Single K+-channel current measurements from brain synaptosomes in lipid bilayers

Ion channels from a rat brain preparation enriched in presynaptic nerve terminals (synaptosomes) were incorporated into planar lipid bilayers. Experiments examined macroscopic (channel-ensemble) currents as well as single-channel currents. Four single-channel conductances (ranging from 10 to 40 pS) were usually observed, each with distinct kinetic properties. All the observed channels selected for K+ over Cl-. These K+ channels may contribute to the resting K+ conductance of brain nerve terminals. Furthermore, this report demonstrates that the properties of ion channels from mammalian brain can be studied in planar lipid bilayers and suggests that this system can be readily extended to many additional investigations on the electrical properties of brain membranes.

Transmembranous incorporation of photoelectrically active bacteriorhodopsin in planar lipid bilayers

Various methods to incorporate bacteriorhodopsin in black lipid membranes are reported. Both purple membrane patches and monomeric bacteriorhodopsin were used as starting material. The incorporation of bacteriorhodopsin into planar lipid bilayers was achieved by the following methods. (i) Purple membrane patches were transferred from water to solutions of lipids in n-alkanes. Black membranes were formed from such organic suspensions. (ii) Lipid layers containing solvent and purple membranes were spread on an air/water interface. These layers were used to form planar bilayers. (iii) Vesicles containing purple membranes or monomeric bacteriorhodopsin were spread on an air/water interface and, from the resulting layer, bilayers were formed. On illumination, steady-state photocurrents were observed in all three cases, indicating that these methods lead to functional transmembranous integration of the protein in the planar black lipid membrane. The influence of an applied electric field on the pumping process was studied on membranes formed by using method i. At approximately 200 mV, the photocurrent tends to zero. Furthermore, it was possible to make planar lipid bilayers photoelectrically active by adding vesicles containing monomeric bacteriorhodopsin to the bathing solution. Because, in this case, only transient photocurrents were observed, it can be concluded that the vesicles are attached to but not fused with the black lipid membrane.

Relaxation studies of ion transport systems in lipid bilayer membranes

Relaxation techniques have been widely used in kinetic studies of chemical reactions in homogeneous solution (Eigen & DeMayer, 1963). The principle of this method is well known: an external variable such as temperature or pressure is suddenly changed and the time course of a state parameter of the system such as concentration is recorded as it approaches a new steady value. Relaxation techniques can also be used for studying the rate of elementary processes in membranes. This method has proved particularly useful for the investigation of ion transport systems (ion carriers, channels, pumps) in artificial planar bilayer membranes. In this review we describe different relaxation techniques which have been developed for this purpose during the last years, as well as applications to a number of ion transport systems.

Plasma levels of androstenedione, epitestosterone, testosterone and oestrogens in cows at parturition

The concentration of total unconjugated oestrogens in the peripheral plasma of cows increased from 2.00 +/- 0.20 (S.E.M.) ng/ml 7 days before to 3.87 +/- 0.50 ng/ml 1 day before parturition and declined to values below 0.30 ng/ml on day 2 post partum. The concentrations of androstenedione, epitestosterone and testosterone remained approximately constant at 0.90 +/- 0.10, 0.92 +/- 0.08 and 0.40 +/- 0.20 ng/ml respectively during the last week of pregnancy. After parturition the concentration of the three androgens declined rapidly. Similar to data obtained in goats we did not find a marked increase in androgen levels in the peripheral blood of cows before parturition.

[The long Q-T syndrome without Q-T prolongation (author's transl)]

Case description of a 30 year-old woman with recurrent attacks of symptomatic ventricular tachyarrhythmias but without Q-T prolongation in the electrocardiogram. All conventional antiarrhythmic agents failed to suppress the arrhythmia and electrical defibrillation had to be carried out 99 times. Blockade of the left stellate ganglion with lidocaine followed by stellectomy led to successful control of the dangerous rhythm disturbance for a follow-up period of 1.5 years. As in patients with the classical long Q-T syndrome stellate ganglion blockade and stellectomy might be indicated in cases with the so called latent Q-T syndrome and life threatening ventricular arrhythmias resistant to antiarrhythmic agents.

Effect of immunization of male rabbits against androstenedione

After active immunization of male rabbits against androstenedione and with increasing antibody titre the concentration of androstenedione and testosterone in peripheral blood increased more than 1000-fold resp. 20-fold above values of control animals. On histological examination of the testes a marked Leydig's cell hyperplasia was found, though there was no difference in testis weight.

