An in-vivo model to examine the electromyographic activity of isolated myometrial tissue from pregnant sheep

Strips (2.5 x 3.5 cm) of myometrium alone (MYO) or endometrium/myometrium (ENDO/MYO) were removed from the pregnant horn of sheep (Day 110 of gestation) and transplanted to sites within the omental fat. These explants developed regular bursts of electromyographic (EMG) activity over a period of 7-10 days, as well as a dose-dependent stimulatory response to oxytocin (50-200 mU i.v.). The frequency (per 2 h) of EMG bursts in the MYO (5.3 +/- 0.2) and ENDO/MYO (5.2 +/- 0.3) explants was significantly greater (P less than 0.05) than that of the uterine myometrium (3.0 +/- 0.1), while burst duration (min) in MYO (4.1 +/- 0.2) and ENDO/MYO (4.1 +/- 0.2) explants was significantly (P less than 0.05) less than in the uterine myometrium (7.3 +/- 0.1). The EMG bursts were asynchronous between the explants and uterus, although systemic administration of oxytocin produced a synchronous burst of EMG activity in all three tissues. No differences in EMG activity or responsiveness were apparent between MYO and ENDO/MYO explants. Histological examination of the explant tissue revealed the presence of smooth muscle fibres regularly orientated into two layers; some loss of endometrial tissue was apparent in ENDO/MYO explants. To validate the mechanical integrity of this model we examined the in-vitro contractile activity of myometrial strips prepared from the explants. The strips developed regular spontaneous contractions and demonstrated a dose-dependent stimulation in response to the addition of oxytocin (10(-10) to 10(-4) M) to the bath fluid. These results suggest that spontaneous contractures during pregnancy are probably not due to pulsatile release of stimulants into the systemic circulation, or the direct diffusion of stimulants from intrauterine tissues to the myometrium but are probably caused by factors within the myometrium itself.     

Anti-inflammatory and utero-relaxant effects in human myometrium of new generation phosphodiesterase 4 inhibitors

The anti-inflammatory and utero-relaxant effects of two potent phosphodiesterase 4 (PDE4) inhibitors of the latest generation: cilomilast (one of the most advanced PDE4 inhibitors in clinical development, reportedly more selective for PDE4D) and compound A (which displays 12-fold greater selectivity toward PDE4B and/or PDE4A than toward PDE4D) were evaluated in human uterine smooth muscle. We first established that these compounds exhibit greater efficacy in inhibiting total cAMP-PDE activity in pregnant versus nonpregnant myometrium (E(max) = 78.0% +/- 3.6% and 80.3% +/- 2.2% in pregnant versus 57% +/- 4.7% and 70.5% +/- 5.9% in nonpregnant women for compound A and cilomilast, respectively; P < 0.05 for both compounds), confirming the prominent participation of PDE4 isoforms in cAMP hydrolysis in the near-term pregnant myometrium. Using pregnant myometrial explants, we have shown that both these drugs and also rolipram, the prototype PDE4 inhibitor, produce concentration-dependent inhibition of lipopolysaccharide (LPS) induced tumor necrosis factor alpha (TNFalpha) release with similar potency in each case (pD2 = 8.0 +/- 0.5, 7.9 +/- 0.2, and 7.6 +/- 0.2 for compound A, cilomilast, and rolipram, respectively). The maximum inhibition produced is 65%. Pretreatment with forskolin or 8-bromo-cAMP mimics the PDE4 inhibitor effect. Furthermore, compound A and cilomilast both produce concentration-dependent inhibition of the spontaneous contractions of myometrial strips and are more potent in pregnant than in nonpregnant myometrium (pD2 = 7.3 +/- 0.7 and 8.1 +/- 0.3 in pregnant versus 6.2 +/- 0.9 and 6.6 +/- 0.1 in nonpregnant myometrium for compound A and cilomilast, respectively; P < 0.05 for both compounds). This demonstrates that the PDE4 isoforms involved in the mechanism of contraction are different in the pregnant and nonpregnant myometrium. Our study highlights the importance of developing PDE4 inhibitors for the pharmacological management of infection-induced preterm labor.     

