamino)methyl]-2-chloro-4-hydroxybenzyl alcohol L-tartrate), a newly developed, potent and selective beta 2-adrenoceptor agonist, as well as ritodrine and isoproterenol, on the spontaneous rhythmic contraction in uteri isolated from late pregnant, middle pregnant and non-pregnant (dioestrous and oestrous) rats. The three agonists inhibited the spontaneous rhythmic contraction at all the stages in a concentration-dependent manner. The pD2 value for HSR-81 was greater in late pregnancy than in dioestrus and oestrus. In the uterine preparations of late pregnancy and dioestrus, ICI-118,551 (1-(7-methylindan-4-yloxy)-3-isopropyl-aminobutan-2-ol , a selective beta 2-adrenoceptor antagonist) and atenolol (a selective beta 1-adrenoceptor antagonist) produced a parallel rightward shift of the concentration-response curves for HSR-81. The pKB values for ICI-118,551 and atenolol suggest that the inhibitory effect of HSR-81 was mediated through beta 2-adrenoceptors in the two stages. In the membranes prepared from rat uteri in late pregnancy and dioestrus, the equilibrium dissociation constant for [125I]iodocyanopindolol binding was not significantly different between the two stages. The three beta-adrenoceptor agonists and the two antagonists competed for the specific [125I]iodocyanopindolol binding and the pKi values were not significantly different between the two stages. However, the maximum number of binding sites was significantly greater in late pregnancy than in dioestrus. The configuration of the competition curves and the pKi values for the two antagonists confirmed the fact that these membranes contain predominantly beta 2-adrenoceptor subtype. These results indicate that the potent inhibition of the spontaneous rhythmic contraction by HSR-81 in the pregnant uterus may be due to the increased number of beta 2-adrenoceptors.

The inhibitory effects of amrinone on isolated rat uterus

Amrinone, a new cardiotonic drug, has received attention as a better therapeutic agent than the cardiac glycosides in the treatment of congestive heart failure. In this study, the effects of amrinone on isolated rat uterus and its probable mechanism of action were investigated. At two different concentrations (0.1 and 0.5 mM), the inhibitory effects of amrinone on the spontaneous contractions of rat uterus were noted. In addition, amrinone (0.5 mM) was found to inhibit the tonic contractions induced by potassium sulphate (K2SO4)-Ringer solution (91.74%) and calcium chloride (CaCl2) (93.04%). These inhibitory effects were compared with regulators of the phosphodiesterase enzyme (PDE). It was concluded that amrinone could behave as a calcium antagonist and PDE inhibitor.

Transient electrocardiographic changes suggesting myocardial ischaemia in newborn infants following tocolysis with beta-sympathomimetics

Serial electrocardiograms (ECGs) were studied prospectively in 80 apparently healthy newborn infants; 30 infants exposed in utero to prolonged tocolytic therapy (21 to ritodrine and 9 to isoxsuprine) and 50 infants non-exposed in utero to drugs (control group) matched for gestational age, Apgar score, and birth weight. Duration of exposure to tocolysis was at least 30 days (30-180 days) with an oral dosage of 10 mg 3 times daily. ECGs were graded for changes suggestive of ischaemia using the arbitrary grading system described by Jedeikin et al. In all infants with ECG features of myocardial ischaemia, serum creatine-phosphokinase iso-enzyme (CK-MB) activity was measured. Six out of 21 infants to ritodrine and six out of nine infants exposed to isoxsuprine showed a degree of ECG ischaemia which persisted for several weeks. No control infant presented grade 2 or 3 ECG changes after the 5th day of life. The results of this study seem to show that prolonged tocolytic therapy with beta-sympathomimetics has side-effects on the fetal myocardium and suggest that this treatment be reserved only for selective cases and/or for short periods of time.

