Mechanisms of beta-adrenergic desensitization in rat myometrium

In vitro relaxant and spasmolytic effects of constituents from Viburnum prunifolium and HPLC quantification of the bioactive isolated iridoids

ETHNOPHARMACOLOGICAL RELEVANCE

Viburnum prunifolium is a North America shrub used in ethnomedicine because of its spasmolytic, sedative, and anti-asthmatic properties.

AIM OF THE STUDY

Contrasting results were reported in past literature about the active principles of this plant. Our aim was to clarify this matter by evaluating the relaxant and spasmolytic activities of the main constituents obtained from the drug.

MATERIALS AND METHODS

The pharmacological assays were carried out on rabbit jejunum spontaneous contractions and on guinea-pig carbachol-precontracted trachea.

RESULTS

Cumulative concentration (1-100 microg/ml) of Viburnum prunifolium methanolic extract (MeOH extract), its purified fractions soluble in ethylacetate (EtOAc fraction) and in n-butanol (BuOH fraction), and the iridoid glucosides (2 x 10(-5) to 4 x 10(-4)M): 2'-O-acetyldihydropenstemide (1), 2'-O-trans-p-coumaroyldihydropenstemide (2), 2'-O-acetylpatrinoside (3), and patrinoside (4), isolated from EtOAc fraction (1 and 2) and BuOH fraction (3 and 4), induced both relaxant effect of rabbit jejunum spontaneous contractions and spasmolytic effect on guinea-pig carbachol (5.5 x 10(-7)M)-precontracted trachea. Propranolol (10(-6)M) antagonised all Viburnum prunifolium tested components relaxant and spasmolytic effects. At non-relaxing concentrations (0.5 microg/ml), MeOH extract and its fractions induced a potentiating effect of isoprenaline cumulative concentrations also in both isolated tissues.

CONCLUSION

In both tissues, the order of potency was EtOAc fraction>BuOH fraction>MeOH extract and 1>2>3>4 suggesting that the major iridoids of EtOAc fraction may be considered among the most active compounds. HPLC analysis of the bioactive iridoids indicates that 1 and 2 are present for 7.38% and 14.90% in EtOAc fraction, and 3 and 4 for 18.47% and 8.86% in BuOH fraction. By comparing the values of EC(50) of the fractions and compounds isolated from them, we may assume that the iridoids play a significant role in the biological activity of the corresponding fractions.

Lymphocyte beta 2-adrenoceptors and adenosine 3':5'-cyclic monophosphate during and after normal pregnancy

1. The beta 2-sympathomimetics, used to inhibit preterm labour, bind predominantly to beta 2-adrenoceptors, activating adenylate cyclase to form adenosine 3':5'-cyclic monophosphate (cyclic AMP), a messenger substance which inhibits the enzyme cascade triggering smooth muscle contraction. beta 2-Adrenoceptor density and cyclic AMP formation can be used as markers of beta 2-adrenergic effect. 2. The present study addresses the influence of pregnancy on the beta-adrenoceptor system. beta 2-Adrenoceptor density and cyclic AMP concentrations (basal and evoked by isoprenaline) in circulating lymphocytes were determined at three points in gestation (16, 29 and 37 weeks) and 9 weeks post partum in 22 normal pregnancies. (-)-[125Iodo]-cyanopindolol was used as the ligand to identify a homogeneous population of beta 2-adrenoceptors on lymphocytes. B- and T-cell fractions were estimated from the same samples. 3. beta 2-Adrenoceptor density decreased significantly during gestation until week 37 (P < 0.01), then increased post partum (P < 0.005). Cyclic AMP concentrations (basal and evoked by isoprenaline) were significantly lower after 16 weeks of gestation than post partum (P < 0.05). 4. The results, which cannot be explained in terms of a shift in the lymphocyte (B- and T-cell) ratio, indicate that beta-adrenoceptor density and function are reduced in normal pregnancy and only return to normal post partum. These findings may be of significance in devising future tocolytic therapy with beta 2-adrenoceptor agonists.

