Downregulation of surface sodium pumps by endocytosis during meiotic maturation of Xenopus laevis oocytes

How to make a good egg!: The need for remodeling of oocyte Ca(2+) signaling to mediate the egg-to-embryo transition

The egg-to-embryo transition marks the initiation of multicellular organismal development and is mediated by a specialized Ca(2+) transient at fertilization. This explosive Ca(2+) signal has captured the interest and imagination of scientists for many decades, given its cataclysmic nature and necessity for the egg-to-embryo transition. Learning how the egg acquires the competency to generate this Ca(2+) transient at fertilization is essential to our understanding of the mechanisms controlling egg and the transition to embryogenesis. In this review we discuss our current knowledge of how Ca(2+) signaling pathways remodel during oocyte maturation in preparation for fertilization with a special emphasis on the frog oocyte as additional reviews in this issue will touch on this in other species.

Pranlukast, a cysteinyl leukotriene type 1 receptor antagonist, attenuates the progression but not the onset of silica-induced pulmonary fibrosis in mice

BACKGROUND

Although cysteinyl leukotrienes (CysLTs) have been implicated in the etiology of acute inflammatory diseases, recent studies have suggested that they also directly stimulate fibroblasts. However, their precise role in the pathogenesis of pulmonary fibrosis is unclear.

METHODS

In this study, we evaluated the effect of both short- and long-term treatment with pranlukast, a CysLT type 1 (CysLT(1)) receptor antagonist, on silica-induced pulmonary fibrosis in mice, which is characterized by persistent progression of fibrosis in the chronic phase. Pranlukast (30 mg/kg/day) was administered orally to mice for 2 or 10 weeks after intratracheal silica instillation.

RESULTS

Pranlukast treatment for 10 weeks significantly attenuated the progression of pulmonary fibrosis, and decreased the content of CysLTs and LTB(4), which were markedly increased in the bronchoalveolar lavage fluid (BALF) and lung tissues of silica-instilled mice in the chronic phase. However, pranlukast treatment for 2 weeks neither affected the acute inflammatory response induced by silica instillation nor inhibited the onset of fibrosis. The expression of TGF-β1 and TNF-α was not affected by pranlukast treatment for either 2 or 10 weeks.

CONCLUSIONS

Pranlukast attenuates the progression of pulmonary fibrosis in the chronic phase but has no effect on the acute inflammatory response or on the onset of pulmonary fibrosis. The antifibrotic effect of pranlukast may be exhibited by antagonizing the direct profibrotic effect of CysLTs, without affecting the expression of other profibrotic cytokines such as TGF-β1 and TNF-α, and also by decreasing the production of CysLTs and LTB(4).

A marked animal-vegetal polarity in the localization of Na(+),K(+) -ATPase activity and its down-regulation following progesterone-induced maturation

The stage-VI Xenopus oocyte has a very distinct animal-vegetal polarity with structural and functional asymmetry. In this study, we show the expression and distribution pattern of Na(+),K(+) -ATPase in stage-VI oocytes, and its changes following progesterone-induced maturation. Using enzyme-specific electron microscopy phosphatase histochemistry, [(3) H]-ouabain autoradiography, and immunofluorescence cytochemistry at light microscopic level, we find that Na(+),K(+) -ATPase activity is mainly confined to the animal hemisphere. Electron microscopy histochemical results also suggest that polarized distribution of Na(+),K(+) -ATPase activity persists following progesterone-induced maturation, and it becomes gradually more polarized towards the animal pole. The time course following progesterone-induced maturation suggests that there is an initial up-regulation and then gradual down-regulation of Na(+),K(+) -ATPase activity leading to germinal vesicle breakdown (GVBD). By GVBD, the Na(+),K(+) -ATPase activity is completely down-regulated due to endocytotic removal of pump molecules from the plasma membrane into the sub-cortical region of the oocyte. This study provides the first direct evidence for a marked asymmetric localization of Na(+),K(+) -ATPase activity in any vertebrate oocyte. Here, we propose that such asymmetry in Na(+),K(+) -ATPase activity in stage-VI oocytes, and their down-regulation following progesterone-induced maturation, is likely to have a role in the active state of the germinal vesicle in stage-VI oocytes and chromosomal condensation after GVBD.

Regulation of store-operated Ca2+ entry during the cell cycle

Cytoplasmic Ca(2+) signals are central to numerous cell physiological processes, including cellular proliferation. Historically, much of the research effort in this area has focused on the role of Ca(2+) signals in cell-cycle progression. It is becoming clear, however, that the relationship between Ca(2+) signaling and the cell cycle is a 'two-way street'. Specifically, Ca(2+)-signaling pathways are remodeled during M phase, leading to altered Ca(2+) dynamics. Such remodeling probably better serves the large variety of functions that cells must perform during cell division compared with during interphase. This is clearly the case during oocyte meiosis, because remodeling of Ca(2+) signals partially defines the competence of the egg to activate at fertilization. Store-operated Ca(2+) entry (SOCE) is a ubiquitous Ca(2+)-signaling pathway that is regulated during M phase. In this Commentary, we discuss the latest advances in our understanding of how SOCE is regulated during cell division.

Involvement of leukotrienes in the pathogenesis of silica-induced pulmonary fibrosis in mice

The authors investigated the role of leukotrienes (LTs) in the pathogenesis of silica-induced pulmonary fibrosis in mice during the progression from acute to chronic phases. Intratracheal instillation of silica particles induced progressive pulmonary fibrosis. The tissue content of cysteinyl (Cys) LTs and LTB(4) was markedly increased in the acute phase after silica instillation, concurrently with the up-regulation of LTB(4) receptor, transforming growth factor (TGF)-beta1, and tumor necrosis factor (TNF)-alpha, along with down-regulation of the CysLT type 2 receptor. Importantly, the tissue content of CysLTs and mRNA levels of TGF-beta1 and TNF-alpha were increased in the fibrotic lung in the chronic phase. Furthermore, strong immunohistochemical staining for the CysLT type 1 receptor, TNF-alpha, and TGF-beta1, but not for the CysLT type 2 receptor, was codetected in the pathological lesions during both acute and chronic phases. These findings suggest that an increase in LT production in the lung and modulation of homeostatic balance among LT receptors may contribute to the progression of pulmonary fibrosis.

Endogenous transport systems in the Xenopus laevis oocyte plasma membrane

Oocytes of the South African clawed frog Xenopus laevis are widely used as a heterologous expression system for the characterization of transport systems such as passive and active membrane transporters, receptors and a whole plethora of other membrane proteins originally derived from animal or plant tissues. The large size of the oocytes and the high degree of expression of exogenous mRNA or cDNA makes them an optimal tool, when compared with other expression systems such as yeast, Escherichia coli or eukaryotic cell lines, for the expression and functional characterization of membrane proteins. This easy to handle expression system is becoming increasingly attractive for pharmacological research. Commercially available automated systems that microinject mRNA into the oocytes and perform electrophysiological measurements fully automatically allow for a mass screening of new computer designed drugs to target membrane transport proteins. Yet, the oocytes possess a large variety of endogenous membrane transporters and it is absolutely mandatory to distinguish the endogenous transporters from the heterologous, expressed transport systems. Here, we review briefly the endogenous membrane transport systems of the oocytes.

