Human chorionic gonadotropin secretion from the early human placenta: in vitro regulation by progesterone and its antagonist

Local actions of placental neurohormones and sex steroids have been proposed to play a role in the regulation of human chorionic gonadotropin (hCG) release from the human placenta. Accordingly, we utilized an in vitro perifusion system and cultures of placental explants to investigate short- and long-term effects of progesterone and its respective antagonist on hCG secretion from the human first-trimester placenta. Tissue slices (100 mg) obtained from legal pregnancy terminations of 9-12 weeks of gestation were continuously perifused and the effluent collected in fractions of 2-20 min. After initial perifusion periods of 30-140 min, either progesterone, a progesterone antagonist (ZK 98-299), or both were added to the perifusion medium at final concentrations of 10(-4)-10(-9) mol/l. Administration was either continuous or intermittent in 10-min pulses. Furthermore, 50-mg pieces of placental explants were cultured in multiwell tissue culture plates for up to 5 days. During the perifusion studies, hCG (determined by enzyme immunoassay) was released in a pulsatile fashion. This hCG pulsatility was decreased in response to both progesterone and progesterone antagonist at all concentrations tested. In contrast, intermittent administration of either treatment increased the hCG secretion. Secretion of hCG was not affected when progesterone and its antagonist were co-administered at equimolar concentrations. These observations demonstrate the diverging effects of progesterone and its antagonist on hCG secretion from the human first-trimester placenta in vitro, depending on the experimental conditions. Thus, progesterone-modulated hCG secretion appears to be regulated in a complex manner, its fine tuning involving other, as yet uninvestigated intraplacental factors.

Oligomerization of Escherichia coli haemolysin (HlyA) is involved in pore formation

Coexpression of pairs of nonhaemolytic HlyA mutants in the recombination-deficient (recA) strain Escherichia coli HB101 resulted in a partial reconstitution of haemolytic activity, indicating that the mutation in one HlyA molecule can be complemented by the corresponding wild-type sequence in the other mutant HlyA molecule and vice versa. This suggests that two or more HlyA molecules aggregate prior to pore formation. Partial reconstitution of the haemolytic activity was obtained by the combined expression of a nonhaemolytic HlyA derivative containing a deletion of five repeat units in the repeat domain and several nonhaemolytic HlyA mutants affected in the pore-forming hydrophobic region. The simultaneous expression of two inactive mutant HlyA proteins affected in the region at which HlyA is covalently modified by HlyC and the repeat domain, respectively, resulted in a haemolytic phenotype on blood agar plates comparable to that of wild-type haemolysin. However, complementation was not possible between pairs of HlyA molecules containing site-directed mutations in the hydrophobic region and the modification region, respectively. In addition, no complementation was observed between HlyA mutants with specific mutations at different sites of the same functional domain, i.e. within the hydrophobic region, the modification region or the repeat domain. The aggregation of the HlyA molecules appears to take place after secretion, since no extracellular haemolytic activity was detected when a truncated but active HlyA lacking the C-terminal secretion sequence was expressed together with a nonhaemolytic but transport-competent HlyA mutant containing a deletion in the repeat domain.

[Blood screening for Down's syndrome in women under 35 years of age with an age-independent index]

The need for alternatives to amniocentesis, beset by 1% abortion rate, has been increasingly voiced over the past years and also includes the under 35 age group. However, the known age-dependent screening programs such as Alpha program according to Wald (1988) and the Dermalog Program according to Norgaard-Pederson (1990) only yield a detection rate of 50 or, at best, 70% (Müller, 1992) for this age group. We tried to improve this result by index calculation. Compared to the tested age-dependent screening programs the Ulm Index achieves a 25 to 40% higher detection rate (85%). For pregnant women beyond 35 years the detection rate of Down's syndrome can even be raised to 95-100% by a combination of the Alpha or Dermalog program with the Ulm Index, without entailling an increase in the rate of false-positive results.

