Effect of estradiol-17 beta and prostaglandins on rat myometrial gap junctions

Gap junctions

Gap junctions are essential to the function of multicellular animals, which require a high degree of coordination between cells. In vertebrates, gap junctions comprise connexins and currently 21 connexins are known in humans. The functions of gap junctions are highly diverse and include exchange of metabolites and electrical signals between cells, as well as functions, which are apparently unrelated to intercellular communication. Given the diversity of gap junction physiology, regulation of gap junction activity is complex. The structure of the various connexins is known to some extent; and structural rearrangements and intramolecular interactions are important for regulation of channel function. Intercellular coupling is further regulated by the number and activity of channels present in gap junctional plaques. The number of connexins in cell-cell channels is regulated by controlling transcription, translation, trafficking, and degradation; and all of these processes are under strict control. Once in the membrane, channel activity is determined by the conductive properties of the connexin involved, which can be regulated by voltage and chemical gating, as well as a large number of posttranslational modifications. The aim of the present article is to review our current knowledge on the structure, regulation, function, and pharmacology of gap junctions. This will be supported by examples of how different connexins and their regulation act in concert to achieve appropriate physiological control, and how disturbances of connexin function can lead to disease.

Blastocyst-mediated induction of endometrial connexins: an inflammatory response?

As a prerequisite for successful embryo implantation in mammals, before implantation ovarian hormones regulate the transformation of the endometrium into the receptive phase. During the implantation process, gene expression in the receptive endometrium is additionally modulated by the presence of a blastocyst. During this complex differentiation process, in humans as well as in rodents, gap junction connexin 26 (Cx26) is suppressed in the uterine epithelium and Cx43 is suppressed in the endometrial stromal cells during the receptive phase. In rodents, a blastocyst-mediated induction of Cx26 takes place locally in the uterine epithelial cells of the implantation chamber surrounding the blastocyst, followed by an increase in Cx43 in the cells of the developing decidua. The Cx26 induction is dependent on the presence of a blastocyst and occurs even before adhesion and invasion of the trophoblast takes place. The signal cascades involved in this blastocyst-mediated connexin induction are still elusive. The process of implantation is considered as a proinflammatory response, and inflammatory factors have been shown to be involved in the implantation process. In fact, Cx26 expression can be induced in the receptive rat endometrium by mediators of the inflammatory cascade including prostaglandin-F2α and IL1β by an ER-independent pathway similar to the blastocyst-mediated connexin induction at the time of implantation. Thus, in the receptive endometrium induction of connexin expression may also be induced by mediators of the inflammatory signaling cascade, and the implantation-related induction of intercellular communication may in part be due to an inflammatory response.

Physiology of myometrial function: intercellular coupling and its role in uterine contractility

The mammalian uterus is composed of a preponderance of small smooth muscle cells usually aligned in two layers. The number of muscle cells in the human uterus at term is estimated at 200 billion, each minute fusiform cell measuring about 5–10μm in diameter and about 200μm in length. The main function of the uterus is to harbour the developing fetus during pregnancy and then to contract vigorously during labour to expel the products of conception. In order for the uterus to contract rhythmically and forcefully, a mechanism must exist to allow interaction between muscle cells in order to achieve synchronous activity. Phasic or cyclical patterns of contractile activity of the uterus cannot be accounted for by stimulation or inhibition from the nervous or endocrine systems. Since myometrial cells are dependent upon action potentials for their contractile processes, some system must be present between the muscle cells for the propagation of action potentials between them. The observation that gap junctions occur in large numbers between myometrial cells during parturition is thought to be significant in this regard and they are considered to play an essential role in parturition and in the control and co-ordination of uterine contractility. In this brief review, we will discuss the role of gap junctions in the modulation of myometrial contractility and the mechanisms that regulate their synthesis and permeability.

Connexin43 and gap junction content in the porcine myometrium during the estrous cycle

The amount of the gap junction protein Connexin43 in myometrium from pig uteri at specific stages of the estrous cycle was studied using 2 different immunochemical methods. Western blot analysis showed the presence of Connexin43 protein in 43 and 47 kD bands at all stages of the cycle. Immunohistochemistry demonstrated only a few small junctional complexes in the myometrium during the luteal phase of the cycle. Gap junctions appeared to increase both in number and size during proestrus and estrus.

Enzyme histochemistry of the regressing rat decidua and metrial gland

In order to understand more about participation of the basal placental zones in processes of regression and degradation as well as separation on the cellular level, the cell metabolism of the rat decidua and metrial gland was investigated enzyme histochemically in cryosections for activities of oxyradical-forming enzymes and hydrolyzing enzymes. Additionally, plastic sections were studied to facilitate the recognition of cell types. Decidual stromal cells and fibroblasts formed the vast majority amongst many cell types in the decidua and metrial gland. High activities of enzymes involved in purine degradation and oxyradical generation were demonstrated in decidual stromal cells and fibroblasts. Microsomal alanyl aminopeptidase and various acid hydrolases were shown to be extremely active in decidual stromal cells. The abundance of these enzyme activities in the decidua and metrial gland in contrast to other placental areas suggests, that these enzymes may have specialized functions in connection with regression and degradation processes finally contributing to placental separation.