HCO3(-)/Cl(-) exchange inactivation and reactivation during mouse oocyte meiosis correlates with MEK/MAPK-regulated Ae2 plasma membrane localization

Background

Germinal Vesicle (GV) stage mouse oocytes in first meiotic prophase exhibit highly active HCO₃⁻/Cl⁻ exchange—a class of transport nearly ubiquitously involved in regulation of intracellular pH and cell volume. During meiosis, however, oocyte HCO₃⁻/Cl⁻ exchange becomes inactivated during first metaphase (MI), remains inactive in second metaphase (MII), and is reactivated only after egg activation. Previous work using pharmacological manipulations had indicated that activity of the MEK/MAPK signaling pathway was negatively correlated with HCO₃⁻/Cl⁻ exchange activity during meiosis. However, the mechanism by which the exchanger is inactivated during meiotic progression had not been determined, nor had the role of MEK/MAPK been directly established.

Methodology/Principal Findings

Expression of a constitutively active form of MEK (MAP kinase kinase), which prevented the normal downregulation of MAPK after egg activation, also prevented reactivation of HCO₃⁻/Cl⁻ exchange. Conversely, suppression of endogenous MAPK activity with dominant negative MEK activated the normally quiescent HCO₃⁻/Cl⁻ exchange in mature MII eggs. A GFP-tagged form of the HCO₃⁻/Cl⁻ exchanger isoform Ae2 (Slc4a2) was strongly expressed at the GV oocyte plasma membrane, but membrane localization decreased markedly during meiotic progression. A similar pattern for endogenous Ae2 was confirmed by immunocytochemistry. The loss of membrane-localized Ae2 appeared selective, since membrane localization of a GFP-tagged human dopamine D1 receptor did not change during meiotic maturation.

Conclusions

Direct manipulation of MAPK activity indicated that GFP-tagged Ae2 localization depended upon MAPK activity. Inactivation of HCO₃⁻/Cl⁻ exchange during the meiotic cell cycle may therefore reflect the loss of Ae2 from the oocyte plasma membrane, downstream of MEK/MAPK signaling. This identifies a novel role for MEK/MAPK-mediated cytostatic factor (CSF) activity during meiosis in membrane protein trafficking in mouse oocytes, and shows for the first time that selective retrieval of membrane proteins is a feature of meiosis in mammalian oocytes.

Internalization of plasma membrane Ca2+-ATPase during Xenopus oocyte maturation

A transient increase in intracellular Ca(2+) is the universal signal for egg activation at fertilization. Eggs acquire the ability to mount the specialized fertilization-specific Ca(2+) signal during oocyte maturation. The first Ca(2+) transient following sperm entry in vertebrate eggs has a slow rising phase followed by a sustained plateau. The molecular determinants of the sustained plateau are poorly understood. We have recently shown that a critical determinant of Ca(2+) signaling differentiation during oocyte maturation is internalization of the plasma membrane calcium ATPase (PMCA). PMCA internalization is representative of endocytosis of several integral membrane proteins during oocyte maturation, a requisite process for early embryogenesis. Here we investigate the mechanisms regulating PMCA internalization. To track PMCA trafficking in live cells we cloned a full-length cDNA of Xenopus PMCA1, and show that GFP-tagged PMCA traffics in a similar fashion to endogenous PMCA. Functional data show that MPF activation during oocyte maturation is required for full PMCA internalization. Pharmacological and co-localization studies argue that PMCA is internalized through a lipid raft endocytic pathway. Deletion analysis reveal a requirement for the N-terminal cytoplasmic domain for efficient internalization. Together these studies define the mechanistic requirements for PMCA internalization during oocyte maturation.

Molecular characterization of a novel cell surface ADP-ribosyl cyclase from the sea urchin

The sea urchin is an extensively used model system for the study of calcium signalling by the messenger molecules NAADP and cyclic ADP-ribose. Both are synthesized by ADP-ribosyl cyclases but our molecular understanding of these enzymes in the sea urchin is limited. We have recently reported the cloning of an extended family of sea urchin ADP-ribosyl cyclases and shown that one of these enzymes (SpARC1) is active within the endoplasmic reticulum lumen. These studies suggest that production of messengers is compartmentalized. Here we characterize the properties of SpARC2. SpARC2 catalyzed both NAADP and cyclic ADP-ribose production. Unusually, the NAD surrogate, NGD was a poor substrate. In contrast to SpARC1, heterologously expressed SpARC2 localized to the plasma membrane via a glycosylphosphatidylinositol (GPI)-anchor. Transcripts for SpARC2 were readily detectable in sea urchin eggs and a majority of the endogenous membrane bound activity was found to be GPI-anchored. Our data reveal striking differences in the properties of sea urchin ADP-ribosyl cyclases and provide further evidence that messenger production may occur outside of the cytosol.

Ca2+ signaling differentiation during oocyte maturation

Oocyte maturation is an essential cellular differentiation pathway that prepares the egg for activation at fertilization leading to the initiation of embryogenesis. An integral attribute of oocyte maturation is the remodeling of Ca2+ signaling pathways endowing the egg with the capacity to produce a specialized Ca2+ transient at fertilization that is necessary and sufficient for egg activation. Consequently, mechanistic elucidation of Ca2+ signaling differentiation during oocyte maturation is fundamental to our understanding of egg activation, and offers a glimpse into Ca2+ signaling regulation during the cell cycle.

Calcium signaling differentiation during Xenopus oocyte maturation

Ca(2+) is the universal signal for egg activation at fertilization in all sexually reproducing species. The Ca(2+) signal at fertilization is necessary for egg activation and exhibits specialized spatial and temporal dynamics. Eggs acquire the ability to produce the fertilization-specific Ca(2+) signal during oocyte maturation. However, the mechanisms regulating Ca(2+) signaling differentiation during oocyte maturation remain largely unknown. At fertilization, Xenopus eggs produce a cytoplasmic Ca(2+) (Ca(2+)(cyt)) rise that lasts for several minutes, and is required for egg activation. Here, we show that during oocyte maturation Ca(2+) transport effectors are tightly modulated. The plasma membrane Ca(2+) ATPase (PMCA) is completely internalized during maturation, and is therefore unable to extrude Ca(2+) out of the cell. Furthermore, IP(3)-dependent Ca(2+) release is required for the sustained Ca(2+)(cyt) rise in eggs, showing that Ca(2+) that is pumped into the ER leaks back out through IP(3) receptors. This apparent futile cycle allows eggs to maintain elevated cytoplasmic Ca(2+) despite the limited available Ca(2+) in intracellular stores. Therefore, Ca(2+) signaling differentiates in a highly orchestrated fashion during Xenopus oocyte maturation endowing the egg with the capacity to produce a sustained Ca(2+)(cyt) transient at fertilization, which defines the egg's competence to activate and initiate embryonic development.

Mechanical Stretch Alters Alveolar Type II Cell Mediator Release toward a Proinflammatory Pattern

Increased mechanical stretch of alveolar type II (ATII) cells occurs during mechanical ventilation. The effects of three patterns of stretching rat ATII cells (frequency [min-1]-Deltasurface area [%]: S40-13, S60-13, S40-30) were compared with those in static cultures at 12, 18, and 24 h. Cell viability and expression of cyclooxygenase-2,5-lipoxygenase, inducible nitric oxide synthase (iNOS), and endothelial nitric oxide synthase (eNOS) were characterized. Supernatants were analyzed for eicosanoids, nitrite, cytokines, and stimulatory effects on rat lymphocytes. S40-13 simulates normal breathing; the other patterns increased amplitude and frequency. There were no significant differences between S40-13 and static cultures. S60-13 only significantly increased the supernatant nitrite (11.2+/-1.6 versus 3.9+/-0.4 microM at 24 h). S40-30 significantly reduced the number of trypan blue-excluding cells, increased the supernatant concentration of TXB2 (4.1+/-0.61 versus 2.2+/-0.36 pg/ml), 6-keto-PGF1alpha (8.7+/-1.0 versus 6.7+/-0.52 pg/ml), cysteinyl-LT (12.2+/-2.0 versus 6.1+/-0.75 pg/ml) and nitrite (7.2+/-1.7 versus 3.9+/-0.4 microM). S40-30 did not alter the release of tumor necrosis factor-alpha and monocyte chemotactic protein-1, but significantly reduced the concentration of the anti-inflammatory interleukin-10 (20.8+/-13.3 versus 130+/-21.5 pg/ml). Expression of cyclooxygenase-2/5-lipoxygenase was increased/decreased; expression of iNOS/eNOS was unchanged by high-amplitude stretch. Supernatants from S40-30 experiments caused lymphocyte activation measured by CD71 and CD54 surface expression. Continuing mechanical distension of ATII cells contributes to an inflammatory response by a shift in the balance of pro- and anti-inflammatory mediators.