Abscisic acid-lipid interactions: a phospholipid monolayer study

Lipid monolayer studies were performed on a Langmuir trough in the absence and in the presence of the plant hormone abscisic acid (ABA). The ABA-induced effects on the lipid monolayers can be summarized as follows: (i) ABA as the free acid (pH below 5.3) increased the molecular area and slightly decreased the surface pressure in the collapse points of monolayers made of saturated, unsaturated and of mixed lipids; ABA as the anion showed only minor effects. (ii) The ABA-induced area increase of the lipid monolayers decreased when the surface pressure increased, but some ABA remained in the monolayers made of unsaturated phospholipids even at collapse pressure. (iii) The incorporation of ABA into the monolayers could be inhibited by adding the plant sterol beta-sitosterol to the monolayer forming phospholipids. (iv) There was no substantial difference of ABA action on plant phospholipids as compared with other phospholipids. (v) ABA had a much stronger influence on unsaturated phospholipids than on saturated ones. (vi) ABA decreased the phase-transition temperature of saturated phospholipids. These results, which agree with those obtained from phospholipid vesicle studies, indicate that the physical state of the lipid is important for the ability of ABA penetrating into the lipid monolayer. Finally, a possible relevance of these results is discussed in terms of the action of ABA on guard cell membranes of plants.

Conversion of the FhuA transport protein into a diffusion channel through the outer membrane of Escherichia coli

The FhuA receptor protein is involved in energy-coupled transport of Fe3+ via ferrichrome through the outer membrane of Escherichia coli. Since no energy source is known in the outer membrane it is assumed that energy is provided through the action of the TonB, ExbB and ExbD proteins, which are anchored to the cytoplasmic membrane. By deleting 34 amino acid residues of a putative cell surface exposed loop, FhuA was converted from a ligand specific transport protein into a TonB independent and nonspecific diffusion channel. The FhuA deletion derivative FhuA delta 322-355 formed stable channels in black lipid membranes, in contrast to wild-type FhuA which did not increase membrane conductance. The single-channel conductance of the FhuA mutant channels was at least three times larger than that of the general diffusion porins of E. coli outer membrane. It is proposed that the basic structure of FhuA in the outer membrane is a channel formed by beta-barrels. Since the loop extending from residue 316 to 356 is part of the active site of FhuA, it probably controls the permeability of the channel. The transport-active conformation of FhuA is mediated by a TonB-induced conformational change in response to the energized cytoplasmic membrane. The ferrichrome transport rate into cells expressing FhuA delta 322-355 increased linearly with increasing substrate concentration (from 0.5 to 20 microM), in contrast to FhuA wild-type cells, which displayed saturation at 5 microM. This implies that in wild-type cells ferrichrome transport through the outer membrane is the rate-limiting step and that TonB, ExbB and ExbD are only required for outer membrane transport.

Chronobiology of prolactin secretion in women: diurnal and sleep-related variations in the pituitary lactotroph sensitivity

While a nocturnal rise accounts for the marked circadian variability of prolactin (PRL) secretion in humans, the mechanisms subserving this neuroendocrine manifestation are still obscure. Since gonadotropin-releasing hormone (GnRH) stimulates PRL under physiological conditions, we questioned whether changes in the pituitary lactotroph sensitivity to GnRH during the 24-hour cycle may contribute to the expression of circadian PRL rhythmicity. Accordingly, 8 women were studied in the early follicular phase of their cycles (days 2-5) on 6 occasions in random order: during daytime between 10.00 and 14.00 h ('day' studies), at night between 22.00 and 02.00 h, when the women were awake ('night' studies), and finally, during the identical night hours, when the women were asleep ('sleep' studies). On all occasions, blood was collected at 10-min intervals for 4 h, while either GnRH (25 micrograms i.v. bolus) or saline (as control) was injected twice within 2 h. As assessed by the net PRL increments (differences between unstimulated nadir and stimulated peak) and the areas under the PRL response curves, the PRL secretion was not substantially altered following GnRH stimulations during the day studies. In contrast, PRL release was markedly enhanced (p < 0.05 or less vs. day studies), when GnRH was administered during the night studies. This GnRH-stimulated PRL release was even further increased (p < 0.01 vs. day, p < 0.05 vs. night or saline studies), when GnRH had been given during sleep.(ABSTRACT TRUNCATED AT 250 WORDS)

Anion transport through the phosphate-specific OprP-channel of the Pseudomonas aeruginosa outer membrane: effects of phosphate, di- and tribasic anions and of negatively-charged lipids

