Effect of gap junction number and permeability on intercellular coupling in rat myometrium

Connexins: key mediators of endocrine function

The appearance of multicellular organisms imposed the development of several mechanisms for cell-to-cell communication, whereby different types of cells coordinate their function. Some of these mechanisms depend on the intercellular diffusion of signal molecules in the extracellular spaces, whereas others require cell-to-cell contact. Among the latter mechanisms, those provided by the proteins of the connexin family are widespread in most tissues. Connexin signaling is achieved via direct exchanges of cytosolic molecules between adjacent cells at gap junctions, for cell-to-cell coupling, and possibly also involves the formation of membrane "hemi-channels," for the extracellular release of cytosolic signals, direct interactions between connexins and other cell proteins, and coordinated influence on the expression of multiple genes. Connexin signaling appears to be an obligatory attribute of all multicellular exocrine and endocrine glands. Specifically, the experimental evidence we review here points to a direct participation of the Cx36 isoform in the function of the insulin-producing β-cells of the endocrine pancreas, and of the Cx40 isoform in the function of the renin-producing juxtaglomerular epithelioid cells of the kidney cortex.

Contractile activity of rabbit uterus at the end of pregnancy and a method for its registration

We developed a method for synchronous registration of electrical activity of the myometrium in pregnant rabbit females and electrocardiograms of the female and fetus on a paper tape and on PC in the online mode using an original pickup for registration of intrauterine pressure in awaken animals in a natural (sitting) posture. The method causes no pregnancy complications and is adequate for evaluation of uterine contractility, female and fetal status in different variants of the experiment.

Effect of oestradiol and progesterone on the instant and directional velocity of microsphere movements in the rat oviduct: gap junctions mediate the kinetic effect of oestradiol

The oviducal transport of eggs to the uterus normally takes 72–96 h in the rat, but this is reduced to less than 20 h after a single injection of oestradiol (E2). This accelerated transport is associated with an increased frequency of pendular movements in the isthmic segment of the oviduct, with increased levels of the gap junction (GJ) component Connexin (Cx) 43, and is antagonised by progesterone (P). In the present study, we investigated the effect of these hormones on the instant and directional velocity of pendular movements and the role of the GJ and its Cx43 component in the kinetic response of the oviduct to E2 and P. Using microspheres as egg surrogates, microsphere instant velocity (MIV) was measured following treatment with E2, P or P + E2, which accelerate or delay egg transport. Microspheres were delivered into the oviduct of rats on Day 1 of pregnancy and their movement within the isthmic segment was recorded. Oestrogen increased MIV with faster movement towards the uterus. After P or P + E2, MIV was similar to that in the control group. Two GJ uncouplers, namely 18α- and 18β-glycyrrhetinic acid, blocked the effect of E2 on MIV. Connexin 43 mRNA levels increased over that seen in control with all treatments. In conclusion, the effects of E2 on MIV resulted in faster movements that produced accelerated egg transport towards the uterus. Gap junctions are probably involved as smooth muscle synchronisers in this kinetic effect of E2, but the opposing effects of E2 and P are not exerted at the level of Cx43 transcription.

Uterine Electromyography Characteristics for Early Diagnosis of Mifepristone-Induced Preterm Labor

OBJECTIVE

Differentiating uterine contractions leading to preterm birth from ineffective uterine activity is difficult with current tools. Uterine electromyographic activity is recordable and consists of bursts (group of action potentials) characterized by characteristics that are different during pregnancy and labor. Our aim was to identify the chronology of the changes in uterine pressure and electromyographic characteristics during mifepristone-induced preterm labor in pregnant rats and to determine the earliest characteristic to change.

METHODS

On day 17 of gestation, intrauterine catheter and electromyography electrodes were implanted in the uterus. On day 18, rats were allocated for treatment with mifepristone or placebo. Intrauterine pressure and electromyography integral activities and electromyography mean were calculated before treatment and 6, 12, 18, 20, 22, and 24 hours after treatment. After mathematical transformation, burst analysis was performed by using power density spectrum energy, peak amplitude, and frequency.