Interactions between the sympathetic nervous system and the RAAS in heart failure

Therapy for heart failure caused by left ventricular systolic dysfunction is based on interference with maladaptive activation of the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS). Agents that block beta-adrenergic receptors, decrease angiotensin-II formation, and antagonize the effects of angiotensin II and aldosterone have been shown to improve morbidity and mortality in this syndrome. Therefore, from a theoretical point of view, it would be desirable to actually diminish the degree of overactivity of these two homeostatic systems. There are compelling physiologic arguments and much experimental data to suggest that beta-adrenergic blockade may diminish activity of the RAAS. Conversely, angiotensin-converting enzyme inhibitors, angiotensin-II antagonists, and aldosterone antagonists may diminish activity of the SNS. Some clinical trials data may be interpreted in a fashion that suggests that part of the benefit of interfering with each system may relate to diminishing activity of the other. If true, combined therapy may lead to a virtuous cycle in which diminishing the adverse effects of each individual system is combined with reduced activity of the other. Such a cycle may be one factor underlying the impressive clinical results of recent neurohormonally based therapeutic trials and reinforces the need to look beyond acute hemodynamic effects of therapeutic agents when assessing their long-term impact in heart failure.

Characterization of urea transport in Bufo arenarum oocytes

Xenopus laevis oocytes have been extensively used for expression cloning, structure/function relationships, and regulation analysis of transporter proteins. Urea transporters have been expressed in Xenopus oocytes and their properties have been described. In order to establish an alternative system in which urea transporters could be efficiently expressed and studied, we determined the urea transport properties of ovarian oocytes from Bufo arenarum, a toad species common in Argentina. Bufo oocytes presented a high urea permeability of 22.3 x 10(-6) cm/s, which was significantly inhibited by the incubation with phloretin. The urea uptake in these oocytes was also inhibited by mercurial reagents, and high-affinity urea analogues. The urea uptake was not sodium dependent. The activation energy was 3.2 Kcal/mol, suggesting that urea movement across membrane oocytes may be through a facilitated urea transporter. In contrast, Bufo oocytes showed a low permeability for mannitol and glycerol. From these results, we propose that one or several specific urea transporters are present in ovarian oocytes from Bufo arenarum. Therefore, these oocytes cannot be used in expression studies of foreign urea transporters. The importance of Bufo urea transporter is not known but could be implicated in osmotic regulation during the laying of eggs in water.

Prostaglandin E2 synthesis and suppression of fibroblast proliferation by alveolar epithelial cells is cyclooxygenase-2-dependent

Alveolar epithelial cells (AECs) may influence neighboring fibroblasts by the elaboration of prostaglandin E(2) (PGE(2)). This prostanoid can be synthesized via "constitutive" cyclooxygenase (COX)-1 and "inducible" COX-2 enzyme isoforms. We compared AECs isolated from wild-type (WT), COX-1 knockout (KO), and COX-2 KO mice to determine the contribution of COX isoforms to AEC PGE(2) synthesis and capacity for suppression of fibroblast proliferation in co-cultures. WT AECs constitutively expressed both COX-1 and COX-2 isoforms by immunoblot analysis. COX-1 KO cells and WT cells comparably augmented PGE(2) synthesis following incubation with lipopolysaccharide or interleukin-1, whereas COX-2 KO cells were unable to do so. Surprisingly, however, constitutive generation of PGE(2) was also dramatically reduced only in COX-2 KO cells. When co-cultured with WT murine lung fibroblasts, AECs from WT and COX-1 KO animals suppressed serum-induced fibroblast proliferation, whereas COX-2-deficient AECs caused a modest enhancement in fibroblast proliferation. These results indicate that PGE(2) synthetic capacity in AECs is predominantly COX-2-dependent under both basal and stimulated conditions. They also demonstrate conclusively that AECs can modulate fibroblast function by the elaboration of suppressive prostanoids. These alterations in AEC phenotype likely contribute to the propensity for pulmonary fibrosis observed in COX-2-deficient mice.

Dopamine-induced inhibition of Na+-K+-ATPase activity requires integrity of actin cytoskeleton in opossum kidney cells

The present study evaluated the importance of the association between Na+-K+-ATPase and the actin cytoskeleton on dopamine-induced inhibition of Na+-K+-ATPase activity. The approach used measures the transepithelial transport of Na+ in monolayers of opossum kidney (OK) cells, when the Na+ delivered to Na+-K+-ATPase was increased at the saturating level by amphotericin B. The maximal amphotericin B (1.0 microg mL-1) induced increase in short-circuit current (Isc) was prevented by ouabain (100 microM) or removal of apical Na+. Dopamine (1 microM) applied from the apical side significantly decreased (29 +/- 5% reduction) the amphotericin B-induced increase in Isc, this being prevented by the D1-like receptor antagonist SKF 83566 (1 microM) and the protein kinase C (PKC) inhibitor chelerythrine (1 microM). Exposure of OK cells to cytochalasin B (1 microM) or cytochalasin D (1 microM), inhibitors of actin polymerization, from both cell sides reduced by 31 +/- 4% and 36 +/- 3% the amphotericin B-induced increase in Isc and abolished the inhibitory effect of apical dopamine (1 microM), but not that of the PKC activator phorbol-12,13-dibutyrate (PDBu; 100 nM). Colchicine (1 microM) failed to alter the inhibitory effects of dopamine. The relationship between Na+-K+-ATPase and the concentration of extracellular Na+ showed a Michaelis-Menten constant (Km) of 44.1 +/- 13.7 mM and a Vmax of 49.6 +/- 4.8 microA cm-2 in control monolayers. In the presence of apical dopamine (1 microM) or cytochalasin B (1 microM) Vmax values were significantly (P < 0.05) reduced without changes in Km values. These results are the first, obtained in live cells, showing that the PKC-dependent inhibition of Na+-K+-ATPase activity by dopamine requires the integrity of the association between actin cytoskeleton and Na+-K+-ATPase.

Induction of maturation-promoting factor during Xenopus oocyte maturation uncouples Ca(2+) store depletion from store-operated Ca(2+) entry

During oocyte maturation, eggs acquire the ability to generate specialized Ca(2+) signals in response to sperm entry. Such Ca(2+) signals are crucial for egg activation and the initiation of embryonic development. We examined the regulation during Xenopus oocyte maturation of store-operated Ca(2+) entry (SOCE), an important Ca(2+) influx pathway in oocytes and other nonexcitable cells. We have previously shown that SOCE inactivates during Xenopus oocyte meiosis. SOCE inactivation may be important in preventing premature egg activation. In this study, we investigated the correlation between SOCE inactivation and the Mos-mitogen-activated protein kinase (MAPK)-maturation-promoting factor (MPF) kinase cascade, which drives Xenopus oocyte maturation. SOCE inactivation at germinal vesicle breakdown coincides with an increase in the levels of MAPK and MPF. By differentially inducing Mos, MAPK, and MPF, we demonstrate that the activation of MPF is necessary for SOCE inactivation during oocyte maturation. In contrast, sustained high levels of Mos kinase and the MAPK cascade have no effect on SOCE activation. We further show that preactivated SOCE is not inactivated by MPF, suggesting that MPF does not block Ca(2+) influx through SOCE channels, but rather inhibits coupling between store depletion and SOCE activation.