One way cross tolerance between cromakalim and salbutamol in the uterus of the rat in vivo

1. Cross tolerance between the potassium (K+) channel opener, cromakalim and the beta 2-adrenoceptor agonist, salbutamol, was investigated in the uterus of the non-pregnant rat in vivo. Uterine sensitivity to salbutamol was similar in both vehicle-treated and cromakalim-tolerant rats. In salbutamol-tolerant rats, uterine responses to cromakalim were markedly decreased compared with saline-infused rats, such that maximum inhibition of uterine contractions was less than 40%. 2. Propranolol treatment and salbutamol tolerance each produced similar reductions in sensitivity of the uterus to salbutamol of approximately 10 fold. The same dose of propranolol did not influence uterine sensitivity to cromakalim, which suggests that the relaxant action of cromakalim is not due to a direct or indirect activation of uterine beta 2-adrenoceptors. 3. Salbutamol produced a marked (11.7 fold) increase in uterine adenosine 3':5'-cyclic monophosphate (cyclic AMP) concentrations measured ex vivo, which was completely inhibited by propranolol pretreatment, but was unaffected by glibenclamide pretreatment. Cromakalim did not increase uterine cyclic AMP concentrations, suggesting that stimulation of adenylate cyclase is not significant in the uterine relaxant action of cromakalim. 4. The lack of propranolol antagonism of cromakalim and of cromakalim-induced changes in uterine cyclic AMP concentrations suggests that the cross tolerance observed between salbutamol and cromakalim may be at the level of K(+)-channels.

cAMP-dependent phosphorylation of soluble and crude microtubule fractions of rat cerebral cortex after prolonged desmethylimipramine treatment

We have analyzed the cAMP-dependent phosphorylation system in the cerebral cortex and hippocampus of rats after acute and chronic administration of desmethylimipramine. Prolonged desmethylimipramine administration modified the cAMP-dependent endogenous phosphorylation of a protein band with apparent molecular weight 280 kDa from the cerebrocortical-soluble fraction. The effect appeared to be specific and associated with the chronic but not the acute administration of desmethylimipramine since we did not obtain any modification in other endogenous cAMP phosphoproteins of either the particulate or soluble fraction of the cerebral cortex. 280 kDa was identified as the soluble microtubule associated protein 2 on the basis of molecular weight, endogenous phosphorylation and immunological recognition. Prolonged desmethylimipramine administration did not induce any modification in the soluble cAMP-dependent endogenous phosphorylation of 280 kDa in other brain areas such as hippocampus, striatum or cerebellum, suggesting a region-specific effect of chronic desmethylimipramine treatment. Microtubule-associated protein 2 is a neuronal protein highly enriched in the dendritic portion of neurons and represents one of the major substrates in the cell for the type II cAMP protein kinase. Since the type II cAMP protein kinase that catalyzes the phosphorylation of microtubule-associated protein 2 copurifies with microtubules, we performed endogenous phosphorylation using increasing concentrations of cAMP in a crude microtubule preparation where microtubule-associated protein 2 appeared to be more concentrated. Under our conditions the maximal effect occurred at 1 microM cAMP, revealing increased 32P incorporation in microtubule-associated protein 2 from a crude microtubule preparation obtained from the cerebral cortex of rats treated with desmethylimipramine. Photoaffinity labelling with 8-azido-[32P]cAMP of the various fractions obtained during the preparation of crude microtubules (S1, S2 and crude microtubules) revealed an increase in the labelling of a protein band with apparent molecular weight of 52 kDa after desmethylimipramine treatment. The labelling of a 47 kDa protein band, which is also present in S1 and S2 fractions was, however, not altered by drug treatment. In conclusion, our studies demonstrated that prolonged desmethylimipramine treatment elicited specific changes in the phosphorylation system associated with a crude microtubule fraction.

Pharmacological characterization of mare uterus motility with special reference to calcium antagonists and beta-2-adrenergic stimulants

1. Uterine motility was studied in vitro in the myometrial tissue obtained from pregnant and non-pregnant mares. 2. The spontaneous contractions of the preparations were not modified by tetrodotoxin, by anticholinergics, antiadrenergics, histamine H1 and H2 blockers, antiserotoninergic and opioid antagonists; but disappeared in Ca2+ and Na+ free medium. 3. beta 2-adrenergic stimulants like salbutamol and hexoprenaline and the calcium channel blockers nifedipine and verapamil were effective inhibitors of the amplitude of phasic contractions (ID50S for salbutamol and nifedipine were 7.7 nM and 14.6 nM, respectively in oestrus preparations). 4. The above data indicated that the mare myometrium contractility in vitro is very sensitive to the action of beta 2 mimetic compounds and calcium antagonists; nifedipine, in particular, seems to be a very promising alternative to beta 2 stimulants in the tocolytic therapy.