RESULTS

As expected, delivery rate within 24 hours was higher in the mifepristone-treated group. Changes in electromyography integral activity and mean, power density spectrum energy, and intrauterine pressure integral activity occurred late during preterm labor, in a range of 2-4 hours before delivery. Electromyography peak frequency of the power density spectrum exhibited early changes, with a shift from low to high frequencies starting at 12 hours before delivery.

CONCLUSION

Electromyography peak frequency of the power density spectrum from individual bursts was the first characteristic to change after antiprogestin treatment, preceding any change in intrauterine pressure, making it a potentially useful marker for the early diagnosis of preterm labor.

Junctional and nonjunctional effects of heptanol and glycyrrhetinic acid derivates in rat mesenteric small arteries

1 Heptanol, 18alpha-glycyrrhetinic acid (18alphaGA) and 18beta-glycyrrhetinic acid (18betaGA) are known blockers of gap junctions, and are often used in vascular studies. However, actions unrelated to gap junction block have been repeatedly suggested in the literature for these compounds. We report here the findings from a comprehensive study of these compounds in the arterial wall. 2 Rat isolated mesenteric small arteries were studied with respect to isometric tension (myography), [Ca2+]i (Ca(2+)-sensitive dyes), membrane potential and--as a measure of intercellular coupling--input resistance (sharp intracellular glass electrodes). Also, membrane currents (patch-clamp) were measured in isolated smooth muscle cells (SMCs). Confocal imaging was used for visualisation of [Ca2+]i events in single SMCs in the arterial wall. 3 Heptanol (150 microm) activated potassium currents, hyperpolarised the membrane, inhibited the Ca2+ current, and reduced [Ca2+]i and tension, but had little effect on input resistance. Only at concentrations above 200 microm did heptanol elevate input resistance, desynchronise SMCs and abolish vasomotion. 4 18betaGA (30 microm) not only increased input resistance and desynchronised SMCs but also had nonjunctional effects on membrane currents. 18alphaGA (100 microm) had no significant effects on tension, [Ca2+]i, total membrane current and synchronisation in vascular smooth muscle. 5 We conclude that in mesenteric small arteries, heptanol and 18betaGA have important nonjunctional effects at concentrations where they have little or no effect on intercellular communication. Thus, the effects of heptanol and 18betaGA on vascular function cannot be interpreted as being caused only by effects on gap junctions. 18alphaGA apparently does not block communication between SMCs in these arteries, although an effect on myoendothelial gap junctions cannot be excluded.

Varied effects of 1-octanol on gap junctional communication between ovarian epithelial cells and oocytes of Oncopeltus fasciatus, Hyalophora cecropia, and Drosophila melanogaster

In insect gap junctions, species-specific differences occur in response to the purported gap junction uncoupling agent, 1-octanol. Changes in gap junctional communication between oocytes and their epithelial cells following treatment with 1-octanol were assayed in Oncopeltus fasciatus (the milkweed bug), Hyalophora cecropia (the American silk moth), and Drosophila melanogaster. In all three species, microinjection of untreated control follicles with Lucifer yellow CH revealed extensive dye coupling among epithelial cells and between epithelial cells and their oocytes. Also for all three species, treatment with octanol appeared to completely block dye coupling and increase oocyte input resistance. The effect on electrical coupling varied. In Drosophila, octanol diminished the electrical coupling from 64% (0.64 coupling coefficient) in controls to 53% in treated follicles. In Hyalophora, the coupling ratio remained the same following treatment. In Oncopeltus, octanol actually increased the electrical coupling ratio from 84% in controls to 94% in treated follicles. While 0.5 mM octanol left some Oncopeltus epithelial cells dye coupled to the oocyte, the electrical coupling ratio was increased slightly more by this concentration than by 1 or 5 mM octanol solutions, although the differences were not significant. While input resistance (R(o )) increased in all three following treatment with octanol, there was considerable difference in the magnitude of the response. Average oocyte R(o ) for Oncopeltus increased the least of the three species, rising from 196-240 kOhm. Both Hyalophora, with a nearly fourfold increase from 230-900 kOhm or more, and Drosophila, with a twofold increase from 701 kOhm to over 1.2 MegOhm showed much larger changes. Results shown here indicate that insect gap junctions have more varied responses to this common gap junction antagonist than have been reported for their vertebrate counterparts. Arch.