Rat alveolar type II cells inhibit lung fibroblast proliferation in vitro

Fibroblasts stimulate alveolar type II epithelial cell differentiation and proliferation in vitro and during lung development. However, little is known about the effects of adult type II cells on fibroblasts. We investigated the effect of adult rat type II cells on proliferation of adult human lung fibroblasts. Fibroblasts were suspended within rat tail collagen which was gelled on a floating polycarbonate filter, and type II cells were cultured on Matrigel. In this coculture system, alveolar type II cells inhibited fibroblast proliferation and indomethacin blocked the inhibitory effect on fibroblast growth. Prostaglandin (PG) E2, the major PG secreted by type II cells, inhibited fibroblast proliferation and was increased during the period of inhibition of fibroblast proliferation. Incubation with arachidonate showed that most of the PGE2 in the coculture system was produced by the fibroblasts. In addition, we found that rat type II cells also inhibited rat fibroblasts and that inhibition of fibroblast growth by type II cells could be stimulated by keratinocyte growth factor. We conclude that in this coculture system, type II cells inhibit fibroblast proliferation by secreting a factor(s) that stimulates PGE2 production by fibroblasts, and that PGE2 directly inhibits fibroblast proliferation.

Of mice, frogs and flies: generation of membrane asymmetries in early development

Embryonic development begins with cleavage of the fertilized egg. Cleavage comprises two major processes: cytokinesis and formation of a polarized epithelial cell layer. The focus of this review is comparison of the generation of membrane polarity during embryonic cleavage in three different developmental model systems. In mammalian embryos, as exemplified by analysis of the mouse, generation of distinct membrane domains is uncoupled from cleavage divisions and is initiated in a specific developmental phase, called compaction. In Xenopus laevis embryos, generation of polarized blastomeres occurs simultaneously with cytokinesis. The origin of specific membrane domains of X. laevis polar blastomeres, however, can be traced back to oogenesis. Finally, in Drosophila melanogaster, generation of polarized cells occurs at cellularization. The relevance of cell adhesion, cell junctions and cytocortical scaffolds will be discussed for each of the model systems. Despite enormous morphologic differences, the three models share many common features; in particular, many important molecular interactions are conserved.

Store-operated calcium entry inactivates at the germinal vesicle breakdown stage of Xenopus meiosis

Store-operated calcium entry (SOCE) is the predominant Ca(2+) influx pathway in non-excitable cells and is activated in response to depletion of intracellular Ca(2+) stores. We have studied SOCE regulation during Xenopus oocyte meiosis. SOCE can be measured readily in stage VI Xenopus oocytes arrested at the G(2)-M transition of the cell cycle, either by Ca(2+) imaging or by recording the SOCE current. However, following meiotic maturation, SOCE can no longer be activated by store depletion. We have characterized the time course of SOCE inactivation during oocyte maturation, and show that SOCE inactivates almost completely, in a very short time period, at the germinal vesicle breakdown stage of meiosis. This acute inactivation offers an opportunity to better understand SOCE regulation.

Assembly of tight junctions during early vertebrate development

Tight junction formation during development is critical for embryonic patterning and organization. We consider mechanisms of junction biogenesis in cleaving mouse and Xenopus eggs. Junction assembly follows the establishment of cell polarity at 8-cell (mouse) or 2-cell (Xenopus) stages, characterized by sequential membrane delivery of constituents, coordinated by embryonic (mouse) or maternal (Xenopus) expression programmes. Cadherin adhesion is permissive for tight junction construction only in the mouse. Occludin post-translational modification and membrane delivery, mediated by delayed ZO-1 alpha(+)isoform expression in the mouse, provides a mechanism for completion of tight junction biogenesis and sealing, regulating the timing of blastocoel cavitation.

Low-dose angiotensin II increases free isoprostane levels in plasma

Chronic intravenous infusion of subpressor doses of angiotensin II causes blood pressure to increase progressively over the course of several days. The mechanisms underlying this response, however, are poorly understood. Because high-dose angiotensin II increases oxidative stress, and some compounds that result from the increased oxidative stress (eg, isoprostanes) produce vasoconstriction and antinatriuresis, we tested the hypothesis that a subpressor dose of angiotensin II also increases oxidative stress, as measured by 8-epi-prostaglandin F(2alpha) (isoprostanes), which may contribute to the slow pressor response to angiotensin II. To test this hypothesis, we infused angiotensin II (10 ng/kg per minute for 28 days via an osmotic pump) into 6 conscious normotensive female pigs (30 to 35 kg). We recorded mean arterial pressure continuously with a telemetry system and measured plasma isoprostanes before starting the angiotensin II infusion (baseline) and again after 28 days with an enzyme immunoassay. Angiotensin II infusion significantly increased mean arterial pressure from 121+/-4 to 153+/-7 mm Hg (P<0. 05) without altering total plasma isoprostane levels (180.0+/-24.3 versus 147.0+/-29.2 pg/mL; P=NS). However, the plasma concentrations of free isoprostanes increased significantly, from 38.3+/-5.8 to 54.7+/-10.4 pg/mL (P<0.05). These results suggest that subpressor doses of angiotensin II increase oxidative stress, as implied by the increased concentration of free isoprostanes, which accompany the elevation in mean arterial pressure elevation. Thus, isoprostane-induced vasoconstriction and antinatriuresis may contribute to the hypertension induced by the slow pressor responses of angiotensin II.

Phosphorylation of the alpha-subunits of the Na+/K+-ATPase from mammalian kidneys and Xenopus oocytes by cGMP-dependent protein kinase results in stimulation of ATPase activity

Phosphorylation of Na+/K+-ATPase by cGMP-dependent protein kinase (PKG) has been studied in enzymes purified from pig, dog, sheep and rat kidneys, and in Xenopus oocytes. PKG phosphorylates the alpha-subunits of all animal species investigated. Phosphorylation of the beta-subunit was not observed. The stoichiometry of phosphorylation estimated for pig, sheep and dog renal Na+/K+-ATPase is 3.5, 2.2 and 2.1 mol Pi per mol alpha-subunit, respectively. Proteolytic fingerprinting of the pig alpha1-subunits phosphorylated by PKG using specific antibodies raised against N-terminus or C-terminus reveals that phosphorylation sites are located within the intracellular loop of the alpha-subunit between the 35 kDa N-terminal and 27 kDa C-terminal fragments. Phosphorylation sites within the alpha1-subunit of the purified Na+/K+-ATPase do not appear to be easily accessible for PKG since incorporation of Pi requires 0.2% of Triton X-100. Administration of cGMP and PKG in the presence of 5 mm ATP, which prevents inactivation of the Na+/K+-ATPase by detergent, leads to stimulation of hydrolytic activity by 61%. Administration of 50 microm of cGMP or dbcGMP in yolk-free homogenates of Xenopus oocytes leads to stimulation of ouabain-dependent ATPase activity by 130-198% and to incorporation of 33P into the alpha-subunit without the detergent. Hence, PKG plays regulatory role in active transmembraneous transport of Na+ and K+ via phosphorylation of the catalytic subunit of the Na+/K+-ATPase.