Possible mechanisms of inhibitory action of protamine on contractile activity of rat aorta

Experiments were performed to determine possible mechanisms of inhibitory action of protamine chloride on noradrenaline (10 microM)-, KCl (40 mM)-, BaCl2 (1 mM)- and CaCl2 (10 mM)-induced contractions in rat aorta. Protamine, La3+ and gallopamil (D600), inhibited the K+-induced contractions more effectively than the noradrenaline-induced responses on the basis of the concentrations giving 40% inhibition. Lanthanum (1-5 mM) reduced tissue Ca content in both normal and Ca2+-depleted Tris-buffered solutions and produced an increase in 45Ca efflux from the aortic strip into the Ca2+-depleted Tris solution. Protamine (1-5 mg ml-1) reduced tissue Ca content in normal Tris solution, but to a lesser extent than La3+ in the Ca2+-depleted solution. Furthermore, protamine (3 mg ml-1) produced no increase in 45Ca efflux from aorta. These results suggest that protamine chloride may preferentially inhibit the Ca2+ influx stimulated by K+ depolarization and that its inhibitory action on rat aorta may be due to non-specific displacement of the superficially located bound Ca2+ of the cell membrane, which can also be readily removed by treatment with Ca2+-depleted solution.

Pharmacological investigation of ritodrine hydrochloride, a beta 2-adrenoceptor stimulant

Ritodrine hydrochloride (ritodrine) has been effectively prescribed for the prevention of premature labor. The present study was carried out to investigate the mode of action of ritodrine on the uterus and heart in comparison with those of isoxsuprine and isoproterenol. 1) Ritodrine (10(-8)-10(-6) M) suppressed the spontaneous motility of pregnant rat uterus and showed positive chronotropic action at the doses of 10(-6)-10(-4) M in guinea-pig atria. 2) In the Ca2+-free, K+-rich Tyrode solution, ritodrine suppressed the Ca2+ induced contracture of pregnant rat uterus, while it potentiated the carbachol induced contraction. 3) Ritodrine increased the amount of cyclic AMP in the uterus but not in heart. This action of ritodrine was suppressed by pretreatment with propranolol (10(-6) M). 4) These results suggest that ritodrine causes actions through activation of cyclic AMP production, as in the case of isoproterenol, and it acts more selectively on beta 2-adrenoceptors than on beta 1-adrenoceptors.

Gap junction formation and regulation in myometrium

Myometrial tissues from pregnant rats were examined by electron microscopy for the presence of gap junctions after incubation in vitro with a variety of agents. Gap junctions were present in low frequency or absent prior to incubation in vitro. The junctions were present in control tissues in high frequency after 48 h incubation. The addition of cycloheximide or actinomycin D inhibited the incorporation of [3H]leucine into TCA-precipitable proteins and prevented gap junction formation. A prostacyclin analog (carbacyclin), a thromboxane synthesis inhibitor, and indomethacin also prevented gap junction formation. Oxytocin had no effect on gap junction formation but isoxsuprine decreased their number and increased their size. Isoxsuprine and isoproterenol also produced electron opaque crystals associated with the gap junctions. Dibutyryl cAMP treatment but not monobutyryl cGMP also increased the size of gap junctions. Based upon this and previous studies, we propose at least four sites for regulation of gap junctions and possible control of labor.

Does activation of cyclic AMP dependent phosphorylation induced by beta-adrenergic agent control the tone of vascular muscle?

1. The relaxing action of the beta-adrenergic agent, isoprenaline, on the porcine coronary artery was investigated in relation to the cyclic AMP level, the endogenous binding of cyclic AMP to the regulatory unit of cyclic AMP dependent protein kinase or the phosphorylation as a result of activation of protein kinase of the muscle homogenate in Krebs solution and excess [K]o solution. These relations were also compared with those of the rat cardiac muscle, in which isoprenaline showed a positive inotropic action. 2. Excess [K]o decreased the cyclic AMP level in proportion to the amplitude of K-induced contracture in the porcine coronary artery. Isoprenaline increased the cyclic AMP level in Krebs solution, while it had no effect in excess [K]o. 3. In the porcine coronary artery, the particulate fraction possessed only 5% of the total cyclic AMP dependent protein kinase, while in the rat cardiac muscle, the particulate fraction was 25% of the total protein kinase. 4. The cyclic AMP dependent protein kinase in the particulate fraction of the porcine coronary artery was already saturated with the endogenous cyclic AMP. However, the binding of cyclic AMP to the protein kinase in the particulate fraction in the cardiac muscle and in the cytosol fraction of both tissues were increased in accordance with the cyclic AMP level. In the coronary artery, the protein kinase in the cytosol fraction was bound to a greater extent with cyclic AMP than was measured in the rat cardiac muscle. 5. In the rat cardiac muscle, isoprenaline enhanced the phosphorylation, detected by autoradiography of SDS gel electrophoresis in individual fractions of phosphorylated protein, while little enhancement was observed in the porcine coronary artery. 6. These observations led to the conclusion that in the porcine coronary artery, beta-adrenergic agent increases the levels of cyclic AMP but does not increase the phosphorylation. If the phosphorylation catalysed by cyclic AMP dependent protein kinase was utilized for Ca mobilization in the cell, the change in the cyclic AMP level would probably not have a causal relation to the muscle tone. This conclusion, however, may not be applicable in the case of the cardiac muscle.