The mechanism of anion transport through the phosphate-starvation inducible OprP-channel of Pseudomonas aeruginosa outer membrane was studied in planar lipid bilayer membranes. The single-channel conductance of OprP was 160 pS in 100 mM chloride solution. Addition of other anions, in particular of phosphate, di and tribasic anions lead to a strong decrease of the chloride conductance. The decrease was used to calculate the stability constants for the binding of these ions to the binding site of the channel on the basis of a one-site two-barrier model. The stability constant of the binding of phosphate to the site was 11,000 M-1 at neutral pH. Surprisingly, di- and tribasic anions, such as sulfate and citrate had a much lower affinity to the binding site inside the channel. Although the single-channel conductance was dependent on the external pH, the stability constants for phosphate binding decrease only slightly for increasing the pH. The use of negatively-charged lipids instead of neutral ones in the planar lipid bilayers had no influence on the single-channel conductance of the OprP-channel, suggesting that the channel is shielded from the influence of surrounding molecules. Its permeability properties are probably not influenced by negatively-charged lipopolysaccharide molecules.

Diurnal variation in the elimination rate of human growth hormone (GH): the half-life of serum GH is prolonged in the evening, and affected by the source of the hormone, as well as by body size and serum estradiol

The half-lives of endogenous and exogenous (biosynthetic monomeric) GH were compared in the morning and evening in healthy young men (n = 10). In group A, a bolus of GHRH was injected either at 0800 or at 2000 h, whereas in group B hGH was injected iv after suppression of endogenous GH by somatostatin. GH was sampled every 10 min and the t1/2 for GH was determined by deconvolution analysis (two compartments). The GH elimination half-life was shorter in the morning: for endogenous GH, t1/2 was 23 +/- 1.1 min (mean +/- SE) in the morning compared to 26 +/- 1.7 min in the evening (P < 0.02). T1/2 correlated negatively with estradiol (r = -0.78; P < 0.01) and positively with sex hormone-binding globulin (r = 0.71; P < 0.03). The half-life of exogenous 22-kilodalton GH was shorter compared to endogenous GH (P < 0.002), and diurnal variation was even more pronounced: t1/2 was 14 +/- 1.0 min in the morning and 19 +/- 1.0 min in the evening (P < 0.01). These effects were not due to differences in GH distribution volumes. The half-life of exogenous GH was significantly affected by weight (r = -0.8; P < 0.01) and height (r = 0.67; P < 0.05). We conclude that in young males, the rate of GH disappearance from the circulation depends on both diurnal mechanisms as well as the source or structural composition of the hormone. Body size and sex steroids contribute to the variability of GH clearance in healthy man.

Characterization of the chloride carrier in the plasmalemma of the alga Valonia utricularis: the inhibition by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid

The effect of the anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the Cl(-)-transport system located in the plasmalemma of cells of the giant marine alga Valonia utricularis was studied by using the charge pulse relaxation technique. Analysis of the biphasic relaxation patterns in terms of the kinetic model published previously (Wang, J., Wehner, G., Benz, R. and Zimmermann, U. (1991) Biophys. J. 59, 235-248) demonstrated that extracellular DIDS dramatically reduced the translocation rate, KAS, of the Cl(-)-carrier complex (maximal inhibition 79%). The translocation rate of the free carrier molecules, KS, as well as the total surface concentration of the carrier, No, were not affected. A Hill-plot of DIDS inhibition on KAS yielded an half-maximal inhibition concentration (IC50) of 3.9 x 10(-5) M and Hill-coefficient of 1.61, suggesting a co-operative binding of the inhibitors to the Cl(-)-carrier. The maximal inhibition of DIDS was dependent on the extracellular Cl(-)-concentration. This inhibition was not competitive to chloride, since it increased and did not decrease with increasing chloride concentration. The DIDS effect decreased with increasing pH-value (investigated pH range between 6.5 and 10). Intravascular DIDS or SITS (4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid) had no effect on the biphasic voltage relaxation pattern. These results showed that the binding sites of DIDS must be located on the outer surface of the plasmalemma of V. utricularis and, in turn, supported previous conclusions that the Cl(-)-carrier (which is assumed to be part of the turgor-pressure-sensing mechanism) is only located in the outer membrane.