Dopamine-induced endocytosis of Na+,K+-ATPase is initiated by phosphorylation of Ser-18 in the rat alpha subunit and Is responsible for the decreased activity in epithelial cells

Dopamine inhibits Na+,K+-ATPase activity in renal tubule cells. This inhibition is associated with phosphorylation and internalization of the alpha subunit, both events being protein kinase C-dependent. Studies of purified preparations, fusion proteins with site-directed mutagenesis, and heterologous expression systems have identified two major protein kinase C phosphorylation residues (Ser-11 and Ser-18) in the rat alpha1 subunit isoform. To identify the phosphorylation site(s) that mediates endocytosis of the subunit in response to dopamine, we have performed site-directed mutagenesis of these residues in the rat alpha1 subunit and expressed the mutated forms in a renal epithelial cell line. Dopamine inhibited Na+,K+-ATPase activity and increased alpha subunit phosphorylation and clathrin-dependent endocytosis into endosomes in cells expressing the wild type alpha1 subunit or the S11A alpha1 mutant, and both effects were blocked by protein kinase C inhibition. In contrast, dopamine did not elicit any of these effects in cells expressing the S18A alpha1 mutant. While Ser-18 phosphorylation is necessary for endocytosis, it does not affect per se the enzymatic activity: preventing endocytosis with wortmannin or LY294009 blocked the inhibitory effect of dopamine on Na+,K+-ATPase activity, although it did not alter the increased alpha subunit phosphorylation induced by this agonist. We conclude that dopamine-induced inhibition of Na+, K+-ATPase activity in rat renal tubule cells requires endocytosis of the alpha subunit into defined intracellular compartments and that phosphorylation of Ser-18 is essential for this process.

Forskolin-induced down-regulation of Na+,K(+)-ATPase activity is not associated with internalization of the enzyme

Activation by protein kinase A by forskolin phosphorylates and inactivates Na+,K(+)-ATPase in COS-7 cells (Cheng et al. 1997b). In this study we show, using [3H]ouabain binding, that forskolin-induced inhibition of Na+,K(+)-ATPase activity is not because of internalization of the enzyme. The effect of forskolin on Na+,K(+)-ATPase activity was examined by two independent methods, ouabain-sensitive 86Rb+ uptake in intact cells and ATP hydrolysis in microsomal preparations from cells. The change in number of functional pumps on cell surface before and after protein kinase A activation was assessed by [3H]ouabain binding measured under equilibrium conditions. Cells, which had been ATP-depleted by antimycin A and 2-deoxyglucose treatment, served as a positive control for the internalization of Na+,K(+)-ATPase. Activation of protein kinase A with forskolin in combination with the phosphodiesterase inhibitor 3-isobutyl-1-methyl xanthine, inhibited Na+,K(+)-ATPase activity, but this treatment had no effect on specific ouabain binding. No change in ouabain binding was found following activation of protein kinase C by phorbol ester or diacyl glycerol analogue treatment in cells. These data suggest that protein kinase A phosphorylation and inhibition of Na+,K(+)-ATPase activity does not lead to any internalization of the enzyme in COS-7 cells.

Receptor-mediated inhibition of renal Na(+)-K(+)-ATPase is associated with endocytosis of its alpha- and beta-subunits

The mechanisms involved in receptor-mediated inhibition of Na(+)-K(+)-ATPase remain poorly understood. In this study, we evaluate whether inhibition of proximal tubule Na(+)-K(+)-ATPase activity by dopamine is linked to its removal from the plasma membrane and internalization into defined intracellular compartments. Clathrin-coated vesicles were isolated by sucrose gradient centrifugation and negative lectin selection, and early and late endosomes were separated on a flotation gradient. Inhibition of Na(+)-K(+)-ATPase activity by dopamine, in contrast to its inhibition by ouabain, was accompanied by a sequential increase in the abundance of the alpha-subunit in clathrin-coated vesicles (1 min), early endosomes (2.5 min), and late endosomes (5 min), suggesting its stepwise translocation between these organelles. A similar pattern was found for the beta-subunit. The increased incorporation of both subunits in all compartments was blocked by calphostin C. The results demonstrate that the dopamine-induced decrease in Na(+)-K(+)-ATPase activity in proximal tubules is associated with internalization of its alpha- and beta-subunits into early and late endosomes via a clathrin-dependent pathway and that this process is protein kinase C dependent. The presence of Na(+)-K(+)-ATPase subunits in endosomes suggests that these compartments may constitute normal traffic reservoirs during pump degradation and/or synthesis.

Biophysical properties of epithelial water channels

The biophysical models describing the structure of water pores or channels have evolved, during the last forty years, from a pure 'black box' approach to a molecular based proposal. The initial 'sieving pore' in which water and other molecules were moving together was replaced by a more restrictive model, where water is moving alone in a 'single file' mode. Aquaporins discovery and cloning [G.M. Preston, T.P. Carroll, W.B. Guggino, P. Agre, Science 256 (1992) 365] leaded to the 'hour-glass model' and other alternative proposals, combining information coming from molecular biology experiments and two dimensional crystallography. Concerning water transfers in epithelial barriers the problem is quite complex, because there are at least two alternative pathways: paracellular and transcellular and three different driving forces: hydrostatic pressure, osmotic pressure or 'transport coupled' movements. In the case of ADH-sensitive epithelia it is more or less accepted that regulated water channels (AQP2), that can be inserted in the apical membrane, coexist with basolateral resident water channels (AQP3). The mechanism underlying the so-called 'transport associated water transfer' is still controversial. From the classical standing gradient model to the ion-water co-transport, different hypothesis are under consideration. Coming back to hormonal regulations, other than the well-known regulation by neuro-hypophysis peptides, a steroid second messenger, progesterone, has been recently proposed [P. Ford, G. Amodeo, C. Capurro, C. Ibarra, R. Dorr, P. Ripoche, M. Parisi, Am. J. Physiol. 270 (1996) F880].

Cytoplasmic ATP-dependent regulation of ion transporters and channels: mechanisms and messengers

Many ion transporters and channels appear to be regulated by ATP-dependent mechanisms when studied in planar bilayers, excised membrane patches, or with whole-cell patch clamp. Protein kinases are obvious candidates to mediate ATP effects, but other mechanisms are also implicated. They include lipid kinases with the generation of phosphatidylinositol phosphates as second messengers, allosteric effects of ATP binding, changes of actin cytoskeleton, and ATP-dependent phospholipases. Phosphatidylinositol-4,5-bisphosphate (PIP2) is a possible membrane-delimited messenger that activates cardiac sodium-calcium exchange, KATP potassium channels, and other inward rectifier potassium channels. Regulation of PIP2 by phospholipase C, lipid phosphatases, and lipid kinases would thus tie surface membrane transport to phosphatidylinositol signaling. Sodium-hydrogen exchange is activated by ATP through a phosphorylation-independent mechanism, whereas ion cotransporters are activated by several protein kinase mechanisms. Ion transport in epithelium may be particularly sensitive to changes of cytoskeleton that are regulated by ATP-dependent cell signaling mechanisms.

Angiotensin converting enzyme inhibition and sympathetic activity in healthy subjects

BACKGROUND

One suggested mechanism for the reduction in mortality rates resulting from the use of angiotensin converting enzyme inhibitors in congestive heart failure is the inhibition of the angiotensin II-mediated norepinephrine release. Direct evidence for this mechanism is lacking in humans.

SUBJECTS AND METHODS

We examined the effects of captopril, 25 mg three times a day, or matched placebo for 7 days on sympathetic activity during a 10 mEq/day sodium diet in seven healthy male subjects aged 30 +/- 3 (SEM) years. A tritiated norepinephrine radioisotope dilution technique was used to measure sympathetic activity, both at rest and during isometric handgrip exercise.

