Ion Movements in Gramicidin Channels

Diffusion and kinetic approaches to describe permeation in ionic channels

Diffusion and reaction rate theories provide convenient approaches for describing permeation in ionic channels. Both approaches have strengths and weaknesses. Diffusion theories realistically approximate the physical process of ion movement within an aqueous pore, but those theories do not handle ionic interactions easily. Reaction rate theories are easy to use, and they provide a mathematical summarization of the data that is valuable for communicating experimental results. During ion permeation, however, the basic assumptions of reaction rate theory are not met. Therefore, rate theories do not provide true physical descriptions of ion permeation.

Solid-state 13C-NMR studies of the effects of sodium ions on the gramicidin A ion channel

End-to-end helical dimers of gramicidin A form transmembrane pores in lipid bilayers, through which monovalent ions may pass. The groups within the peptide that interact with these ions have been studied by application of solid-state spectroscopic methods to a series of gramicidin A analogues synthesized with 13C in selected peptide carbonyl groups. The resonances of D-Leu10, D-Leu12 and D-Leu14 analogues were perturbed in the presence of 0.16 M sodium ions, whereas the resonances of the carbonyls of Gly2, Ala3, D-Leu4 and Val7, which are closer to the formylated N-terminal end of the peptide, were unaffected. The observed changes in chemical shift anisotropy are indicative of a change in orientation of the abovementioned leucine carbonyls.

Electrical and chemical excitability appear one week before birth in the embryonic rat spinal cord

Embryonic rat spinal cord cells were acutely dissociated with the enzyme papain, stained with a voltage-sensitive oxonol dye and incubated with various pharmacological agents. Changes in the fluorescence intensity and, by inference, membrane potential of the cells were analyzed in a flow cytometer. Veratridine caused depolarization of the cells in a TTX-sensitive manner from as early as embryonic day 13. Depolarizing responses to muscimol and kainate appeared slightly later, at embryonic days 14 and 15, and were blocked by the antagonists bicuculline and CNQX, respectively. Responses to veratridine and kainate did not occur in sodium-free medium. The emergence of these excitable membrane properties coincides with postmitotic differentiation and synaptic development in the embryonic spinal cord.

Effect of acyl chain length on the structure and motion of gramicidin A in lipid bilayers

The transmembrane ion transport properties of gramicidin A have previously been shown to dependent on the nature of its lipid environment. Solid-state NMR spectroscopic studies of 13C-labelled analogues of gramicidin in oriented multilayers of phosphatidylcholine have shown that variation of the lipid hydrocarbon chain length has no effect on the structure or orientation of the peptide backbone.

Properties of gramicidin A channels in erythrocyte membranes

Studies for the cation permeability properties of the gramicidin A channel in erythrocyte membranes are presented. It is shown that gramicidin A interacts with the membrane in a cooperative manner, creating aggregates of the antibiotic molecules in the lipid lattice of the membrane. Cationic channels exist in these aggregates with the following order of selectivity: Rb+ greater than Cs+ greater K+ greater than Na+. The cation permeability of the channels depends on the media surrounding the membrane. This finding has been explained on the basis of Hodgkin-Keynes theory for single-file ion diffusion through extra-narrow pores.

Dielectric spectroscopy of L-alpha-lysolecithin-packaged gramicidin A

The complex permittivities of L-alpha-lysolecithin in the absence and presence of the gramicidin A ion channel were measured over the temperature range 0-60 degrees C and over the frequency range 1-1000 MHz. One dielectric relaxation/loss has been observed. It is located at 103.3 MHz (1.54 ns) for a micellar 0.4 M L-alpha-lysolecithin solution at 20 degrees C, whereas it is shifted to 71.7 MHz (2.22 ns) for a lamellar L-alpha-lysolecithin-gramicidin A aqueous solution (0.4 M L-alpha-lysolecithin, 0.0308 M gramicidin A) at 20 degrees C. The dielectric relaxation decreases and the relaxation time increases when gramicidin A is incorporated into L-alpha-lysolecithin. These dielectric changes are related, in part, to the micellar-to-lamellar lipid phase transition induced by the incorporation of gramicidin A into lysolecithin. We suggest that the diffuse rotational motion of the polar head group of L-alpha-lysolecithin contributes to the dielectric relaxation/loss at around 100 MHz.

Dielectric relaxation spectroscopy on dimyristoylphosphatidylcholine-packaged gramicidin A

The complex permittivities of aqueous suspensions of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and of DMPC packaged gramicidin A' (DMPC-GA) have been determined over the frequency range of 1 MHz to 1 GHz and the temperature range of 0-60 degrees C. A dielectric relaxation/loss has been observed at about 66 MHz for the DMPC suspension (30 degrees C) and at about 57 MHz for the DMPC-GA suspension (30 degrees C). This dielectric relaxation/loss has been attributed to the rotational mobility of the zwitterionic group of DMPC. The temperature dependence (from 60 degrees C to 0 degrees C) of this dispersion/absorption process of the DMPC suspension indicates a sharp reduction of the dielectric relaxation at about 20 degrees C. This dielectric change is related to the conversions of shape and structure of bilayer aggregates. This sharp reduction of the dielectric relaxation disappears or broadens when GA is incorporated into the DMPC aqueous suspension. The interpretation of these results is that the GA addition into the DMPC aqueous suspension induces a small decrease of the rotational mobility of the zwitterionic group above the lipid phase transition, and a small increase of the rotational mobility of the zwitterionic group below the lipid phase transition.