Smooth muscle physiology and effect of bladder and urethra muscle length/tension on response to stimulation. Part I. Review

With particular reference to the lower urinary tract, a review of basic anatomy and physiology of smooth muscle is presented. The relationship as altered by electrica and pharmacologic stimulation is discussed.

Differences in contractile responses to acetylcholine and serotonin in rat stomach fundus strips

The differences in contractile responses to acetylcholine (ACh) and serotonin (5-HT) were studied in the rat fundus strips. At low temperatures, the contractile response to 5-HT was reduced more noticeably than to ACh. The contraction induced by ACh at 4 degrees C was not inhibited by La3+. Removal of extracellular Ca2+ reduced the response to 5-HT more significantly than that to ACh.. La3+, verapamil and D-600 inhibited the 5-HT response more than the ACh response. In Ca2+-free solution the contractile response to ACh of a fundus strip preloaded with Ca2+ was lar ger than that to 5-HT. These results suggest that the Ca2+ movements induced by ACh and by 5-HT are different, and that the difference in Ca2+ movements may cause the difference in contractile responses to the two agonists at low temperatures.

Effects of beta-adrenergic agonists on rat uterine motility and cAMP level in vivo

Intravenous injection of isoprenaline and salbutamol into rats caused an inhibition of the spontaneous contractions of uterus and increased the uterine cAMP level in a dose-dependent manner. The effects of isoprenaline (0.036 nmol/kg) were diminished or completely prevented by propranolol (0.07-7 mumol/kg). Intravenous infusion of the beta-adrenergic agonists during one hour increased the uterine cAMP level. The highest level of cAMP measured was at 3 min. after start of infusion, thereafter it declined first rather fast but later more slowly. This time pattern of cAMP was very similar to what was seen during incubation of uterine tissue with isoprenaline in vitro. Pretreatment with isoprenaline three times daily 4-7 days before the experiments, decreased both the degree of inhibition and the increase of cAMP in uterus caused by an intravenous injection of isoprenaline. A slight effect on the cAMP rise was detectable after one day of pretreatment, and it seemed to have reached its maximal effect after 4 days, since no further decline in the cAMP response was seen when the pretreatment was prolonged. A dessensibilization to beta-adrenergic agonists was thus seen in uterus after excessive stimulation of isoprenaline in vivo.

Role of Ca in isoproterenol-induced increases in cAMP levels in rat uterus

The influence of alteration in the Ca2+ environment of the tissue on isoproterenol-induced increases in cAMP levels and relaxation was studied in rat uterus. In muscles depolarized with 47.5 mM K+ (with or without Na+), the ability of isoproterenol to increase cAMP levels and to produce relaxation was found to be inversely related to external calcium concentration. The pretreatment of the muscle with D600 or EGTA restored the cAMP response to isoproterenol in the depolarized uterus to a level observed in nondepolarized muscle. The study with Ro 20-1724, a phosphodiesterase (PDE) inhibitor indicated that the failure of isoproterenol to elevate cAMP levels in the depolarized uterus could not be related to the activation of PDE by Ca2+. The exposure of rat uterus to a zero-Ca2+ solution accentuated the increases in cAMP levels produced by isoproterenol. These results have raised the question of a possible regulatory role of Ca2+ in beta-adrenoceptor-induced increases in cAMP levels in uterine smooth muscle.