RESULTS

Captopril blunted the increase in mean arterial pressure during isometric handgrip exercise (placebo, from 81 +/- 4 to 112 +/- 2 mm Hg; captopril, from 78 +/- 3 to 101 +/- 2 mm Hg; p < 0.01). However, the increase in systemic norepinephrine spillover during isometric handgrip exercise was not blunted by captopril. Captopril had no effect on resting mean arterial pressure or systemic norepinephrine spillover.

CONCLUSIONS

Captopril did not attenuate baseline or static exercise-stimulated sympathetic activity in healthy subjects. These findings would indicate that angiotensin converting enzyme inhibition does not decrease sympathetic activity at rest or during the stimulus of isometric handgrip exercise.

Autoprotective mechanisms in the CNS: some new lessons from white matter

Anoxia/ischemia in the CNS is a common and devastating phenomenon. It is possible that the best hopes for protection against anoxic/ischemic injury may involve recruiting and/or augmenting any autoprotective systems that evolution has provided for the CNS. We describe here the existence of such an autoprotective system present in CNS white matter. White matter is both well suited to studying extrasynaptic systems, such as the system we describe here, and is a highly appropriate target for research into anoxic-ischemic injury in its own right. We show that white matter contains functional GABAB and adenosine receptors that respond to an anoxic efflux of GABA and adenosine by recruiting a convergent intracellular mechanism involving protein kinase C (PKC). The net result of this receptor-mediated cascade is an increase in resistance to anoxia, which presumably allows CNS white matter to tolerate better a common class of ischemic events that are located solely in white matter and that comprises approximately 25% of all strokes seen clinically.

The Ca(2+)-induced leak current in Xenopus oocytes is indeed mediated through a Cl- channel

Defolliculated oocytes of Xenopus laevis responded to removal of external divalent cations with large depolarizations and, when voltage clamped, with huge currents. Single channel analysis revealed a Cl- channel with a slope conductance of about 90 pS at positive membrane potentials with at least four substates. Single channel amplitudes and mean channel currents had a reversal potential of approximately -15 mV as predicted by the Nernst equation for a channel perfectly selective for Cl-. Readdition of Ca2+ immediately inactivated the channel and restored the former membrane potential or clamp current. The inward currents were mediated by a Ca2+ inactivated Cl- channel (CaIC). The inhibitory potency of Ca2+ was a function of the external Ca2+ concentration with a half maximal blocker concentration of about 20 microM. These channels were inhibited by the Cl- channel blockers flufenamic acid, niflumic acid and diphenylamine-2-carboxylate (DPC). In contrast, 4,4'-acetamido-4'-isothiocyanatostilbene-2, 2'-disulfonicacid (SITS), another Cl- channel blocker, led to activation of this Cl- channel. Like other Cl- channels, the CaIC was activated by cytosolic cAMP. Extracellular ATP inhibited the channel while ADP was without any effect. Injection of phorbol 12-myristate 13-acetate (PMA), a protein kinase C activating phorbol ester, stimulated the Cl- current. Cytochalasin D, an actin filament disrupting compound, reversibly decreased the clamp current demonstrating an influence of the cytoskeleton. The results indicate that removal of divalent cations activates Cl- channels in Xenopus oocytes which share several features with Cl- channels of the CLC family. The former so-called leak current of oocytes under divalent cation-free conditions is nothing else than an activation of Cl- channels.

Cytokine-induced neutrophil chemoattractant production by primary rat alveolar type II cells

This study was designed to determine the production of the chemokine cytokine-induced neutrophil chemoattractant (CINC) by primary rat alveolar type II (ATII) cells upon stimulation with exogenous and endogenous proinflammatory factors. Cultures of primary rat ATII cells were exposed to lipopolysaccharide (LPS), interleukin-1 beta (IL-1 beta) or tumor necrosis factor-alpha (TNF alpha) over a 16 hour period and the production of CINC both apically and basolaterally was measured by ELISA. Compared to unstimulated (UNS) cultures, LPS, IL-1 beta and TNF alpha were found to significantly increase the level of CINC detected in culture by two, four and sixteen hours post stimulation, respectively. ATII cells also demonstrated a polar secretion of CINC. The accumulation of CINC basolaterally was significantly more than apically; 133%, 45%, 117% and 123% for UNS, IL-1 beta, LPS and TNF alpha respectively. We demonstrated that primary rat ATII cells may participate in the chemokine network during inflammation by the production of CINC upon stimulation with endogenous and exogenous factors.

Involvement of the GTP binding protein Rho in constitutive endocytosis in Xenopus laevis oocytes

To study an endocytotic role of the GTP-binding protein RhoA in Xenopus oocytes, we have monitored changes in the surface expression of sodium pumps, the surface area of the oocyte and the uptake of the fluid-phase marker inulin. Xenopus oocytes possess intracellular sodium pumps that are continuously exchanged for surface sodium pumps by constitutive endo- and exocytosis. Injection of Clostridium botulinum C3 exoenzyme, which inactivates Rho by ADP-ribosylation, induced a redistribution of virtually all intracellular sodium pumps to the plasma membrane and increased the surface area of the oocytes. The identical effects were caused by injection of ADP-ribosylated recombinant RhoA into oocytes. The C3 exoenzyme acts by blocking constitutive endocytosis in oocytes, as determined using a mAb to the beta 1 subunit of the mouse sodium pump as a reporter molecule and oocytes expressing heterologous sodium pumps. In contrast, an increase in endocytosis and a decrease in the surface area was induced by injection of recombinant Val14-RhoA protein or Val14-rhoA cRNA. PMA stimulated sodium pump endocytosis, an effect that was blocked by a specific inhibitor of protein kinase C (Gö 16) or by ADP-ribosylation of Rho by C3. Similarly, the phorbol ester-induced increase in fluid-phase endocytosis in oocytes was inhibited by Gö 16, C3 transferase, or by injection of ADP-ribosylated RhoA. In contrast to C3 transferase, C. botulinum C2 transferase, which ADP-ribosylates actin, had no effect on sodium pump endocytosis or PMA-stimulated fluid-phase endocytosis. The data suggests that RhoA is an essential component of a presumably clathrin-independent endocytic pathway in Xenopus oocytes which can be regulated by protein kinase C.

Angiotensin II augments reflex activity of the sympathetic nervous system during cardiopulmonary resuscitation in pigs

BACKGROUND

During hypotensive states, angiotensin II augments reflex activity of the sympathetic nervous system. The purpose of the present study was to assess the effects of this vasoconstrictor on myocardial blood flow and plasma catecholamine concentrations during and after CPR.

METHODS AND RESULTS

After 4 minutes of ventricular fibrillation and 3 minutes of open-chest CPR, 14 pigs (24 to 26 kg) were randomized into two groups receiving either saline (n = 7) or 0.05 mg/kg angiotensin II (n = 7). Arterial plasma catecholamine concentration was measured with high-pressure liquid chromatography. Organ blood flow was measured with radiolabeled microspheres. During CPR, after drug administration, left ventricular myocardial blood flow was significantly higher in the angiotensin II-treated group than in the control group. During CPR, median epinephrine concentrations before and 90 seconds and 5 minutes after drug administration were 63.0, 35.2, and 22.5 ng/mL, respectively, in the control group and 63.2, 139.8, and 154.2 ng/mL, respectively, in the angiotensin II group (P < .001 at 90 seconds and P < .01 at 5 minutes). At the same times, median norepinephrine concentrations were 52.6, 59.8, and 33.9 ng/mL, respectively, in the control group and 42.5, 98.7, and 111.3 ng/mL, respectively, in the angiotensin II group (P < .01 at 5 minutes). Restoration of spontaneous circulation was possible in all of the angiotensin II-treated pigs, whereas only 3 of the 7 saline-treated pigs could be resuscitated. At 5 minutes after successful resuscitation, epinephrine was 6.8 ng/mL in the control group and 16.1 ng/mL in the angiotensin II group (P < .05).