Surmounting barriers in ionic channels

Biological cells are defined by the membrane that shields their vital molecules from the environment. The lipid bilayer of the membrane is an effective dielectric shield (Parsegian, 1969; Andersen, 1978; Honiget al.1986), preventing penetration by charged molecules: the lipid presents a large electrostatic energy barrier because it cannot neutralize the charge of solute molecules nearly as well as water. This energy barrier inhibits the permeation of solutes with local charge, even metabolites. Of course, metabolites do enter cells, and so physiologists have suspected (for a very long time, Hille, 1984, ch. 8) that the membrane shield is pierced by aqueous channels, through which solutes diffuse (with their local charge substantially neutralized) as they cross the membrane. These aqueous pores now have molecular reality (e.g. Nodaet al.1984; Miller, 1986). Each is formed by a specialized protein, integral to membranes, perhaps shaped like a thick-walled pipe, called ionic channels. Channels control the movement of many important molecules in and out of cells by the ‘gating’ mechanism that controls their opening and closing and by the selective properties of their open channel.

Proton-coupled organic cation transport in renal brush-border membrane vesicles

We had previously proposed that organic cations are transported across the brush-border membrane in the canine kidney by a H+ exchange (or antiport) system (Holohan, P.D. and Ross, C.R. (1981) J. Pharmacol. Exp. Ther. 216, 294-298). In the present report, we demonstrate that in brush-border membrane vesicles the transport of organic cations is chemically coupled to the countertransport of protons, by showing that the uphill or concentrative transport of a prototypic organic cation, N1-methylnicotinamide (NMN), is chemically coupled to the flow of protons down their chemical gradient. In a reciprocal manner, the concentrative transport of protons is coupled to the counterflow of organic cations down their concentration gradient. The transport of organic cations is monitored by measuring [3H]NMN while the transport of protons is monitored by measuring changes in acridine orange absorbance. The functional significance of the coupling is that a proton gradient lowers the Km and increases the Vmax for NMN transport.

Activation of K+ and Cl- channels by Ca2+ and cyclic AMP in dissociated kidney epithelial (MDCK) cells

In dissociated MDCK cells, activators of the cyclic AMP system cause depolarization detectable by changes in fluorescence of the membrane potential sensitive dye bisoxonol. Addition of forskolin (60 microM), vasopressin (2 microM), 8-bromo-cyclic AMP (0.5 mM) or 1-epinephrine (10 microM) depolarized the cells substantially in low Cl- (5 mM) but had little effect in high Cl- (140 mM) solution. These results are consistent with cyclic AMP activation of Cl- channels. The Ca2+-ionophore ionomycin (1 microM) produced a rapid hyperpolarization in low and high Cl- solutions, consistent with K+ channel opening. Using a clonal subline, MDCK-14, the magnitude of the ionomycin hyperpolarization was roughly proportional to the concomitant rise in [Ca2+]i as measured with the intracellular Ca2+ probe indo-I. Both l-epinephrine and isoproterenol appeared to activate the Cl- channels. However only l-epinephrine produced a [Ca2+]i rise and a transient hyperpolarization (due to K+ channel opening), which preceded the depolarization due to Cl- channel opening. The l-epinephrine-induced [Ca2+]i response of the heterogeneous MDCK cell population but not of the clonal subline MDCK-14 was inhibited by removal of extracellular Ca2+. In the latter only the slow secondary phase of the [Ca2+]i rise was affected by Ca2+ removal. It is concluded that l-epinephrine activates K+ and Cl- channels in a sequential manner in MDCK cells by Ca2+ and cAMP signals, presumably via alpha- and beta-adrenergic receptors located on the same cell.

Temperature jump as a new technique to study the kinetics of fast transport of protons across membranes

Application of a temperature jump (2.5 degrees C) to a suspension of liposomes, having phosphate (delta pK/delta T approximately 0.005) as the internal buffer and tris(hydroxymethyl)aminomethane (delta pK/delta T approximately 0.031) as the external buffer, created a delta pH (pHin - pHout) of positive sign in ca. 5 microseconds. Decay of this delta pH was monitored by using the fluorescent pH indicator 8-hydroxy-1,3,6-pyrenetrisulfonic acid entrapped inside the liposome. This technique is useful to study transmembrane proton movement in the time range 5 microseconds-10 s at physiological pH values. The kinetics of proton transport aided by ion carriers such as nigericin, monensin, carbonyl cyanide m-chlorophenylhydrazone (CCCP), and valinomycin were studied by our method. The electrogenic nature of transport by CCCP and valinomycin and electroneutral ion transport by nigericin and monensin were shown. From the kinetics of proton transport aided by gramicidin, the time-averaged single-channel conductance of gramicidin channels was estimated to be (2.1 +/- 0.5) X 10(-16) S for H+ at pH 7.5.