Time course of resensitization of rat uterine beta-adrenoceptor system

The time course of resensitization of the down-regulated beta-adrenoceptor system of rat myometrium was studied. The isoprenaline-induced decrease of myometrial cAMP reversed rather rapidly and after 4 and 8 days no significant change of the cAMP level was demonstrated, however, the cAMP level was higher than in the control myometrium after 12 days. The elevated phosphodiesterase activity decreased to 80% of control activity after 4 days, but returned to control activity after 8 days. The decreased number of beta-adrenoceptors increased gradually after discontinuing the treatment and reached the number of the controls after 12 days. Our results suggest that the resensitization of the beta-adrenoceptor-cAMP-system is rather complex, the normalization of the cAMP content and the phosphodiesterase activity is more rapid than the normalization of the beta-adrenoceptor number which is a rather slow process in myometrial tissue.

The effects of long-term infusion of salbutamol, diltiazem and nifedipine on uterine contractions in the ovariectomized, post-partum rat

The sensitivity of the uterus to the inhibition of contractions by salbutamol, diltiazem or nifedipine was assessed in the ovariectomized, post-partum rat by dose-response curves following bolus intravenous (i.v.) administration. These tests were performed before (day 1), immediately after a 20 h i.v. infusion of salbutamol, diltiazem, nifedipine or appropriate control infusate (day 2) and after a further 20 h infusion of saline (day 3). In a further group of animals sensitivity to nifedipine was assessed before and after a 20 h infusion of salbutamol. Uterine contractions were monitored throughout infusions. Infusion of salbutamol (2 micrograms kg-1 min-1) produced an initial marked inhibition of uterine contractions, an effect which was not maintained despite continued infusion. Contractions reappeared after 2 h of infusion and reached pre-infusion levels by 5 h. The dose-response curve to salbutamol on day 2 was shifted more than 100 fold to the right compared with that on day 1. Sensitivity of the uterus on day 3 did not differ from that on day 1. Nifedipine (25 micrograms kg-1 min-1) produced sustained inhibition of uterine contractions throughout the 20 h of infusion. Sensitivity of the uterus to nifedipine could not, therefore, be tested on day 2; sensitivity on day 3 did not differ from that on day 1. In addition, there was no change in sensitivity of the uterus to nifedipine after a 20 h infusion of salbutamol. 4 Diltiazem (200 Ig kg-' min-') produced a marked initial inhibition of uterine contractions, with a partial return of contractions during continued infusion in 7 out of 12 animals so that mean integral values reached 40% of those pre-infusion. The dose-response curve to diltiazem on day 2 showed a 25 fold shift to the right compared with that on day 1 in 4 out of 12 animals where the test could be performed. Sensitivity of the uterus on day 3 did not differ from that on day 1. 5 These findings suggest that marked but reversible tolerance to the inhibitory actions of salbutamol on uterine contractions occurs during long-term infusion. There was no evidence of tolerance to the uterine actions of nifedipine, but there was evidence of tolerance to diltiazem in some animals.

Tolerance to or facilitation of pharmacological effects induced by chronic treatment with the beta-adrenergic stimulant clenbuterol

The effect in mice of chronic administration of the beta-adrenergic agonist clenbuterol was studied using four pharmacological tests, two of which are predictive of antidepressant activity in man. The suppressive effect of acute administration of the beta-adrenergic agonist in two tests unrelated to antidepressant activity, locomotor activity and investigation of food, was attenuated after chronic treatment. However, in the two tests which are predictive of antidepressant activity, antagonism of reserpine induced hypothermia and potentiation of yohimbine toxicity, the effect of clenbuterol was greater following chronic treatment.