CONCLUSIONS

During CPR, angiotensin II appears to increase coronary perfusion pressure and myocardial blood flow, not only by direct peripheral arteriolar vasoconstriction via angiotensin II receptors but also by inducing a massive catecholamine release with adrenergic peripheral vasoconstriction.

Modulation of cardiac autonomic control in humans by angiotensin II

Angiotensin II (Ang II) exerts an inhibitory action on vagal activity in animals and may also facilitate sympathetic activity. The object of this study was to compare autonomic activity resulting from equivalent steady-state baroreflex activation during intravenous Ang II infusion with that resulting from a control infusion of phenylephrine. Eight healthy subjects aged 22 to 34 years were studied in a single-blind, randomized, prospective crossover study. Autonomic activity was determined by computer analysis of RR interval variability in the time and frequency domains. Despite equal experimental hypertension with Ang II and phenylephrine infusion, at peak infusion rates the mean RR interval was significantly shorter with Ang II (983 +/- 179 milliseconds; mean +/- SD) than with phenylephrine (1265 +/- 187 milliseconds, P < .01). The variability of RR intervals was not significantly different, but the variability (median interquartile difference) of RR interval successive differences was significantly lower with Ang II (66 milliseconds) than with phenylephrine (104 milliseconds, P < .02). Power spectral analysis revealed the power of the 0.25-Hz component in normalized units to be significantly smaller during Ang II infusion (20.5 +/- 12.7 U) than during phenylephrine (38.2 +/- 14.7 U, P < .05), whereas the power of the 0.1-Hz component was significantly greater during Ang II infusion (67.8 +/- 17.1 U) than phenylephrine (38.8 +/- 20.3 U, P < .05). Measures of vagal modulation of heart rate were significantly attenuated, and sympathetic modulation appeared to be increased during Ang II infusion compared with control phenylephrine infusions. These observations may underlie reports of increased vagal activity during angiotensin-converting enzyme inhibitor therapy.

Epithelial cell polarity in early Xenopus development

The Xenopus blastula consists of two morphologically distinct cell types. Polarized epithelial cells build up the embryonic surface and fence off an inner non-polarized cell population. We examined the establishment of this early functional cell diversification in the embryo by single cell analysis, in vitro cell culture, and transplantation experiments. Single blastomeres from a 64-cell embryo (1/64 cells) exhibit several features of polarized cells. The plasma membrane of 1/64 cells consists of an apical domain, which is inherited from the original egg membrane, and a basolateral domain derived from newly formed membrane during cleavage. These are inherent, cell-autonomous properties of the blastomeres, as they form and are maintained in blastomeres raised in the absence of any cell interactions in calcium free medium. Upon in vitro culture a single 1/64 cell gives rise to an aggregate of two different cell types. Cells carrying a part of the former egg membrane domain differentiate into polarized epithelial cells, whereas cells lacking this membrane domain are not polarized. These results demonstrate that the inclusion of the egg membrane, rather than external signals related to the position of a cell in the intact embryo, is required for the apical/basolateral differentiation of the surface epithelium. This view is supported by cell transplantation studies. A single 1/64 cell was implanted into the blastocoel of a stage 8 blastula embryo. The progeny of the implanted cell proliferate within the host embryo and split into two morphologically distinct populations with different cell behaviours. Cells incorporating a part of the egg membrane form coherent patches of polarized epithelial cell sheets in the interior of the host embryo. In contrast, cells lacking egg membrane do not exhibit any characteristics of polarized cells and eventually spread into different regions of the host embryo. Our results show that the egg membrane and/or components of the submembrane cortex play a determinative role in the formation of the blastula epithelium.

Angiotensin II. Adrenergic sympathetic constrictor action in humans

BACKGROUND

Angiotensin II (Ang II) facilitates adrenergic neurotransmission in normotensive and hypertensive subjects, whereas angiotensin-converting enzyme inhibitors have been shown to depress circulating catecholamine concentrations in some studies. We investigated the effect of local intra-arterial infusion of Ang II into the brachial artery of healthy volunteers during blockade of postsynaptic alpha-receptors with phentolamine. The response was compared with that seen with Ang II infused during nitroprusside administration at a dose designed to give a dilator response similar to that with phentolamine.

METHODS AND RESULTS

Ang II (6.25, 25, and 100 pmol/min) was infused alone and then together with sodium nitroprusside (4 micrograms/min) and phentolamine (40 micrograms/min) in eight healthy volunteers. Forearm blood flow was measured by strain-gauge plethysmography. The percentage reduction in forearm blood flow produced by Ang II 100 pmol/min in the phentolamine-predilated vascular bed was significantly lower than that seen in the sodium nitroprusside-predilated forearm bed (28.1 +/- 2.9% versus 52.9 +/- 4.2%; P = .006). Comparison of the rate of change of blood flow in response to quadrupling doses of Ang II during blockade of alpha-receptors with phentolamine and during nitroprusside administration was calculated from the mean slope of the regression line of log-transformed blood flow versus dose of Ang II. The mean slope during nitroprusside administration (-0.16 +/- 0.025) was significantly greater than that during blockade with phentolamine (-0.098 +/- 0.020) (P = .046).

CONCLUSIONS

We conclude that a significant part of the vasoconstrictive action of exogenous Ang II on forearm resistance vessels in humans is sympathetically mediated.

Membrane permeability changes during Rana oocyte maturation

A transition from an open system to a closed one must occur during the complex process of meiotic maturation of the amphibian oocyte. Membrane permeability to urea in Rana oocytes following progesterone stimulation was determined, and the largest decrease was found to coincide with germinal vesicle breakdown. These findings suggest that the timing of the disappearance of membrane permeability correlates with developmental events that prepare the oocyte for a hostile environment.

A mutation of the Drosophila sodium pump alpha subunit gene results in bang-sensitive paralysis

A bang-sensitive enhancer trap line was isolated in a behavioral screen. The flies show a weak bang-sensitive paralysis, recovering after about 7 s. The P element insert is localized at 93B1-2 on the salivary chromosomes, the site of the (Na+,K+)ATPase alpha subunit gene. Molecular characterization demonstrates that the transposon is inserted into the first intron of this gene. This insertion leads to normal-sized transcripts, but reduced levels of expression. This change is also reflected in lower amounts of a normal-sized alpha subunit protein. Mutant flies show a much greater sensitivity to ouabain, likewise indicating, on a functional level, a reduction in Na+ pump activity. Furthermore, the bang-sensitive behavior can also be mimicked by injecting sublethal doses of ouabain into wild-type flies. The molecular and functional evidence indicates that the insertion has produced a hypomorphic mutation of the (Na+,K+)ATPase alpha subunit gene, opening the way to future studies of the regulation of the Na+ pump.