A cation channel in frog lens epithelia responsive to pressure and calcium

Patch-clamp recording from the apical surface of the epithelium of frog lens reveals a cation-selective channel after pressure (about +/- 30 mm Hg) is applied to the pipette. The open state of this channel has a conductance of some 50 pS near the resting potential (-56.1 +/- 2.3 mV) when 107 mM NaCl and 10 HEPES (pH 7.3) is outside the channel. The probability of the channel being open depends strongly on pressure but the current-voltage relation of the open state does not. With minimal Ca2+ (55 +/- 2 microM) outside the channel, the current-voltage relation is nonlinear even in symmetrical salt solutions, allowing more current to flow into the cell than out. The channel, in minimal Ca2+ solution, is selective among the monovalent cations in the following sequence K+ greater than Rb+ greater than Cs+ greater than Na+ greater than Li+. The conductance depends monotonically on the mole fraction of K+ when the other ion present is Li+ or Na+. The single-channel current is a saturating function of [K+] when K+ is the permeant ion, for [K+] less than or equal to 214 mM. When [Ca2+] = 2 mM, the current-voltage relation is linearized and the channel cannot distinguish Na+ and K+.

Two Ca2+ ATPase genes: homologies and mechanistic implications of deduced amino acid sequences

Rabbit genomic DNA contains two genes that encode Ca2+ ATPases of fast twitch and of slow twitch (and cardiac) sarcoplasmic reticulum, respectively. The deduced amino acid sequences of the products of the two genes are highly conserved in putative Ca2+ binding regions, in sectors leading from cytoplasmic domains into transmembrane domains, and in transmembrane helices. A transport mechanism is proposed in which Ca2+ binds to negatively charged groups on amphipathic stalk sectors, becoming occluded during enzyme phosphorylation by bound ATP. Rotation of the stalk sectors is induced as the energy in the phosphorylated enzyme (E1P) is utilized in conformational changes leading to the low energy form, E2P. Rotation leads to disruption of high affinity Ca2+ binding sites and release of Ca2+ into a charge-lined membrane channel. Ca2+ then traverses the membrane by exchange diffusion.

Potassium-39 NMR of K+ interaction with the gramicidin channel and NMR-derived conductance ratios for Na+, K+ and Rb+

A potassium-39 NMR study of potassium ion interaction with the gramicidin transmembrane channel in phospholipid bilayers at high ion activity is reported which allows determination of a weak binding constant, Kwb approximately equal to 8.3/M, and an off-rate constant for the weak site, kwoff approximately equal to 2.6 X 10(7)/sec. These values are interpreted with the aid of additional NMR data as the binding constant for formation of the doubly occupied channel state and the rate constant for an ion leaving the doubly occupied state. Considering the singly occupied channel state for the potassium ion to be "electrically silent" at 1 molar ion activity, as with the sodium ion, the single-channel conductance for 100 mV and 30 degrees C calculated to be 29 pS, and using the same approximation with previous NMR results on the sodium and rubidium ions, reasonable conductance ratios were calculated. Further experimental estimates of the other three constants with the experimental location of binding sites and Eyring rate theory to introduce voltage dependence allowed a more complete calculation of the two-site channel. The single-channel conductance for potassium ion is calculated to be 24 pS at 1 M activity and 26 pS at 0.6 M activity, which compares for diphytanoyl phosphatidylcholine membranes to an experimental most probable single-channel conductance of 25 pS and a mean channel conductance of 20 pS. The calculated conductance ratios using NMR-derived constants were gamma (K)/gamma (Na) = 2.0 and gamma (Rb)/gamma (Na) = 4.3.(ABSTRACT TRUNCATED AT 250 WORDS)

Infrared spectroscopic studies on gramicidin ion-channels: relation to the mechanisms of anesthesia

Fourier transform infrared spectroscopic studies are reported on gramicidin ion-channels in phospholipid bilayers and the effects on the spectra of the anesthetics and related compounds (methoxyflurane, halothane, chloroform, carbon tetrachloride, n-pentane and n-decane) have been determined. The addition of anesthetics containing the 'acidic hydrogen' caused unique changes particularly on the amide I bands at 1639 cm-1 and 1670 cm-1. The 1639 cm-1 band became more intense while the intensity near 1670 cm-1 decreased dramatically. These effects were not observed with carbon tetrachloride, n-pentane and n-decane. The 1670 cm-1 band is interpreted as arising from the carbonyls involved in the head-to-head hydrogen-bonded dimerization where the relationship between chains is analogous to that of the antiparallel beta-pleated sheet structure and the anesthetics with 'acidic hydrogens' are considered to disrupt the hydrogen-bonded dimerization by competitive hydrogen bonding to the carbonyls at the head-to-head junction. As the dimer-monomer equilibrium is the 'on-off' mechanism for gramicidin ion-channel conductance, the results are considered in terms of the mechanism of action of anesthetics and are taken to suggest, for certain anesthetics, a hydrogen-bonding role to protein ion-channel components.