In vitro conversion of steroid hormones in bovine ovarian follicles

In bovine ovarian follicles in vitro conversion of androstenedione and oestrone into oestradiol-17 beta, and of oestradiol-17 beta into oestrone was observed. There was no in vitro conversion of pregnenolone, progesterone, dehydroepiandrosterone and testosterone.

Dicarboxylic acid analogs of gramicidin A: dimerization kinetics and single channel properties

According to the model of Urry, the cation-permeable gramicidin channel is a dimeric helix formed by association of two peptide monomers linked at their amino ends. In this paper the channel properties of gramicidin analogs are described which have been obtained by chemical modification at the coupling site of the two half-channels. In these analogs the amino terminal -CHO group is replaced by -CO(CH2)nCCOH (n = 2, 3, 4, 5, 6). All analogs form conducting channels in black lipid membranes with the same general properties as found for gramicidin A. The observation that the channel-forming activity decreases with increasing pH is consistent with the notion that the half-channels are linked at the amino terminus. The channel lifetime of the different analogs varies between 2 msec and greater than of equal to 50 sec, the longest lifetime being found for the compound with n = 3. The single-channel conductance : formula : (see text) is always smaller than that of gramicidin A, but the reduction of : formula : (see text) depends on the nature of the permeable ion. Ion specificity was studied at 1 M electrolyte by measuring the conductance : formula : (see text) for different permeable ions (Na+, K+, Cs+). The conductance ration : formula : (see text) (Cs+)/ : formula : (see text) (Na+) was found to vary between 2 and 10.5 for the different analogs.

Formation of ionic channels in black lipid membranes by succinic derivatives of gramicidin A

Different succinyl derivatives of Gramicidin A were synthesized and their activity was investigated with different methods on lipid bilayer membranes. The succinyl derivatives of Gramicidin A can be classified as three different types, the O-succinyl derivative, the N-succinyl derivative and the N-O-succinyl derivative of Gramicidin A. An O-pyromellityl-N-succinyl gramicidin was synthesized which can be attributed to the latter class. It was found that O-succinyl gramicidin behaves like the unmodified Gramicidin A despite a charge effect on single-channel conductance, arising from the negative charge of the succinic residue at the mouth of the channel. The activity of N-succinyl and N-O-succinyl gramacidin and of O-pyromellityl-N-succinyl-gramicidin depends strongly on the pH of the electrolyte solution. It is demonstrated that at low pH (less than or equal to 5) the N-succinyl derivatives show high activity, whereas at high pH (greater than or equal to 7) the activity is sharply reduced or disappears totally. From these experiments it can be concluded that, for the formation of a dimeric gramicidin channel, the hydrogen of the formyl group can be replaced by a protonated carboxylic group of a succinic residue. Further results, obtained by measurement of the single-channel conductance and of the reaction rate constants for the channel formation, are discussed in terms of the structural basis of the single stranded model for the gramicidin channel. On this basis the double stranded helix can be excluded and an interesting head-to-head single stranded beta(pi L.D.) helical channel is described which contains carboxyl groups at the head-to-head junction.

Effects of surface charge on the conductance of the gramicidin channel

The electric conductance of the cation-permeable gramicidin channel in negatively charged phosphatidylserine membranes has been studied. At low electrolyte concentrations the single-channel conductance is much larger in the negatively charged membrane than in a neutral membrane. This enhancement of conductance is in agreement with theoretical expectations, although a complete description of the salt concentration dependence of conductance was not possible. The results of these experiments may be compared with previous studies of a negatively charged gramicidin analog (O-pyromellityl gramicidin). It is found that the electrostatic effect on the conductance is much larger for a neutral channel embedded in a negatively charged lipid than for the negatively charged O-pyromellityl analog (with three charges at the channel mouth) embedded in a neutral lipid.