Purification and Characterization of Porin from Corn (Zea mays L.) Mitochondria

Mitochondrial porin from corn (Zea mays L. B 73) shoots was solubilized with lauryl(dimethyl)-amine oxide and purified by chromatography on a hydroxyapatite:celite column. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the purified protein had an apparent molecular mass of 35 kD. When reconstituted in planar lipid bilayer membranes the porin formed ion-permeable channels with single-channel conductance of 2.0 and 4.0 nanosiemens in 1 M KCl. At low transmembrane voltages corn porin had the properties of a general diffusion pore with an estimated effective diameter of 1.6 nm and a small selectivity for anions over cations. The primary structure of corn porin seems to be quite different from that of other mitochondrial porins, because it did not cross-react with monoclonal antibodies against human porin and with polyclonal antibodies against yeast porin. Furthermore, the peptide maps of corn and bovine heart porins were very different. A sequence of 21 amino acids obtained by Edman degradation of peptides generated by porin proteolysis with Staphylococcus aureus V8 protease did not show any significant homology with known sequences of mitochondrial porins. Results of our investigation suggest that corn porin possesses functional properties similar to those of other mitochondrial porins, despite major structural differences.

Use of irreversible electrical breakdown of lipid bilayers for the study of interaction of membranes with surface active molecules

Lipid bilayers were formed in the presence of different macromolecules and high electrical fields were used to induce mechanical rupture of the membranes. The kinetics of pore formation during irreversible breakdown was studied as a function of the macromolecules. We observed that macromolecules having a strong binding affinity to the membrane alter the time course of pore formation significantly. We propose this method as a simple test for adsorption of macromolecules to membranes.

TolC of Escherichia coli functions as an outer membrane channel

Reconstitution experiments were performed with TolC from Escherichia coli outer membrane by using the lipid bilayer membrane technique. TolC was purified by elution of the oligomeric and the monomeric forms out of preparative SDS-PAGE. The oligomeric but not the monomeric form of the protein was able to increase the specific conductance of artificial lipid bilayer membranes. Investigation of the membrane activity in single-channel experiments suggested that TolC formed ion-permeable channels. The channels of 80 pS in 1 M KCl had a much smaller single-channel conductance than the general diffusion pores of E. coli outer membrane (1500 pS). The single-channel conductance was only moderately dependent on the bulk aqueous KCl concentration which indicated either ion binding or charge effects. Titration of TolC-induced membrane conductance with peptides lead to a dose-dependent decrease of the conductance. This result suggested that TolC contained a binding site for peptides. A dissociation constant of 20 mM was calculated for the binding of the tripeptide H-Gly-Gly-Leu-OH to the binding site. The results are consistent with the assumption that TolC acts as an outer membrane channel for peptides.

Characterization of the channel formed by the mycobacterial porin in lipid bilayer membranes. Demonstration of voltage gating and of negative point charges at the channel mouth

We studied the channel formed by the mycobacterial porin from the cell wall of Mycobacterium chelonae (Trias, J., Jarlier, V., and Benz, R. (1992) Science 258, 1479-1481) by reconstituting the mycobacterial porin and cell wall extracts in lipid bilayer membranes. The channel exhibited two different states in lipid bilayer membranes at 10 mV of applied voltage. One was characterized by a steplike appearance while the other showed a fast, voltage-dependent, flickering behavior between a closed and an open state. The channel was voltage-gated, and starting at 40 mV of applied voltage the mycobacterial porin channel switched to a closed configuration in an asymmetric fashion. The channel was cation-selective and had 2.5-point negative charges at both sides of the channel. Identical channels were observed when membranes were reconstituted with cell wall extracts, suggesting that there is only one porin species in the mycobacterial cell wall.

Crystallization and preliminary X-ray diffraction analysis of ScrY, a specific bacterial outer membrane porin

The sucrose-specific outer membrane porin ScrY of Salmonella typhimurium was isolated from Escherichia coli K-12 strain KS 26 containing the plasmid pPSO112. The protein was purified to homogeneity by differential extraction of the cell envelope in the presence of the detergents sodium dodecyl sulfate and lauryl (dimethyl)-amine oxide (LDAO). The porin had apparent molecular weights of 58 kDa and 120 kDa for the monomer and for the trimer, respectively, on SDS/PAGE. The purified trimers were crystallized using poly(ethylene glycol) 2000 and the detergents octylglucoside (OG) and hexyl-(dimethyl)-amine oxide (C6DAO). X-ray diffraction of the crystals showed reflections to 2.3 A. The space group of the crystals was R3 and the lattice constants of the hexagonal axes were a = b = 112.85 A and c = 149.9 A. The crystal volume per unit of protein molecular weight was 3.47 A3/Da.