Intracellular trafficking pathways in the assembly of connexins into gap junctions

Trafficking pathways underlying the assembly of connexins into gap junctions were examined using living COS-7 cells expressing a range of connexin-aequorin (Cx-Aeq) chimeras. By measuring the chemiluminescence of the aequorin fusion partner, the translocation of oligomerized connexins from intracellular stores to the plasma membrane was shown to occur at different rates that depended on the connexin isoform. Treatment of COS-7 cells expressing Cx32-Aeq and Cx43-Aeq with brefeldin A inhibited the movement of these chimera to the plasma membrane by 84 +/- 4 and 88 +/- 4%, respectively. Nocodazole treatment of the cells expressing Cx32-Aeq and Cx43-Aeq produced 29 +/- 16 and 4 +/- 7% inhibition, respectively. In contrast, the transport of Cx26 to the plasma membrane, studied using a construct (Cx26/43T-Aeq) in which the short cytoplasmic carboxyl-terminal tail of Cx26 was replaced with the extended carboxyl terminus of Cx43, was inhibited 89 +/- 5% by nocodazole and was minimally affected by exposure of cells to brefeldin A (17 +/-11%). The transfer of Lucifer yellow across gap junctions between cells expressing wild-type Cx32, Cx43, and the corresponding Cx32-Aeq and Cx43-Aeq chimeras was reduced by nocodazole treatment and abolished by brefeldin A treatment. However, the extent of dye coupling between cells expressing wild-type Cx26 or the Cx26/43T-Aeq chimeras was not significantly affected by brefeldin A treatment, but after nocodazole treatment, transfer of dye to neighboring cells was greatly reduced. These contrasting effects of brefeldin A and nocodazole on the trafficking properties and intercellular dye transfer are interpreted to suggest that two pathways contribute to the routing of connexins to the gap junction.

Effects of heptanol on the neurogenic and myogenic contractions of the guinea-pig vas deferens

1. The effects of the putative gap junction uncoupler, 1-heptanol, on the neurogenic and myogenic contractile responses of guinea-pig vas deferens were studied in vitro. 2. Superfusion of 2.0 mM heptanol for 20-30 min produced the following reversible changes in the biphasic neurogenic contractile response (8 trials): (i) suppression of both phases; (ii) delayed development of both the first as well as the second phase, accompanied by complete temporal separation of the two phases; (iii) prominent oscillations of force during the second (noradrenergic) phase only. 3. To eliminate prejunctional effects of heptanol, myogenic contractions were evoked by field stimulation of the vas in the presence of suramin (200 microM) and prazosin (1 microM). Heptanol (2.0 mM) abolished these contractions reversibly. 4. These results show that (i) heptanol inhibits both excitatory junction potential (EJP)-dependent and non EJP-dependent contractions of the vas; (ii) a postjunctional site of action of heptanol, probably intercellular uncoupling of smooth muscle cells, contributes to the inhibition of contraction.

The initiation, continuation, and termination of spontaneous episodes of circus movements in the pregnant myometrium of the rat

OBJECTIVES

The objectives of this study were to determine the mechanisms responsible for the initiation and the termination of episodes of circus movements and to describe the characteristics of these circuits in rat myometrium.

STUDY DESIGN

By means of simultaneous recordings from 240 sites in Wistar rats at 17, 19, or 21 days' gestation, a spatial and temporal analysis was performed on the electrical activation from the beginning until the end of 22 episodes of circus movements.

RESULTS

The initiation of reentry was caused by (1) transient unexcitable areas at the beginning of the burst or (2) lines of conduction blocks. Circus movements were terminated by (1) reversal of recruitment (decruitment) at the end of the burst, (2) occurrence of lines of conduction blocks, or (3) interruption of the circuit. On average circuits rotated for 6 revolutions during 2 seconds. In most cases circuits remained stable; sometimes circuits drifted along the tissue and occasionally multiple wavelets were seen instead of a single circuit.