The influence of methylene blue on the spontaneous contractity of the non-pregnant human myometrium and on the myometrial response to DEA/NO

Nitric oxide (NO) is a potent inhibitor of spontaneous contractions of the human non-pregnant myometrium; however, the precise mechanism by which NO causes the myometrial smooth muscles to relax remains unclear. The aim of this study was to determine the influence of methylene blue (MB) on myometrial contractions and the response of the myometrium to DEA/NO in vitro. Concentration-response curves to DEA/NO were constructed in the absence and presence of MB (5x10(-6), 10(-4) and 10(-2) mol/l) and 5x10(-3) mol/l cystamine. Cystamine did not counteract the DEA/NO-induced relaxation of the myometrial strips. MB itself, excluding the lowest concentration, caused noticeable changes in spontaneous activity. The changes involved a concentration-dependent increase in the frequency of contractions, and a decrease in their amplitude. In conclusion, our results confirm that NO relaxes the human myometrium via a cGMP-independent mechanism. The results obtained in the presence of MB may be misleading because of its complex influence on myometrial contractile activity.     

In vitro effects of polychlorinated biphenyls (PCBs) on the contractility of bovine myometrium from the periovulatory stage of the estrous cycle

Strips of longitudinal myometrium from cows were obtained on days 19-21 and 1-5 of the estrous cycle and incubated (aerated atmosphere; 4 degrees C; 24, 48 or 72 h) with a mixture of PCBs Aroclor (Ar) 1248 or with one of three PCBs (77, 126 or 153), all at doses of 10 or 100 ng/ml. The force and frequency of spontaneous and oxytocin (OT; 10(-7)M)-stimulated contractions of each strip was registered by means of HSE Schuler Organbath. Contractions of myometrial strips in the presence and absence of PCBs were observed after 24, 48 and 72 h of incubation. All PCBs significantly affected myometrial contractions. A mixture of PCBs increased the spontaneous force of contractions after 24 h but decreased after 48 h. Individual congeners of PCB also amplified the force of contractions and in most cases this effect was dose-dependent. Response of myometrium to PCB-126 and PCB-153 or PCB-77 appeared after 24 h or 48 h of incubation. Incubation of myometrial strips with PCB congeners markedly amplified OT-stimulated contractions. This effect was less evident when tissue was pre-treated with a higher dose of PCBs. Pre-treatment with estrogen-like PCB-153 increased the spontaneous and OT-evoked frequency of myometrial contractions from days 19-21. The spontaneous force of myometrial strips' contractions as well as the effects evoked by PCBs and OT was higher before than after ovulation. In summary, PCBs affected both the force and frequency of uterine contractions. Thus, it can be concluded that PCBs may impair both ovum fertilization and blastocyst implantation in cows.     

Aging does not affect voluntary activation of the ankle dorsiflexors during isometric, concentric, and eccentric contractions.

This study examines the age-related deficit in force of the ankle dorsiflexors during isometric (Iso), concentric (Con), and eccentric (Ecc) contractions. More specifically, the contribution of neural and muscular mechanisms to the loss of voluntary force was investigated in men and women. The torque produced by the dorsiflexors and the surface electromyogram (EMG) from the tibialis anterior and the soleus were recorded during maximal Iso contractions and during Con and Ecc contractions performed at constant angular velocities (5-100 degrees/s). Central activation was tested by the superimposed electrical stimulation method during maximal voluntary contraction and by computing the ratio between voluntary average EMG and compound muscle action potential (M wave) induced by electrical stimulation (average EMG/M wave). Contractile properties of the dorsiflexor muscles were investigated by recording the mechanical responses to single and paired maximal stimuli. The results showed that the age-related deficit in force (collapsed across genders and velocities) was greater for Iso (20.5%; P < 0.05) and Con (38.6%; P < 0.001) contractions compared with Ecc contractions (6.5%; P > 0.05). When the torque produced during Con and Ecc contractions was expressed relative to the maximal Iso torque, it was significantly reduced in Con contractions and increased in Ecc contractions with aging, with the latter effect being more pronounced for women. In both genders, voluntary activation was not significantly impaired in elderly adults and did not differ from young subjects. Similarly, coactivation was not changed with aging. In contrast, the mechanical responses to single and paired stimuli showed a general slowing of the muscle contractile kinetics with a slightly greater effect in women. It is concluded that the force deficit during Con and Iso contractions of the ankle dorsiflexors in advanced age cannot be explained by impaired voluntary activation or changes in coactivation. Instead, this age-related adaptation and the mechanisms that preserve force in Ecc contractions appeared to be located at the muscular level.     

Unusual venoconstrictor effects of angiotensin II.