The somatotropin-somatomedin axis in adult patients with Turner syndrome: measurement of stimulated GH, GH-BP, IGF-I, IGF-II and IGFBP-3 in 25 patients

So far, few studies have addressed the regulation of GH and GH-dependent growth factors in adult patients with Turner syndrome. We therefore studied a group of 25 genetically proven patients with Turner syndrome (age 20-50 years) and 10 control women (25-48 years). Turner patients were significantly shorter (148.7 +/- 1.1 cm vs. 169.1 +/- 2.3 cm; mean +/- SE; p < 0.0001) and more overweight [body mass index (BMI)] 25.6 +/- 1.3 vs. 21.4 +/- 0.6 in controls; p < 0.01). No significant differences were present when the integrated GH response to stimulation with arginine and the serum levels of GH-binding protein (GH-BP), IGF-I, IGF-II and binding protein 3 for IGFs (IGFBP-3) were compared between the two groups. However, more detailed analysis revealed significant abnormalities of the somatotropic axis in Turner patients. Pituitary GH secretion was negatively and serum GH-BP positively related to the degree of overweight in normal patients. In Turner patients, no such relationship was present, while IGF-II significantly increased with BMI. IGFBP-3 was positively related to adult height in normal women but not in Turner patients. While serum testosterone values did not affect any of the somatotropic parameters measured, there was a previously unreported, inverse relation between serum estradiol and GH-BP in controls but not in Turner patients. While adult patients with Turner syndrome do not display endocrine features of GH insufficiency, a detailed analysis reveals several abnormalities of the interrelation between anthropometric parameters, sex steroids and the pituitary-somatomedin axis.

Topology of PhoE porin: the 'eyelet' region

A model for the topology of the PhoE porin has been proposed according to which the polypeptide traverses the outer membrane sixteen times mostly as amphipathic beta-sheets, thereby exposing eight loops at the cell surface. Until now, no evidence has been obtained for the surface exposure of the third loop. Recently, the structure of porin of Rhodobacter capsulatus has been determined. The proposed model of PhoE is very similar to the structure of the R. capsulatus porin, which has an 'eyelet' region, extending into the interior of the pore. The proposed third external loop of PhoE might form a similar 'eyelet' region. To determine the location of the predicted third external loop of PhoE, multiple copies of an oligonucleotide linker encoding an antigenic determinant of VP1 protein of foot-and-mouth disease virus (FMDV) were inserted. All hybrid proteins were properly inserted in the outer membrane. The monoclonal antibody MA11, directed against the linear FMDV epitope, was able to bind only to intact cells expressing a hybrid PhoE protein with at least three copies of the FMDV epitope present. Antibiotic sensitivity tests and single-channel conductance measurements revealed that the insertions influenced the channel size. These results are consistent with a location of the third loop of PhoE within the pore channel.

Kinetics of pore size during irreversible electrical breakdown of lipid bilayer membranes

The kinetics of pore formation followed by mechanical rupture of lipid bilayer membranes were investigated in detail by using the charge-pulse method. Membranes of various compositions were charged to a sufficiently high voltage to induce mechanical breakdown. The subsequent decrease of membrane voltage was used to calculate the conductance. During mechanical breakdown, which was probably caused by the widening of one single pore, the membrane conductance was a linear and not exponential function of time after the initial starting process. In a large number of experiments using various lipids and electrolytes, the characteristic opening process of the pore turned out to be independent of the actual membrane potential and electrolyte concentration. Our theoretical analysis of the pore formation suggested that the voltage-induced irreversible breakdown is due to a decrease in edge energy when the pore had formed. After initiation of the pore, the electrical contribution to surface tension is negligible. The time course of the increase of pore size shows that our model of the irreversible breakdown is in good agreement with mechanical properties of membranes reported elsewhere.