Mechanism of beta-adrenergic relaxation of smooth muscle

The mechanism of beta-adrenergic relaxation was investigated in isolated smooth muscle cells. Beta-adrenergic agents stimulate cyclic AMP-dependent phosphorylation, enhance Na+/K+ transport and induce relaxation. The stimulation of Na+/K+ transport is obligatory for relaxation, and we suggest that this stimulation induces relaxation through enhanced Na+/Ca2+ exchange.

Effects of isoprenaline on the time course of the cyclic AMP level in rat uterus

Isoprenaline markedly relaxed rat uterine muscle and increased its content of cyclic AMP. The cyclic AMP concentration reached its highest value, about ten times the control after 3 min., after which it declined rather quickly at first but later more slowly. After 60 min. the cyclic AMP content reached twice the prestimulation value. The muscle remained relaxed all the time. If the incubation medium from the uterus treated with isoprenaline for 60 min. was added to a fresh muscle, a normal response was elicited. Addition of a new dose of isoprenaline to a muscle previously incubated with isoprenaline for 60 min. only gave a minimal rise in cyclic AMP after 3 min. The presence of catechol or corticosterone, which both decreased the inactivation of isoprenaline, showed no effect on the decrease of the isoprenaline-stimulated cyclic AMP level. Nor could the decrease in the cyclic AMP response be prevented in the presence of GTP,F-, adenosine or in Ca2+-deficient medium. The addition of theophylline and puromycin both lead to a slower decline of cyclic AMP content, but after 60 min. the value returned to near that seen ater only isoprenaline. Our studies on isoprenaline-induced desensitization of rat uterus have led us to the conclusion that the change in the capacity of the preparations to accumulate cyclic AMP may be a result of either changes in the beta-adrenoceptors or in the cyclase-system.

The effect of intravenous salbutamol upon plasma and urinary potassium during premature labour

Plasma potassium and urinary potassium excretion were measured in eight patients in premature labour treated with salbutamol, and in ten patients at full term in spontaneous labour. There was a significant fall in plasma potassium concentration following salbutamol infusion, but no change during labour in the control groups. Urinary potassium excretion was similar in both groups.

Agonist induced release of intracellular Ca2+ in the rabbit aorta

The effects of hormonal agonists (norepinephrine, angiotensin, and histamine) on 45Ca efflux from the rabbit aorta were studied using a Ca-EGTA buffered efflux medium. Each caused a transient stimulation of efflux rate which probably reflected the release of an intracellular 45Ca store. The size of the stimulation of efflux correlated with the size of the initial rapid phase of contraction. The norepinephrine-sensitive intracellular Ca fraction was estimated to be greater than 21 mumoles/Kg wet tissue weight. This fraction is separate from intracellular Ca which is accumulated during relaxation. Evidence is presented for the lack of cyclic nucleotide involvement in the release of Ca2+, and possible alternative modes of coupling are discussed.

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.

Effect of calcium removal on monoamine-elicited depressions of cultured tuberal neurons

The role of extracellular calcium in monoamine responses of central neurons was ivestigated using explant cultures of tuberal hypothalamus. The spontaneous activity of neurons in cultures was recorded in balanced salt and calcium-deficient salt solutions. This increased firing rate was counteracted by the addition of magnesium. Addition of magnesium also regularized the pattern of firing. Iontophoretic application of putative monoamine neurotransmitters reversibly decreased the rate of firing in both normal and calcium-deficient salt solutions. These results suggests that monoamine inhibitions are not primarily mediated by transmembrane calcium fluxes.