CONCLUSIONS

Spatial nonhomogeneities in conduction caused by transient unexcitable areas play a major role in both the initiation and the termination of episodes of circus movements in the pregnant rat myo-metrium. In addition, most circuits remained in a single location, although sometimes meandering or even fibrillatory activity was seen.

Ineffectiveness of oxytocin on intercellular communication between term pregnant human myometrial cells before labor

OBJECTIVE

The effects of the uterotonic agent oxytocin on intercellular communication between term pregnant human myometrial cells before labor were studied to investigate its contribution to the synchronicity of uterine contractions.

STUDY DESIGN

The membrane potential and the input resistance of myometrial cells were measured with intracellular micropipettes, and dye-coupling assays were concomitantly performed while the tissues were perfused with three concentrations of oxytocin (10(-8) mol/L, 10(-7) mol/L, and 10(-6) mol/L). The results were compared with those obtained from tissues perfused with gap junctional uncoupling agent octanol and those with Tyrode's solution (control).

RESULTS

Octanol increased input resistance and inhibited dye coupling but did not affect membrane potential compared with controls. Oxytocin depolarized membrane potential at all studied concentrations. The mean input resistance values and detectable dye propagation with oxytocin were similar to controls.

CONCLUSION

Oxytocin does not contribute to gap junctional intercellular communication in term pregnant human myometrium before labor but may affect contractions by depolarizing membrane potential.

Intercellular communication between dorsal root ganglion cells and colonic smooth muscle cells in vitro

The mechanism(s) by which intestinal smooth muscle tension is signaled to extrinsic primary afferent neurons is poorly understood. In order to characterize myocyte-neuron communication, we developed a coculture system using rat dorsal root ganglion (DRG) neurons and myocytes obtained from the circular muscle layer of the rat distal colon. Both cell types maintained their phenotype in culture, as demonstrated by positive immunocytochemical staining for neuron-specific enolase and smooth muscle actin. Myocytes showed mechanosensitivity in the form of increases in [Ca2+]i in response to light mechanical touch of the plasma membrane. This increase in [Ca2+]i was independent of extracellular Ca2+ and passed as a propagated wave from muscle cells into adjacent DRG neurites. The inhibitory effect of octanol on this intercellular propagation suggests propagation of [Ca2+]i gradients via heterologous gap junctions. This preparation may serve a useful model system for the study of the interaction of visceral afferents and their target cells.

Effects of heptanol on electrical activity in the guinea-pig vas deferens

1. The effects of the putative intercellular uncoupling agent I-heptanol on electrical activity in the guinea-pig vas deferens were studied by use of intracellular and extracellular recording techniques. 2. At concentrations of 0.5, 1 and 2 mM, heptanol rapidly, monotonically and reversibly attenuated intracellularly recorded excitatory junction potential (e.j.p.) amplitude without affecting its time course, while spontaneous excitatory junction potentials (s.e.j.ps) were left unaffected. 3. Heptanol did not affect either the extracellularly recorded evoked excitatory junction current (e.j.c.), or the nerve terminal impulse that preceded it. These observations indicate that heptanol does not affect nerve impulse conduction, neurotransmitter release, or the postjunctional receptors involved in the production of the e.j.p. 4. E.j.ps appear to be suppressed by heptanol due to its intercellular uncoupling effects. Therefore, functional intercellular coupling may be necessary for the generation of the e.j.p. in smooth muscle.

Characterization of an oxytocin-induced rise in [Ca2+]i in single human myometrium smooth muscle cells