Cumulative dose-response curves to angiotensin II were performed on helical strips from canine lateral saphenous vein. Threshold concentrations were in the range of 10(-18)-10(-17)M. Increases in angiotensin from 10(-17)-10(-12)M failed to elicit further increases in tension. Subsequent increases in angiotensin concentration from 10(-11)-10(-7)M again produced dose-related increases in tension. Repeated dose-response curves in the same strips showed reduced maximal response. Responses to low concentrations of angiotensin were attenuated by low concentrations of phentolamine. These results suggest that, at extremely low concentrations angiotensin produced marked contractions in canine saphenous vein strips by releasing endogenous norepinephrine from sympathetic nerve terminals.     

Comparative aspects of adrenergic receptors in the hearts of lower vertebrates

The cardiac adrenoceptors of lower vertebrates were characterized in atrial preparations. Adrenaline (A) potentiated the force and frequency of contraction in the spontaneously beating atria of the frog, trout and flounder and in electrically paced atrial strips from the shark. The inotropic responses of A were most pronounced at the lower temperatures for the frog and trout, while A enhanced frequency to a greater extent at higher temperatures in the frog atria. Atrial alpha-receptors activated by A at 8 degrees C could not be detected in any of the species under study. The apparent affinities for the inotropic and chronotropic responses of agonist in the frog (15 degrees C) and trout (8 degrees C) atria were: Iso greater than Sal greater than or equal to A greater than NA. A cocaine-sensitive uptake for A and NA was apparent in these atria, consistent with sympathetic innervation. The affinities for the catecholamines in the flounder and shark atria were not increased by cocaine, in accordance with absence of sympathetic innervation of the atria in these species. These atria were also insensitive to corticosterone. The affinities for A and NA were on the other hand higher in the sympathetically non-innervated atria of the flounder than in the innervated atria of the frog and trout. The apparent orders of relative affinities for agonists were Iso greater than A = NA greater than Sal for the flounder, and of the relative potencies Iso = A greater than NA greater than Sal for the shark atrium. The results are consistent with the hypothesis that catecholamines enhance cardiac performance in lower vertebrates chiefly via "adrenaline" receptors which resemble the beta 2-type of mammalian adrenoceptors in many respects. Unlike that in mammals, cardiac adrenaline receptors in the frog and trout are activated by the sympathetic neurotransmitter ("innervated" receptors). On the other hand, the adrenaline receptors of the flounder and shark are responding to the circulating catecholamines ("humoral" receptors). However, the flounder atrium, with equal affinities for A and NA, appears as an exception to the rule by having a mixed population of humoral beta 1- and beta 2-adrenoceptors, indicating a role for circulating NA in cardiac regulation in this species.     

The actions of prostaglandins and cyclo-oxygenase inhibition on the resistance vessels supplying the human fetal placenta

The resistance arteries supplying individual exchange villi of the full-term human fetal placenta were examined for their reactivity to various prostaglandins (PG's) as well as for their ability to synthesize biologically active PG's. PGA1, PGF2 alpha, PGE2 and PGE1 produced dose-dependent contractions between 10(-7) and 10(-5)M. The order of potency observed was PGA1 approximately PGF2 alpha greater than PGE2 greater than PGE1. TXB2 was without activity in this preparation. Prostacyclin (PGI2) produced a dose-dependent relaxation of pre-contracted strips between 10(-8)M and 10(-5)M. Arachidonic acid (A.A.) produced stable dose-dependent contractions (10(-5) M to 10(-3)M) which were totally abolished by pretreatment with 10(-7)M meclofenamate (MF). At no concentration of A.A. was any evidence of vascular relaxation observed. Larger concentrations of MF (greater than 10(-6)M) resulted in a non-specific depression of the placental vascular smooth muscle. Meclofenamate (10(-7)M) pretreatment of strips subjected to dose-response studies using PGF2 alpha, PGE2, bradykinin (B K) and angiotensin II (AII) revealed a significant reduction in tension developed to both BK and AII. This finding suggests that the vasoactive peptides BK and AII stimulate the synthesis of vasoconstricting PG's in the fetal placental resistance arteries which relax in response to PGI2 and contract in response to the other PG's tested.     