Identification of two general diffusion channels in the outer membrane of pea mitochondria

Reconstitution experiments were performed on lipid bilayer membranes in the presence of detergent solubilized mitochondrial membranes of pea seedlings (Pisum sativum). The addition of the detergent-solubilized material to the membranes resulted in a strong increase of the membrane conductance. To identify the proteins responsible for membrane activity the detergent extracts were applied to a hydroxyapatite (HTP) column and the fractions were tested for channel formation. The eluate of the column contained a protein which migrated as a single band with an apparent molecular mass of 30 kDa on SDS-PAGE. This channel was identified as the porin of pea mitochondria since it formed voltage-dependent channels with single-channel conductances of 1.5 and 3.7 nS in 1 M KCl and an estimated effective diameter of about 1.7 nm. Further elution of the column with KCl containing solutions yielded fractions which resulted in the formation of transient channels in lipid bilayer membranes. These channels had a single-channel conductance of 2.2 nS in 1 M KCl and had also the characteristics of general diffusion pores with an estimated effective diameter of 1.2 nm. Zero-current membrane potential measurements suggested that pea porin was anion-selective in the open state. The selectivity of the second channel was investigated by the measurement of the reversal potential. It was also slightly anion-selective. Its possible role in the metabolism of mitochondria is discussed.

The human first-term placenta in vitro: regulation of hCG secretion by GnRH and its antagonist

Gonadotropin releasing hormone (GnRH) has been proposed to play a role in the regulation of human chorionic gonadotropin (hCG) release from the human placenta. To test this assumption, we utilized an in vitro perifusion system, together with cultures of placental explants, to investigate short- and long-term effects of GnRH and its respective antagonist on the hCG secretion from the early human placenta. Tissue slices of human placenta (100 mg), obtained from first-trimester terminations of pregnancies, were continuously perifused and the effluent collected in fractions of 2-20 min. After initial perifusion periods of 30-40 min, either GnRH, a GnRH antagonist (SB-75; Asta Pharma, Frankfurt, Germany) or both compounds at equimolar concentrations were added to the perifusion medium at final concentration of 10(-4)-10(-8) mol/l). Administration was effected either continuously or intermittently in 10-min pulses. Further, 50-mg pieces of placental tissue explants were cultured in tissue culture plates for up to 6 days. During the perifusions, hCG (determined by enzymeimmunoassay) was found to be released spontaneously in a pulsatile fashion. Pulse amplitudes and frequencies of this episodic hCG secretion were increased in response to GnRH, but not affected by GnRH antagonist. Also, GnRH stimulated the hCG secretion during cultures of placental explants. When pharmacological doses of GnRH (10(-4) mol/l) were utilized, this stimulatory effect of GnRH was no longer evident, while perifusion with medium containing GnRH antagonist at identical concentrations stimulated the hCG secretion, indicating an intrinsic agonistic activity of the antagonist.(ABSTRACT TRUNCATED AT 250 WORDS)

Porins in the cell wall of mycobacteria

The cell wall of mycobacteria is an efficient permeability barrier that makes mycobacteria naturally resistant to most antibiotics. Liposome swelling assays and planar bilayer experiments were used to investigate the diffusion process of hydrophilic molecules through the cell wall of Mycobacterium chelonae and identify the main hydrophilic pathway. A 59-kilodalton cell wall protein formed a water-filled channel with a diameter of 2.2 nanometers and an average single-channel conductance equal to 2.7 nanosiemens in 1 M potassium chloride. These results suggest that porins can be found in the cell wall of a Gram-positive bacterium. A better knowledge of the hydrophilic pathways should help in the design of more effective antimycobacterial agents.

Purification, functional characterization, and cDNA sequencing of mitochondrial porin from Dictyostelium discoideum

Porin of Dictyostelium discoideum was extracted from mitochondria with Genapol X-80 and was purified by hydroxyapatite and CM-cellulose chromatography. The purified protein displayed a single band of 30 kDa in SDS-polyacrylamide gel electrophoresis. The formation of channels in artificial lipid bilayer membranes defined its function as a channel-forming component. Its average single-channel conductance was 3.9 nanosiemens in 1 M KCl, which suggested that the effective diameter of the channel is approximately 1.7 nm at small transmembrane potentials. The channel displayed a characteristic voltage dependence for potentials higher than 20 mV. It switched to substates of smaller conductance and a selectivity different to that of the open state. The closed state was stabilized at low ionic strength. The cDNA sequence of mitochondrial porin from D. discoideum was determined. It showed little sequence similarities to other known mitochondrial porins. The functional similarity, however, was striking. Localization of the porin in the mitochondrial outer membrane was confirmed by immunogold labeling of cryosections of fixed cells.