Differential effects of cytoskeletal agents on hemispheric functional expression of cell membrane receptors in Xenopus oocytes

1. We studied the effects of three cytoskeleton-disrupting agents, colchicine (COL), vinblastine (VIN), cytochalasins, on the functional hemispheric expression of native muscarinic and acquired thyrotropin-releasing hormone receptors TRH-Rs). Responses in oocytes of common donors, which express M3-like receptors (M3Rs), were not affected by either COL or VIN on the animal hemisphere. The functional expression of M3Rs on the vegetal hemisphere was inhibited by 50%. Cytochalasin B caused a uniform inhibition (by 31-33%) of receptor functional expression on either hemisphere. 2. Oocytes of variant donors express predominantly M1-like receptors (M1Rs) on the animal and M3Rs on the vegetal hemisphere. In these oocytes, both COL and VIN caused approximately 50% inhibition of functional expression on either hemisphere. Cytochalasin B caused more extensive, though variable inhibition on both hemispheres. Both antitubulin agents had no effect on the functional expression of the TRH-Rs on either hemisphere. Cytochalasin B, however, caused an extensive inhibition of the functional expression of this receptor (by 70-75%). 3. Induction of maturation of oocytes (7-hr incubation with progesterone) resulted in a 66% decrease in the response to TRH, reflecting mainly a decrease on the animal hemisphere. Maturation in the presence of colchicine had no further effect on the activity measured on the animal hemisphere but caused a major increase in the activity on the vegetal hemisphere. This resulted in a dramatic change in animal/vegetal activity ratio (4.8 +/- 1.5 to 0.8 +/- 0.2). 4. It appears that while antitubulin drugs affect the functional expression of the three receptors at the two hemispheres differently, disruption of the microfilaments interferes uniformly with receptor functional expression. We suggest that microfilaments may be involved in a common component of the signal transduction pathway in oocytes or in the anchoring of receptors coupled to the guaninine nucleotide-binding regulatory proteins. Moreover, progesterone-induced changes in the functional organization of the signal transduction pathway appear to be controlled to a large extent by the tubulin component of the cytoskeleton.

Maturation induced internalization of beta 1-integrin by Xenopus oocytes and formation of the maternal integrin pool

A pool of beta 1-integrin, ready to be inserted into the cleavage membranes, is present in the cytoplasm of the Xenopus egg, while its plasma membrane is devoid of this membrane protein (Gawantka et al., 1992). The underlying mechanisms that lead to this specific pattern of beta 1-integrin distribution in the egg have been investigated. beta 1-Integrin is present on the oocyte membrane throughout oogenesis. During maturation the oocyte membrane is cleared of beta 1-integrin via internalization of the protein by the oocyte. Synthesis of beta 1-integrin precursor is stimulated moderately in the maturing oocyte. At the same time processing of the precursor into the mature form of beta 1-integrin and its complexing with a putative alpha-chain is greatly accelerated. This way a maternal integrin pool accumulates in the mature oocyte. It is localized in conspicuous yolk free patches which contain large amounts of endoplasmic reticulum, Golgi complexes and smooth vesicles. We suggest that membrane vesicles harbouring the beta 1-integrin are generated in these cytoplasmic regions and that this store of vesicles provides the material source for the rapid membrane formation during cleavage.

Nonparallel patterns of circadian pancreatic and biliary secretions in fasting rats

We compared the circadian patterns of pancreatic and biliary secretions in fasting rats. For this purpose, indwelling plastic catheters were placed in 10 male Wistar rats (300-320 g) for the collection of biliary and pancreatic secretions. After small samples were taken for analysis, pancreatic and biliary secretions were recirculated into the duodenum by an additional connecting system. The rats were adapted to an inverse night-day cycle by artificial light during the night (8 PM-8 AM) and by darkroom housing at daytime (8 AM-8 PM). During a 24-h fasting period, samples of bile (100 microL) and pancreatic juice (20 microL) were taken every hour for determination of the following parameters: pancreatic and biliary flow rate, protein, amylase, lipase, trypsin, and bile acid output. Peak pancreatic flow rate (1.96 +/- 0.05 mL/h.kg) was achieved toward the end of the dark period at 7 PM. A significant increase of pancreatic secretion could be achieved merely by turning the lights off, a significant decrease by turning the lights on. Similar circadian patterns were found for pancreatic protein, amylase, and lipase output with peak secretions at 7 PM. An increase of nearly 5x was found between minimal (15.64 +/- 0.65 mg/h.kg) and maximal (72.43 +/- 2.83 mg/h.kg) pancreatic protein output. The amplitude was highest for amylase; peak amylase output (13740 +/- 832 U/h.kg) was about 18-fold above minimal output (758 +/- 44.3 U/h.kg). Conversely, the peak of trypsin concentration in pancreatic juice (1095 +/- 17.8 U/mL) occurred during the light period when flow rates were lowest.(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in eicosanoid production by rat alveolar type II cells isolated after silica-induced lung injury

Although alveolar type II cells in primary culture have been shown to produce eicosanoids and exposure of type II cells to silica in vitro alters eicosanoid production, the production of eicosanoids by alveolar type II cells isolated after acute lung injury in vivo has not been evaluated. Therefore, we investigated the production of arachidonic acid (AA) metabolites by alveolar type II cells isolated after silica-induced lung injury. Alveolar type II cells were isolated from rats 14 days after intratracheal silica instillation and from untreated animals. Type II cells were separated into normotrophic and hypertrophic populations by centrifugal elutriation, and secreted eicosanoids were determined under basal and stimulated conditions by enzyme immunoassay on the day of isolation and after 1 day in culture. Under basal conditions, freshly isolated type II cells from silica-treated animals produced more prostaglandin (PG) E2 than 6-keto-PGF1 alpha or thromboxane B2 (TxB2). Production of all three prostanoids increased with increasing cell size. The calcium ionophore A23187 stimulated a less than 2-fold increase in PGE2 and 6-keto-PGF1 alpha production in all groups of cells. In contrast, this calcium ionophore greatly enhanced TxB2 and leukotriene C4 (LTC4) production by normotrophic type II cells from both untreated and silica-treated animals. Incubation with exogenous AA suggested that the increased capability of the hypertrophic cells to synthesize PGE2 and TxB2 was due primarily to an increase in arachidonate availability. The hypertrophic type II cells also appear to have increased prostacyclin synthase activity. There were no differences in the catabolism of PGE2 between the normotrophic and the hypertrophic type II cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Up-regulation of sodium pump activity in Xenopus laevis oocytes by expression of heterologous beta 1 subunits of the sodium pump

Recent evidence suggests that the beta subunit of the Na+ pump is essential for the alpha subunit to express catalytic activity and for assembly of the holoenzyme in the plasma membrane. We report here that injection into Xenopus laevis oocytes of cRNAs specific for beta 1 subunit isoforms of the Na+ pump of four species (Torpedo californica, chicken, mouse and rat) causes a time-dependent increase in the number of ouabain-binding sites, both in the plasma membrane and in internal membranes. Expression of the beta 1 subunit of the Na+ pump of mouse and rat in the oocytes could be substantiated by immunoprecipitation using a polyclonal antiserum against the mouse beta 1 subunit. Scatchard analysis in permeabilized cells disclosed that the affinity for ouabain is unchanged after expression of each of the beta 1 subunits. A proportional increase in ouabain-sensitive 86Rb+ uptake indicates that the additionally expressed ouabain-binding sites on the cell surface represent functional Na+ pumps. The findings support the concept of Geering. Theulaz, Verrey, Häuptle & Rossier [(1989) Am. J. Physiol. 257, C851-C858] that beta 1 subunits expressed in oocytes associate with an excess of endogenous alpha subunits of the Na+ pump to form a hybrid enzyme. In addition, all of the beta 1 isoforms investigated in the present study were also capable of combining with the co-expressed alpha 1 subunit of the Torpedo Na+ pump to produce a functional enzyme. Injection of cRNA encoding for the Torpedo alpha 1 subunit alone had no effect on the ouabain-binding capacity of the surface and intracellular membranes of the oocyte.