Cerebral intraventricular 6-hydroxydopamine prevents vascular changes in the mineralocorticoid hypertensive rat

The effect of cerebral intraventricular administration of 6-hydroxydopamine (6-OHDA) on blood pressure and vascular smooth muscle responsiveness in deoxycorticosterone acetate (DOCA)-treated rats was assessed. Rats treated with 6-OHDA and DOCA had significantly lower systolic blood pressures (142 +/- 8 mm Hg) than rats treated with DOCA alone (185 +/- 5 mm Hg). After 5 weeks of DOCA treatment, femoral arteries and aortae were excised from these rats, cut helically into strips, and placed in a muscle bath to record isometric force. Dose-response curves to serotonin were shifted to the left in femoral arteries from DOCA-treated rats compared to both control and 6-OHDA-DOCA-treated rats (ED50: DOCA = 6.8 X 10(-8) M, control = 27.9 X 10(-8) M, 6-OHDA-DOCA = 13.4 X 10(-8) M). Arachidonic acid, the prostaglandin precursor, produced greater maximal contractions in femoral artery strips of DOCA-treated rats (358 +/- 56 mg) than in those from controls (115 +/- 31 mg). The maximal response to arachidonic acid in arteries from 6-OHDA-DOCA rats (203 +/- 78 mg) was not different from control values. Ouabain produced a greater maximal response in aortic strips from DOCA rats (658 +/- 165 mg) compared to those from control (196 +/- 72 mg) or 6-OHDA-DOCA (309 +/- 87 mg) rats. We conclude that increased vascular responsiveness to serotonin, arachidonic acid, and ouabain in DOCA hypertensive rats is secondary to a central action of the mineralocorticoid.     

Corticotropin-releasing hormone acts on CRH-R1 to inhibit the spontaneous contractility of non-labouring human myometrium at term

AIMS: Corticotropin-releasing hormone (CRH) has been implicated in the mechanisms controlling human parturition. The aims of the present study were to explore effects of CRH on contractility of human term myometrium and compare these effects in labouring and non-labouring myometrial strips. MAIN METHODS: The cumulative effects of CRH (10(-10) to 10(-7) mol/l) on the spontaneous contractility of labouring and non-labouring myometrial samples were evaluated using isometric tension recordings. KEY FINDINGS: CRH exhibited a concentration-dependent relaxant effect on spontaneous contractions in non-labouring term myometrium. This effect was mediated principally via a reduction in the amplitude rather than any changes in the frequency of contractions. The CRH-induced inhibitory effect on contractility could be blocked by pre-treatment with a CRH-R1 antagonist antalarmin, but not by pre-treatment with the CRH-R2 antagonist astressin 2B. CRH had no effect on spontaneous contractions in the labouring myometrium, as no change in either the amplitude or the frequency was observed. SIGNIFICANCE: Our findings indicate that CRH acts on CRH-R1 to inhibit spontaneous contractions in term myometrium from women who were not undergoing labour, but not those who were undergoing labour, supporting the hypothesis that CRH exerts dual effect on myometrium during pregnancy.     

Apamin inhibits NO-induced relaxation of the spontaneous contractile activity of the myometrium from non-pregnant women

There is now considerable evidence for the involvement of K⁺ channels in nitric oxide (NO) induced relaxation of smooth muscles including the myometrium. In order to assess whether apamin-sensitive K⁺ channels play a role in NO – induced relaxation of the human uterus, we have studied the effect of specific blockers of these channels on the relaxation of myometrium from non-pregnant women. In vitro isometric contractions were recorded in uterine tissues from non-pregnant premenopausal women who had undergone hysterectomy. Apamin (10 nM) and scyllatoxin (10 nM) did not alter spontaneous myometrial contractions. However, 15-min pretreatment of the myometrium strips with apamin completely inhibited relaxation caused by diethylamine-nitric oxide (DEA/NO). The pretreatment with scyllatoxin significantly reduced (about 2.6 times) maximum relaxation of the strips induced by DEA/NO (p < 0.05). These results strongly suggest that, beside Ca²⁺ and voltage dependent charybdotoxin-sensitive (CTX-sensitive) K⁺ channels, apamin-sensitive K⁺ channels are also present in the human non-pregnant myometrium. These channels offer an additional target in the development of new tocolytic agents.     