Beta-adrenergic receptors in human myometrium during pregnancy: changes in the number of receptors after beta-mimetic treatment

The concentration of available beta-adrenoceptors has been studied in the myometrium of women treated with terbutaline for premature uterine contractions and in an untreated control group. Myometrial strips were taken at cesarean section from the lower uterine segment and the uterine fundus. The concentration of beta-adrenoceptors was determined with a radioligand binding assay. In untreated women we found no difference in the concentration of beta-adrenoceptors in the uterine fundus compared to that in the lower uterine segment. The cyclic adenosine monophosphate production after beta-adrenoceptor agonist stimulation in vitro was equal in both locations. In the terbutaline-treated women, the binding site concentrations in both the fundus and lower uterine segment were lower compared to those in the control group. The decrease was most pronounced in the fundus where receptor concentration was only half that found in the control group. The results suggest that treatment with beta-mimetics causes a desensitization in the beta-adrenoceptor system of human myometrium during pregnancy. This desensitization may partly explain the limited duration of the relaxant effect of beta-mimetics often seen in the treatment of preterm labor.

Prolonged inhibition of human myometrial contractility by intermittent isoproterenol

Regular, spontaneous contractions of human myometrial strips obtained at the time of elective cesarean section were recorded in a tissue bath in five experiments. Administration of the beta-adrenergic agonist isoproterenol (1 mumol/L) resulted in the rapid onset of myometrial relaxation. In the continuous presence of the beta-agonist, myometrial contractions of amplitude equal to or greater than that of baseline resumed after 10 to 40 minutes (mean +/- SEM = 29.7 +/- 7.3), indicating the development of desensitization of the tissue to the tocolytic effects of isoproterenol. In contrast, intermittent exposure of the myometrial strips to isoproterenol prevented the onset of desensitization and resulted in prolonged inhibition of myometrial contractions. These findings suggest that modification of the present regimen of administration of beta-adrenergic agonists from continuous to intermittent infusion may improve the success of these agents in the prevention of preterm births.

Disappearance of the decrease in biting behavior induced by clenbuterol, a beta-adrenergic agonist, after chronic administration

The beta-adrenergic agonist clenbuterol decreased interest in food in starved mice, 30 minutes after administration. This effect disappeared after repeated treatment with clenbuterol (0.25 mg/kg, twice daily). Three chronic injections were sufficient to prevent the effect of an acute dose of clenbuterol (0.125 mg/kg) up to 45 hours after treatment.

In vitro study of phosphodiesterase-inhibiting drugs: a complement to beta-sympathomimetic drug therapy in premature labor?

Since terbutaline treatment in premature labor has been shown to increase phosphodiesterase activity in the myometrium, the phosphodiesterase-inhibiting effect of theophylline and papaverine on myometrial preparations from pregnant women has been studied. The patients were either treated with terbutaline during the last 3 weeks of pregnancy or not treated. The results showed that papaverine is about 100 times more potent than theophylline in both phosphodiesterase-inhibiting effect and muscle-relaxing effect. No differences in effects were found in myometrial strips from terbutaline-treated or untreated women. A possible improvement in the tocolytic therapy with a combination of a beta-sympathomimetic and a phosphodiesterase-inhibiting drug is suggested.

Relaxant beta-receptors in the trachea, but not prejunctional alpha 2-receptors in the tracheal cholinergic neurones, are subjected to homologous desensitization

Guinea-pigs were pretreated with either isoprenaline, terbutaline or the alpha 2 agonist B-HT 920 in order to asses the hypothesis that beta-and alpha 2 receptors in trachea are subjected to homologous desensitization. In these experiments the beta receptor activity was investigated on tracheal ring preparations contracted with carbacholine. Both in the isoprenaline and the terbutaline pretreated group the relaxant responses to beta agonists were diminished. Pretreatment with B-HT 920 did not affect the sensitivity of the trachea to beta stimulation. In order to asses the responsiveness of alpha 2 receptors the trachea was contracted by electrical field stimulation in the presence of propranolol. During these conditions contractions were mediated by activation of cholinergic neurones and inhibitory effects of alpha stimulation were due to inhibition of the cholinergic neurotransmission by stimulation of prejunctional alpha 2 receptors. In these tests neither isoprenaline, terbutaline nor B-HT 920 pretreatment affected the responsiveness of alpha stimulation to inhibit the electrically induced contractions.