Variations in oestradiol and progesterone receptor levels in the porcine myometrium and cervix during pregnancy and parturition

In vitro binding and exchange methods were used to determine the levels of oestradiol (E) and progesterone (P) receptors in cytosolic and nuclear fractions of cells obtained from the porcine myometrium and cervix during pregnancy and parturition. The concentration of oestradiol cytosolic receptors (ERc) in non-placental and placental myometrium and the cervix was low in early and mid-pregnancy, increasing at late pregnancy and was highest at parturition. The oestradiol nuclear receptors (ERn) followed the same pattern in the 3 sampling areas. The levels of PRc in non-placental regions were low in early pregnancy, increased to late pregnancy but declined during parturition. In placental regions the PRc value was high in early and mid-pregnancy, but decreased at late pregnancy and parturition. The concentration of PRc in the cervix increased significantly at mid-pregnancy and declined afterwards. In early and mid-pregnancy the PRn content was high, and decreased at late pregnancy and parturition in the 3 sampling areas. The data obtained agree with the present theories on the endocrine mechanisms regulating receptor levels in the uterus. Moreover, these data support the concept that the uterine motility during pregnancy and parturition might be influenced by changes in myometrial steroid receptor concentrations.

Modulation of gap junctions in the myometrium of pigs during early gestation, induced abortion and the oestrous cycle

Myometrial samples were biopsied from pigs at early pregnancy, within 12 h after initiated abortion and at first dioestrus post-abortion. Abortion was induced either with an isolated gram-negative bacterial endotoxin or the prostaglandin (PG) F2 alpha analog cloprostenol. The tissues were quantitatively examined for gap junctions in electron micrographs. Very small numbers of gap junctions were present between smooth muscle cells in myometria at 24-27 days of pregnancy. The cell-to-cell junctions increased both in number and size at abortion, irrespective of the agent used as abortifacient. A low number of gap junctions were recorded in samples obtained after reinitiation of the oestrous cycle. The plasma levels of oestradiol-17 beta were low at the three biopsy procedures while the increased numbers of gap junctions at abortion occurred when significantly high levels of PGF2 alpha concomitantly with withdrawal of progesterone were determined in circulating blood plasma. It remains therefore unclear, which role do oestrogens play in the synthesis of gap junctions at induced abortion.

In vitro studies on the control of human myometrial gap junctions

In this study human myometrial tissues were examined for the presence of gap junctions by quantitative electron microscopy before and after incubation in tissue culture media with and without indomethacin. The area of gap junctions was very low in tissues from pregnant women at term but not labor, before incubation. After 24 and 48 h incubation without any treatment, segments of some of the same tissues developed many gap junctions and other tissues contained few junctions. Prostaglandin E (PGE), prostaglandin F (PGF) and prostaglandin F metabolite (PGF metabolite) levels in the media at various times were measured by radioimmunoassay. The prostaglandins increased progressively during the incubation period. Treatment of tissues with indomethacin decreased prostaglandin levels in the media and increased the numbers of gap junctions in those control tissues that developed few junctions over the same incubation interval. We conclude that the capacity of human myometrial tissues to develop gap junctions in vitro may depend upon a maturational stage in preparation for labor. Furthermore, our results suggest that products of the cyclo-oxygenase or lipoxygenase pathways may control the presence of gap junctions in the human myometrium and that changes in synthesis in these patterns may occur as part of the maturational process.

Effects of instrumentation on myometrial activity in the oophorectomized ewe

The influence of an intrauterine pressure catheter on electrical myometrial activity (EMG) was investigated in chronically instrumented oophorectomized sheep. The EMG of non-catheterized (NC) ewes (n = 4) was characterized by long-lasting but infrequently occurring bursts of electrical activity, whereas the EMG of catheterized (C) ewes (n = 4) showed short and frequent bursts. Intra-arterial administration of 0.1 mg of 17 beta-estradiol (E2) was followed in C and NC ewes by an increase in myometrial activity after an initial silent period. E2 did not essentially change the pattern of long and short bursts in either group. Intravenous administration of the prostaglandin-synthesis inhibitor naproxen changed the EMG pattern in the C ewes, both before and after E2, to a pattern resembling the EMG of NC ewes. The results of this study indicate that electrical myometrial activity is markedly influenced by the presence of an intrauterine pressure catheter, probably due to enhanced prostaglandin synthesis.

Hormonal control of gap junctions in the myometrium

Myometrial tissues from immature, ovariectomized mature, and pregnant rats were examined by thin-section or freeze-fracture electron microscopy for the presence of gap junctions following treatment with steroid hormones and inhibitors of prostaglandin synthesis. Gap junctions were present in the myometrium of immature rats after repeated injections of estradiol or diethylstilbestrol (500 micrograms/day). The frequency of gap junctions in immature rats after estradiol treatment was similar to that found during delivery. Concomitant administration of indomethacin or meclofenamate with estradiol (50 and 500 micrograms/day) for 3 or 4 days enhanced the effect of estradiol on gap junctions. Progesterone depressed the response of 500 micrograms/day estradiol and both 50 and 500 micrograms/day doses of estradiol when administered together with the inhibitors of prostaglandin synthesis. Gap junctions were also present in the myometria of ovariectomized mature rats after treatment with estradiol by itself or together with indomethacin, and these effects were suppressed by concomitant progesterone administration. These results indicate that the steroid hormones and prostaglandins may interact to modulate the presence of gap junctions in the myometrium.