Functional adaptation of diaphragm to chronic hyperinflation in emphysematous hamsters

Costal strips of diaphragmatic muscle obtained from animals with elastase-induced emphysema generate maximum tension at significantly shorter muscle fiber lengths than muscle strips from control animals. The present study examined the consequences of alterations in the length-tension relationship assessed in vitro on the pressure generated by the diaphragm in vivo. Transdiaphragmatic pressure (Pdi) and functional residual capacity (FRC) were measured in 22 emphysematous and 22 control hamsters 4-5 mo after intratracheal injection of pancreatic elastase or saline, respectively. In 12 emphysematous and 12 control hamsters Pdi was also measured during spontaneous contractions against an occluded airway. To allow greater control over muscle excitation, Pdi was measured during bilateral tetanic (50 Hz) electrical stimulation of the phrenic nerves in 10 emphysematous and 10 control hamsters. Mean FRC in the emphysematous hamsters was 183% of the value in control hamsters (P less than 0.01). During spontaneous inspiratory efforts against a closed airway the highest Pdi generated at FRC tended to be greater in control than emphysematous hamsters. When control hamsters were inflated to a lung volume approximating the FRC of emphysematous animals, however, peak Pdi was significantly greater in emphysematous animals (70 +/- 6 and 41 +/- 8 cmH2O; P less than 0.05). With electrophrenic stimulation, the Pdi-lung volume curve was shifted toward higher lung volumes in emphysematous hamsters. Pdi at all absolute lung volumes at and above the FRC of emphysematous hamsters was significantly greater in emphysematous compared with control animals. Moreover, Pdi continued to be generated by emphysematous hamsters at levels of lung volume where Pdi of control subjects was zero.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of calcium channel blocker, mibefradil, and potassium channel opener, pinacidil, on the contractile response of mid-pregnant goat myometrium

The present study was undertaken to investigate the in vitro influence of mibefradil, a calcium channel blocker, and pinacidil, a potassium channel opener, on pregnant goat myometrial spontaneous rhythmic contractility and contractions induced with the agonist, oxytocin. Longitudinal strips from the distal region of uterus, collected from goats at midgestation, were mounted in an organ bath for recording isometric contractions. Mibefradil (10(-8)-10(-4) M) or pinacidil (10(-10)-10(-4) M), added cumulatively to the bath at an increment of 1 log unit, caused concentration-dependent inhibition of the spontaneous rhythmic contractions of isolated uterine strips. The rhythmic contraction was, respectively, abolished at 100 and 10 microM concentrations of mibefradil and pinacidil. In a concentration-dependent manner, mibefradil (1 and 10 microM) antagonized the contractions elicited with oxytocin (10(-5)-10(-2) IU). Pretreatment of uterine strips with glibenclamide (10 microM), a selective KATP channel blocker, caused a rightward shift of the concentration-response curve of pinacidil with a concomitant decrease in its pD2 value. Pinacidil (0.3, 1 and 3 microM), in a concentration-related manner, antagonized the oxytocin (10(-5)-10(-2) IU)-induced contractile response. The inhibition of spontaneous rhythmic contractions and antagonism of oxytocin-induced contraction by mibefradil in the pregnant goat myometrium may be related to the antagonism of voltage-dependent Ca2+ channels, while by pinacidil suggests that KATP channel could be a therapeutic target for tocolysis.     

Evidence for an intrinsic control of myometrial contractile periodicity in sheep during pregnancy.

Reduction in concentration of prostaglandins in plasma by administration of sodium meclofenamate to pregnant sheep failed to alter the frequency or duration of electromyographic activity bursts or the response to oxytocin of myometrial tissue transplanted to the omentum. However, a significant (P < 0.05) delay (8.6 +/- 3.8 versus 1.3 +/- 0.3 min) in the myometrial response to oxytocin was observed when the hormone was administered 1 min after a spontaneous burst of electromyographic activity compared with 15 min after a burst, indicating a period of refractoriness. Similarly, the myometrial threshold for electrical stimulation was higher at 10-25% of the interval between contractions than close to the expected time of the next contraction. Stimulation of the myometrium at intervals of 30 s revealed a cycling of the electrical stimulation threshold: significantly higher voltages were required to elicit responses between spontaneous bursts of electromyographic activity (18.0 +/- 2.2 V) than during bursts (11.3 +/- 1.6 V). In contrast, there was no voltage differential in animals close to labour (< 24 h). These data provide no evidence to support a role for prostaglandins in the generation of contractions during pregnancy, but suggest that periodicity of contractions is associated with inherent changes in myometrial responsiveness to stimulation, which could occur as a result of a cycling of the resting membrane potential.     