Differentiation of intestinal smooth muscle relaxation caused by drugs that inhibit phosphodiesterase

In this study a number of chemically unrelated smooth muscle relaxants were tested: a) for potency of phosphodiesterase (PDE)-inhibition, using guinea pig colon-PDE and rat erythrocyte-PDE, b) for potency and duration of relaxation of the isolated guinea-pig colon, c) for their interaction with NH4Cl and orciprenaline as well as with "high calcium". Compared with the spasmolytic effect inhibition of guinea pig colon-PDE or rat erythrocyte-PDE was strong for papaverine and some other relaxants but was low or virtually absent with verapamil, hexobendine and bencyclane. The spasmolytic effect of some PDE-inhibitors was found diminished by the PDE-activator NH4Cl. Most of these drugs enhanced the relaxant effect of the "cyclase activator" orciprenaline; the latter are designated A-type drugs. Verapamil, bencyclane, hexobendine and M 13 did not show this type of interaction with NH4Cl and orciprenaline; they are designated B-type drugs. With A-type drugs--but not with B-type drugs--a highly significant correlation was observed between potency of relaxation and inhibition of colon-PDE (r = 0.85) as well as rat erythrocyte-PDE (r = 0.77). The spasmolytic action of aminophylline and papaverine (A-type drugs) was not inhibited by elevation of extracellular calcium to 9 mM, whereas relaxation induced by verapamil and hexobendine (B-type drugs) at 1.8 mM calcium was found abolished by 9 mM calcium. It is concluded that A-type drugs relax guinea-pig colon by inhibition of PDE and accumulation of cAMP, and that B-type drugs in all probability act by (a) cAMP-independent mechanism(s).

Action of prostaglandin, PGF2alpha, on the uterus of the pregnant rat

The effects of prostaglandin F2alpha (PG) have been studied on the transmembrane potentials and contractions in isolated myometrial strips from pregnant rats. The results showed that: 1. The sensitivity of the myometrium to exogenous PG increases from day 19 to day 22 of gestation. 2. The electrical response to PG, at maximally effective doses (10-6 to 10-5 M) consists of a slow depolarization which upon reaching threshold initiates spike discharge. 3. These actions are most pronounced at term (day 22) and are due to a direction of PG on the myometrial cells. 4. D-600 (a methoxy derivative of verapamil) abolishes spike discharge and the phasic contractions induced by PG but has no effect on the slow depolarization and the accompanying increase in tonic tension. 5. The slow depolarization is dependent upon the presence of sodium in the external environment and is unaffected by the removal of calcium. 6. The spikes (and phasic contractions) are dependent upon the presence of calcium in the external environment.

A quantitative microiontophoretic analysis of the responses of central neurones to noradrenaline: interactions with cobalt, manganese, verapamil and dichloroisoprenaline

1. A new experimental procedure has been devised for the study of pharmacological antagonism in the central nervous sytem using automated microiontophoresis to deliver pulses of agonists and computer-generated histograms to quantify the neuronal response. The system allows study of potential antagonists having direct depressant effects and also of neurones with irregular or slow discharge rates. 2. The histogram analysis reveals the necessity for regular, periodic delivery of agonists during the assessment of agonist-antagonist interactions. Without regular repetitive delivery, many agonists, such as noradrenaline, exhibit an apparent but artifactual decrease in inhibitory potency after an interruption of agonist pulses. Examples of this phenomenon are shown, using cerebellar Punkinje cells and cerebral cortical neurones in rats anaesthetized with halothane. 3. Preliminary results with these computer-generated drug response histograms revealed manganese, cobalt, and verapamil to be generally ineffective as antagonists of noradrenaline, despite their direct depressant effects. 4. Conversely, dichloroisoprenaline (DCI), a beta-adrenoceptor antagonist, was effective in blocking noradrenaline-induced depressions of firing in the cerebral cortex at doses which caused over 50% decrease in spontaneous discharge.

Action of D-600 on spontaneous and electrically stimulated activity of the parturient rat uterus

The effects of D-600, the methoxy derivative of verapamil, on the parturient rat uterus have been studied. At concentrations of 10(-8) to 10(-7) M, D-600 abolishes spontaneous contractions and reduces the force of electrically evoked contractions. The first action is associated with a reduction in amplitude of the slow waves (oscillations of membrane potential which underlie rhythmic bursts of spikes initiating contractions). The second is associated with a reduction in frequency of spikes within the bursts accompanying the electrically stimulated contractions. This reduction results from a decrease in the slope of the prepotential in cells in pacemaker areas of the muscle. D-600 also decreased the height and rising velocity of the conducted action potentials, but these actions cannot account entirely for the reduction in contractile force.