The effect of the uterotonic pituitary hormone oxytocin on the regulation of intracellular calcium concentration ([Ca2+]i) was studied in single cells of a smooth muscle cell line derived from human non-pregnant myometrium. [Ca2+]i was measured with fluorescence microscopy, and by recording the activity of Ca(2+)-activated potassium currents (IK(Ca)) on the whole cell and single channel level. Oxytocin induced a rapid and transient increase in [Ca2+]i that was paralleled by a significant increase in IK(Ca) activity. After removal of extracellular Ca2+, repetitive stimulation with oxytocin did not alter the [Ca2+]i transients initially; however, their amplitude became progressively smaller and the response was eventually abolished completely, indicating that oxytocin increased [Ca2+]i by release of Ca2+ from intracellular stores. Nifedipine did not alter the oxytocin-induced [Ca2+]i-transients suggesting that oxytocin failed to activate Ca2+ entry through voltage-operated Ca2+ channels. Thapsigargin abolished the oxytocin-induced [Ca2+]i transient. Caffeine alone had no effect on [Ca2+]i, however it diminished the oxytocin-induced [Ca2+]i transients. Ryanodine did not affect the oxytocin response indicating that these cells lack release of Ca2+ from the ryanodine receptor release channel. These results demonstrate that oxytocin elicited [Ca2+]i transients predominantly through Ca2+ release from thapsigargin-sensitive stores, presumably by activating an inositol 1,4,5-trisphosphate dependent pathway.

Reciprocal expression of cell-cell coupling and voltage-dependent Na current during embryogenesis of rat telencephalon

Using whole-cell patch-clamp techniques in situ (whole-tissue and tissue slices), we have studied two aspects of rat telencephalic cell development during the period of embryogenesis starting at E12. The first aspect was related to junctional coupling as revealed by low input resistance, intercellular dye spread and pharmacologic blockade. Coupling appeared to decrease with time, both in extent and occurrence. The second aspect dealt with cell excitability as revealed by voltage-dependent Na current (INa) expression. Immature action potentials and their underlying INaS were present in a small proportion of E12 cells. These currents were blocked 36% and 78% by 10(-7) M and 10(-6) M tetrodotoxin (TTX), respectively. From then onward, INaS got larger and more prevalent while no obvious changes in kinetics were observed. At E21, INaS were abolished by 10(-7) M TTX and channel density apparently was sufficient to support overshooting yet still immature action potentials.

Studies of connexin 43 and cell-to-cell coupling in cultured human uterine smooth muscle

OBJECTIVE

The aim of this study was to assess the presence and the permeability of gap junctions between human uterine smooth-muscle cells in culture.

STUDY DESIGN

The uterine smooth muscles obtained from term-pregnant women were cultured. The presence of gap junction was evaluated by immunocytochemistry with gap junction protein antibodies and by measuring input resistance and intercellular spread of lucifer yellow. These measures also evaluated the permeability of gap junctions. Octanol, isoproterenol, dibutyryl cyclic adenosine monophosphate and forskolin were applied to the cultures to assess their effects on the permeability of gap junctions.

RESULTS

During culture, immunocytochemical staining of gap junction protein (connexin 43) was increased and input resistance was decreased on day 2 of culture versus day 21 (18.4 +/- 7.87 M omega day 2; 3.8 +/- 1.76 M omega, day 21; p < 0.001). However, the decrease in input resistance was related to cell density rather than time in culture (16.4 +/- 5.01 M omega, single cells on days 1 and 2; 5.3 +/- 2.35 M omega, high-density cultures on days 1 and 2; p < 0.001). Octanol increased input resistance and intercellular spread of lucifer yellow in confluent cultures; isoproterenol, dibutyryl cyclic adenosine monophosphate, and forskolin did not.

CONCLUSIONS

The increased staining of connexin 43 and the decreased input resistance during culture are evidence of elevated number of gap junctions between cells. The rapid and reversible increase in input resistance and decrease in spread of lucifer yellow by octanol are the result of decreased permeability of gap junctions. These two methods of modulation of gap junctions in human uterine smooth muscles are thought to be major mechanisms for the control of uterine contractility.

Intercellular communication in smooth muscle

The functioning of a group of cells as a tissue depends on intercellular communication; an example is the spread of action potentials through intestinal tissue resulting in synchronized contraction. Recent evidence for cell heterogeneity within smooth muscle tissues has renewed research into cell coupling. Electrical coupling is essential for propagation of action potentials in gastrointestinal smooth muscle. Metabolic coupling may be involved in generation of pacemaker activity. This review deals with the role of cell coupling in tissue function and some of the issues discussed are the relationship between electrical synchronization and gap junctions, metabolic coupling, and the role of interstitial cells of Cajal in coupling.