Complex effects of imatinib on spontaneous and oxytocin-induced contractions in human non-pregnant myometrium

Human myometrium includes two important cell populations involved in its contractility: smooth muscle fibers and interstitial cells. The pacemaking mechanism is not yet identified, but it is possible that myometrial smooth muscle cells contract in response to a signal generated by c-kit positive interstitial cells. The aim of this study was to investigate the effects of imatinib as a c-kit receptor antagonist on the spontaneous or oxytocin (OT) induced contractions of human non-pregnant myometrium in vitro. Myometrial strips were obtained from non-pregnant women (reproductive age) undergoing hysterectomy for benign indications. The strips were suspended in organ baths for recording of isometric tension. Imatinib effects were assessed on spontaneous contraction and after preexposure to OT.Direct exposure of myometrial strips to imatinib inhibits both amplitude and frequency of contractions (80-320 μM) in a dose dependent manner. Amplitude reverted back to 90% of the baseline amplitude by consequent addition of imatinib (until 480 μM). Total inhibition of myometrial contraction was obtained after addition of OT 60 nM. If myometrium was pre-exposed to OT (320 nM), imatinib 80-160 μm increased amplitude, while decreasing frequency. These data provide evidence that telocytes may be involved as modulators of the spontaneous contractions of the non-pregnant human uterus, via a tyrosine-kinase independent signaling pathway.     

Contractile responses of canine isolated pulmonary lobar arteries and veins to norepinephrine, serotonin, and tyramine.

Isolated helical strips of canine intrapulmonary lobar arteries and veins (about 4 mm in diameter) undergo dose-related tension development when exposed to increasing concentrations (10(-8) - 10(-3) M) of norepinephrine (NE), serotonin or 5-hydroxytryptamine (5-HT) and tyramine (Tyr). Venous segments were generally more sensitive while the maximum tension development was greater in the arterial strips, probably owing to their greater thickness. Both strips were more sensitive to 5-HT than NE and only responded to Tyr at high concentrations. Norepinephrine and 5-HT were nearly equally efficacious, whereas Tyr was less so. Responses to the latter were slow to develop, exhibited tachyphylaxis, and were greatly inhibited by phentolamine (10(-8) M), an alpha-adrenergic blocker. Exposure to cocaine (10(-5) M) enhanced submaximal NE responses, inhibited Tyr contractions and had no consistent effect on 5-HT responses. Phentolamine (10(-8) M) was also found to inhibit NE responses without altering 5-HT probably acts on other receptors. Tyramine may, in part, act directly on alpha-adrenergic receptors but may also release NE from surviving adrenergic nerve terminals in the preparation. Cocaine inhibits this effect and potentiates responses to lower levels of NE, presumably by blocking NE uptake into nerve terminals although a post-junctional action cannot be excluded.     

Calcium channels in isolated cells and strips of longitudinal muscle of rat myometrium

The properties of Ca2+ channels in strips and single muscle cells of longitudinal muscle of estrogen-dominated rat myometrium were studied under the effects of elevation of K+ concentration, the partial channel agonist Bay K 8644, and nitrendipine. In isolated strips in 0.5 mM Ca2+, Bay K 8644 (pD2 = 7.8-8.0) lowered the threshold for and enhanced the contractions in response to an elevation of K+ concentration, including the maximum response to K+ elevation alone. Bay K 8644 alone in concentrations up through 10(-6) M did not initiate contractions in 0.5 mM Ca2+ solutions. At higher concentrations (10(-5) M), Bay K 8644 behaved as an antagonist to contractions induced by elevation of K+. In isolated cells 10(-7) M Bay K 8644 enhanced the shortenings to elevated K+ and lowered the threshold K+ concentration required. Also no significant contraction occurred with 10(-7) M Bay K 8644 at normal K+ concentration. In contrast with its effect in isolated strips, no significant increase in maximum shortening (to 60 mM K+) was observed, possibly because cells without a mechanical load were maximally shortened by K+ alone. From these studies, we conclude that Ca2+ channels of isolated strips and cells of rat myometrium behave similarly and have similar properties to those of other smooth muscles in their interactions with elevation of K+, nitrendipine, and Bay K 8644.     