Haemolysin of Escherichia coli: comparison of pore-forming properties between chromosome and plasmid-encoded haemolysins

Lipid bilayer experiments were performed with chromosome-encoded haemolysin of Escherichia coli. The addition of the toxin to the aqueous phase bathing lipid bilayer membranes of asolectin resulted in the formation of transient ion-permeable channels with two states at small transmembrane voltages. One is a prestate (single-channel conductance 40 pS in 0.15 M KCl) of the open state, which had a single-channel conductance of 420 pS in 0.15 M KCl and a mean lifetime of 30 s. Membranes formed of pure lipids were rather inactive targets for this haemolysin. Experiments with different salts suggested that the haemolysin channel was highly cation-selective at neutral pH. The mobility sequence of the cations in the channel was similar if not identical to their mobility sequence in the aqueous phase. The single-channel data were consistent with a wide, water-filled channel with an estimated minimal diameter of about 1 nm. The pore-forming properties of chromosome-encoded haemolysin were compared with those of plasmid-encoded haemolysin. Both toxins share common features, oligomerize probably to form pores in lipid bilayer membranes. Both types of haemolysin channels have similar properties but different lifetimes.

Role of ovarian sex steroids in the regulation of thyrotropin (TSH) secretion of hypogonadal women

The exact role of ovarian sex steroids in the neuroendocrine regulation of thyrotropin (TSH) release in women can only be accurately assessed in the absence of any considerable ovarian sex steroid feedback upon the hypothalamic-pituitary unit. Consequently, the unstimulated episodic and thyrotropin-releasing hormone (TRH) stimulated TSH secretion was evaluated in postmenopausal women before and during sequential ovarian sex steroid replacements. Seven euthyroid women (mean age: 59.4 years) were studied initially without any sex steroid replacement (control studies), then on the last day of a 21-day course of oral estradiol-valeriate (E2) administration (2 mg daily) and finally, on the last day of a 21-day course of oral estradiol-progesterone (E2/P4) replacement (2 mg E2 and 200 mg micronized P4 daily). During all study occasions, blood was sampled at 10 min intervals for 10 h, while TRH (200 micrograms iv) was administered 8 h after initiation of blood collections. Compared to the control conditions, serum E2 and P4 concentrations markedly increased (p less than 0.001) following oral E2 or E2/P4 treatments. Total triiodothyronine (T3) and thyroxine (T4) concentrations and free T3 and T4 equivalents remained unchanged during E2 and E2/P4 regimens. In the unstimulated secretory profiles, TSH was found to be episodically released, with little interindividual variability for each study condition. Since the TSH pulse attributes (pulse amplitudes, frequencies, interpulse intervals, mean TSH concentrations, by Cluster pulse algorithm) did not significantly change during E2 and E2/P4 replacements, the episodic character of TSH secretion virtually remained unchanged by sex steroid replacements.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on human porin. VII. The channel properties of the human B-lymphocyte membrane-derived "Porin 31HL" are similar to those of mitochondrial porins

Porin 31HL was isolated and purified from total membrane preparations of a human B-lymphocyte cell line. The protein showed a single band of apparent molecular mass 31 kDa on SDS-PAGE. Reconstitution of the protein into artificial lipid bilayer membranes defines its function as a channel-forming protein. The distribution of single-channel conductances had two maxima of 2.4 and 4.3 nS in 1M KCl. The channel formed by Porin 31HL of human B-lymphocytes was found to be voltage-dependent and switched to ion-permeable substates at membrane voltage larger than 20mV. In the open state the pore exhibited the characteristics of a general diffusion pore because the mobility sequence of the ions inside the pore was similar to that in the bulk aqueous phase. The effective diameter was estimated to be about 1.7 nm. The properties of the low conductance state of the channel were studied in detail. In this state the pore favored the passage of cations, in contrast to the open state which favored anions slightly. Monoclonal antibodies against the N-terminal end of Porin 31HL blocked its reconstitution but had otherwise no influence on the channel properties. This result suggested that the amphipathic alpha-helical structure at the N-terminal end is probably not involved in channel gating. The channel-forming properties of Porin 31HL were compared to those of porins isolated from mitochondrial outer membranes and to those of the "maxi chloride channel" observed in the cytoplasmic membrane of several eukaryotic cells.