[Primary shape change of platelets in vitro (author's transl)]

Studies with interference contrast microscopy reveal that platelets undergo a typical shape change within 30--60' after venepuncture, i.e. swelling, formation of large tentacles, tiny protrusions and vesicles at the platelet surface. This "shape change" can be observed in citrated blood and PRP, heparinized blood and EDTA-blood as well. It is enhanced by low incubation temperatures (4 degrees C, 10 degrees C) and delayed at 37 degrees C as compared with room temperature. An increased number of primarily shape changed platelets is found if platelets are strongly mechanically irritated at blood sampling. The shape change is partly reversible in vitro, it is completely or almost completely reversible in vivo. Some antiaggregating agents inhibit the in vitro shape change at varying degrees (Bencyclan, SH 869 greater than ASA greater than D-Propranolol). The shape change is partly inhibited after oral or i.v. administration of ASA. A typical transformation of platelets into "spheric" forms can be observed following the addition of Bencyclan, SH 869 and D-Propranolol to PRP in vitro. The spontaneous "primary shape change" which occurs in PRP or blood after blood sampling is probably different from the secondary ADP-induced shape change. The primary shape change may influence the results of different platelet function and aggregating tests. The shape change kinetics of "healthy" subjects and patients with Hodgkin's disease differ significantly. The described method may gain more clinical interest in the future.

l/f noise in black lipid membranes induced by ionic channels formed by chemically dimerized gramicidin A

The noise behavior of lipid bilayer membranes, doped with a chemically dimerized gramicidin A, was investigated. In contrast to normal gramicidin A, which generates a Lorentzian type power spectrum due to the formation and disappearance of conducting dimers, the current power spectrum density Sm(f) obtained with this gramicidin A derivative showed over several orders of magnitude a clear l/f behavior. The intensity of this l/f component was analyzed as a function of the membrane-applied voltage, membrane resistance, electrolyte concentration, and composition. The relationship between the mean-square fluctuation in current and the membrane current mean value was found to follow Hooge's equation, i.e., deltaI2 = alphaI2m/Nf where N is the number of channels and alpha is a constant equal to 1.0 X 10(-2). It is suggested that a l/f type noise was observed because the chemically dimerized form of gramicidin A produces long lasting cation selective channels.

Ion channels formed by chemical analogs of gramicidin A

Channel-forming peptides such as gramicidin A offer the opportunity to study the relationship between chemical structure and transport properties of an ion channel. This article summarizes a number of recent experiments with chemical analogs and derivatives of gramicidin A using artificial lipid bilayer membranes. The introduction of negative charges near the channel mouth leads to an increase in the cation transport rate. Hybrid channels consisting of a neutral and a negatively charged or of a positively and a negatively charged half-channel may be formed. The current-voltage characteristic of these hybrid channels exhibits a pronounced asymmetry. Experiments with charged derivatives of gramicidin A have been used in order to distinguish between different structural models of the dimeric channel; these studies strongly support Urry's model of a single-stranded, head-to-head associated helical dimer. In a further set of experiments gramicidin analogs with modified amino acid sequence were studied. It was found that a single substitution (tryptophan replaced by phenylalanine) leads to marked changes in the conductance of the channel. Analogs with a simplified amino acid sequence such as (L-Trp-D-Leu)7-L-Trp or L-Trp-Gly-(L-Trp-D-Leu)6-L-Trp are able to form cation permeable channels with similar properties as gramicidin A.

Structure of the gramicidin A channel: discrimination between the piL,D and the beta helix by electrical measurements with lipid bilayer membranes

Measurements with different chemically modified gramicidins in lipid bilayer membranes were used to discriminate between the dimeric pi(L,D) helix proposed by Urry and the dimeric parallel or antiparallel helices proposed by Veatch and Blout. Evidence for the pi(L,D) helix was obtained on the basis of the different actions of a negatively charged O-pyromellitylgramicidin and a negatively charged N-pyromellityldesformylgramicidin on lipid bilayer membranes. O-Pyromellitylgramicidin forms ionic channels in lipid membranes when it is applied to both sides of the membrane. In contrast to unmodified gramicidin, O-pyromellitylgramicidin is inactive when it is applied only to one side of the membrane. N-Pyromellityldesformylgramicidin does not form ionic channels in lipid bilayer membranes whether it is applied to one or both sides of the membrane. These results support the view that the gramicidin channel is formed by two pi(L,D) helices. Dimer formation by head-to-head association of two pi(L,D) helices needs six intermolecular hydrogen bonds, which are located at the formyl end of the molecule and which occur deep within the lipid membrane. In the head-to-head associated pi(L,D) helix the absence of the formyl group leads to an inactivation of the peptide, whereas in a parallel or antiparallel double-stranded helix the absence of the formyl group should have only minor effects.