Leukotriene and anti-IgE induced contractions of human isolated trachea: studies with leukotriene receptor antagonists and a novel 5-lipoxygenase inhibitor

The pharmacological actions of three leukotriene D4 (LTD4) receptor antagonists, FPL-55712, L-648,051, and L-649,923, and a novel inhibitor of leukotriene biosynthesis, L-651,896, have been investigated on isolated human tracheal smooth muscle. In the order of potency L-648,051 greater than FPL-55712 greater than L-649,923, these agents antagonized contractions to LTD4 and produced parallel rightward shifts in the dose-response curves. Mean -log KB values against LTD4 were 6.9 +/- 0.1, 6.5 +/- 0.3, and 6.0 +/- 0.1 for L-648,051, FPL-55712, and L-649,923, respectively. FPL-55712 also antagonized contractions to LTC4 (-log KB value, 6.4 +/- 0.3) and this activity was not decreased by the gamma-glutamyl transpeptidase inhibitor, L-serine borate. In the presence of 1 x 10(-7) M atropine, 7 x 10(-6) M mepyramine, and 1.4 x 10(-6) M indomethacin, L-648,051 at 2 x 10(-5) and 2 x 10(-6) M produced complete and partial blockade, respectively, of the contraction to goat anti-IgE. L-649,923 and FPL-55712 produced partial but significant inhibition at 2 x 10(-5) M, whereas the 5-lipoxygenase inhibitor, L-651,896, produced almost complete inhibition at 3.5 and 35 x 10(-6) M. L-Serine borate (15 mM) did not alter the the activity of FPL-55712 versus anti-IgE. These findings indicate that LTD4 receptors mediate contraction of human trachea to exogenously applied and endogenously (anti-IgE) released leukotrienes. LTD4 antagonists, such as L-648,051, may be useful in assessing the role of leukotrienes in respiratory disease.     

Clarithromycin inhibits myometrial contractions in isolated human myometrium independent of stimulus

Erythromycin has a well-known dual effect on the contractility of the gastrointestinal system and recently has also been shown to inhibit contractions of the rat myometrium. The aim of the present study was to investigate the effects of clarithromycin on oxytocin, prostaglandin F2alpha (PGF2alpha) and KCl-induced contractions of human myometrium in vitro. Myometrial strips were obtained from pregnant women undergoing elective Cesarean section and the strips were suspended in a jacketed organ bath filled with Krebs solution at 37 degrees C (pH 7.4) and continuously aired with 95% oxygen and 5% carbon dioxide. Isometric contractions were measured using a force displacement transducer. Oxytocin, PGF2alpha, KCl and clarithromycin were applied to the tissue bath and the amplitude and frequency of contractions were evaluated at 20-min intervals. Freidmann analysis of variance, Kruskal Wallis and Wilcoxon Rank tests were used for statistical analysis of the data. Clarithromycin dose dependently inhibited the amplitude of contractions independent of the stimulus. Pre-treatment with apamin prevented clarithromycin-induced effects on amplitude and frequency of contractions. We conclude that the macrolide antibiotic clarithromycin may have a direct inhibitory effect on contractions of human myometrium.     

Adrenergic-cholinergic interactions in left atria: interaction of carbachol with alpha- and beta-adrenoceptor agonists

The purpose of the present investigation was to determine the nature of the functional interaction of muscarinic agonists with cAMP-generating and cAMP-independent agonists in left atria. Negative inotropic responses of rabbit isolated left atrial strips to the muscarinic agonist carbachol were measured in the absence and presence of equi-active inotropic doses of the beta-adrenoceptor stimulant isoproterenol (Iso), the mixed alpha- and beta-adrenoceptor stimulant phenylephrine (PE) plus 1 microM timolol to block the beta-receptor mediated component of its response, and elevated extracellular Ca2+. Carbachol produced dose-dependent negative inotropic responses in left atrial strips, which were much greater than control in the presence of either Iso, or PE plus timolol. However, carbachol responses were of a similar magnitude to the control in the presence of elevated extracellular Ca2+. In the presence of timolol, PE had no significant effect on cAMP levels in left atrial strips, and inotropic responses to carbachol alone and in combination with PE plus timolol were accompanied by significant increases in cGMP levels but no change in cAMP levels. Carbachol attenuated Iso-induced increases in cAMP levels, but decreases in left atrial tension were proportionally greater than the decreases in cAMP levels produced by carbachol in the presence of Iso. These results suggest that the antiadrenergic effects of muscarinic receptor stimulation may occur by a different mechanism in left atria than has been previously reported in ventricular muscle. While the nature of this mechanism is unknown, it may involve antagonism by muscarinic agents of both alpha- and beta-adrenoceptor mediated increases in Ca2+ influx.