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S Ciani

Publications and source records attributed to S Ciani.

At least 37 records · Page 2Linked to original sources
PubMed1988

Metabolic regulation of the K(ATP) and a maxi-K(V) channel in the insulin-secreting RINm5F cell.

K channels in the cell membrane of the insulin-secreting RINm5F cell line were studied using the patch-clamp technique in cell-attached patch mode. With 140 mM K in the pipette, two channels displaying different conductive and kinetics properties were observed. A voltage-independent, inward-rectifying, 55-pS channel was active at rest (no glucose, -70 mV), but was almost completely inhibited by 5 mM glucose. A 140-pS channel was seen in the absence of glucose only after cell membrane depolarization with high (30 mM) K. This channel was voltage dependent, with a linear slope conductance between -60 and +60 mV, and was completely inhibited only by greater than 15 mM glucose. The former channel we identify as an ATP-sensitive channel previously described in excised patches and refer to it as the K(ATP) channel. The latter, because of its large conductance and voltage-dependent kinetics, will be referred to as the maxi-K(V) channel, adopting a nomenclature previously used to classify highly conductive K channels (Latorre, R., and C. Miller, 1983, Journal of Membrane Biology, 71:11-30). In addition to glucose, mannose and 2-ketoisocaproate, which also initiate insulin secretion and electrical activity in the islet beta cell, reduced the activity of both the K(ATP) and the maxi-K(V) channel. Lactate and arginine, which potentiate but do not initiate insulin secretion or beta cell electrical activity in normal islets, each caused a large reduction in maxi-K(V) channel activity, without consistently affecting the activity of K(ATP) channels. Another agonist that potentiates insulin secretion and electrical activity in normal cells, the tumor-promoting phorbol ester TPA, blocked maxi-K(V) channel activity while stimulating the activity of the K(ATP) channel, thereby implicating phosphorylation in the control of channel activity. These results indicate that metabolic substrates that initiate electrical activity and insulin secretion in normal beta cells reduce the activity of both the K(ATP) and the maxi-K(V) channel, while potentiating agents reduce only the maxi-K(V) channel. The possible role of these two channels in the processes of initiation and potentiation of the beta cell response is discussed.

Adenosine Triphosphate↗
PubMed1987

Permeation of divalent cations through alpha-latrotoxin channels in lipid bilayers: steady-state current-voltage relationships.

alpha-Latrotoxin, a polypeptide neurotoxin known to cause massive release of transmitter from vertebrate nerve terminals, is thought to act by forming cation-selective channels in plasma membranes. This paper describes the steady-state current carried by Ca2+, Sr2+ and Ba2+ through pores of alpha-LaTx molecules incorporated in artificial bilayer membranes made of neutral lipids. Even when the solutions separated by the membrane are identical, the I-V relations rectify strongly, the current being higher when the side to which the toxin is added is positive. The polarity of the rectification is consistent with the hypothesis that the mechanism of action of the toxin is, at least in part, that of promoting inwardly directed flow of cations, and thus, accumulation of Ca2+ and other ions in the intracellular spaces. The dependence of the I-V characteristics on voltage and Ca2+ concentration is well described by a one-site, one-ion model for a channel. Three parameters of the model are deduced: the binding constant of the site for Ca2+, K = 1.5 M-1 (or K = 7 M-1 when activities are used instead of concentrations); the "electrical" distance of the site from the toxin-containing solution, alpha = 0.3; the free energy difference between the two barrier peaks, delta F = 0.26 kT. The values of the parameters deduced by studying the channel in the presence of Ca2+ give theoretical curves that also fit the data with Sr2+ and Ba2+, indicating a low level of discrimination among these three cations.

Arthropod Venoms↗
PubMed1987

Regulation by cell metabolism and adenine nucleotides of a K channel in insulin-secreting B cells (RIN m5F).

A potassium channel in membranes of the insulin-secreting B-cell line, RIN m5F, was studied using the patch-clamp technique. In cell-attached patches, and for pipette solutions containing 140 mM KCl, the I-V curves exhibited a pronounced rectification, with the conductance being higher when the current flowed from the electrode into the cell (50 pS). Addition of glucose (5-7 mM) to the bath was sufficient to abolish the channel activity, while low doses of the metabolic inhibitor 2,4-dinitrophenyl enhanced it, indicating a link between channel activity and cell metabolism. In excised inside-out patches, the activity of a channel with similar conductive properties was inhibited by ATP in a dose-dependent fashion, the half-maximal concentration being approximately 70 microM. ADP also acted as an inhibitor, but with much lower potency. However, when ADP was present, the blocking effectiveness of ATP was reduced, suggesting a nonadditive interaction of the two nucleotides with the channel. In conclusion, complete closure of the K channel by glucose (5-7 mM) suggests that this channel may be responsible for the early phase of the B-cell depolarization, which is seen in normal cells for glucose increments up to similar values. In addition, since ATP and ADP are both present in the cells, the reduction by ADP of the blocking efficacy of ATP in excised patches may help one to understand why, in intact cells, the channel is still functional for intracellular ATP concentrations that in excised patches would completely suppress its activity.

2,4-Dinitrophenol↗
PubMed1986

Effects of the external pH on Ca channels: experimental studies and theoretical considerations using a two-site, two-ion model.

Some effects of the external pH on Ca channels were studied in a hybridoma cell line (mAb-7B), by using the whole-cell configuration of the patch-clamp technique. As the pH was lowered, both the activation and the inactivation curves shifted toward less negative membrane potentials, suggesting a pH-induced decrease of an external negative surface potential, sensed by the mechanism of gating. The potential for half-activation, V1/2, and that for half-inactivation, Vh, were related by a straight line with a slope of one. The inward current varied exponentially with V1/2, as would be expected if the field inside the channel and the Ca2+ concentration at the entrance were sensitive to the surface potential. However, the reversal potential and the outward current were unaltered by changes in the pH. Under the hypothesis that the channel senses the surface potential, all these results, as well as the nernstian behavior of the reversal potential with respect to Ca2+, observed in previous studies, are accounted for by a three-barrier, two-ion model for a channel, provided it is assumed that the potential in the channel drops almost entirely across the barrier adjacent to the external solution.

Animals↗
PubMed1985

Experimental and theoretical studies on Tl+ interactions with the cation-selective channel of the sarcoplasmic reticulum.

This paper presents an experimental study and a theoretical interpretation of the effects of thallous ion on the electrical properties of the cation-selective channel of the sarcoplasmic reticulum (SR channel). The properties of this channel in solutions which do not contain thallous ion are consistent with the predictions of Läuger's theory for singly occupied pores (P. Lüger, 1973, Biochim. Biophys. Acta 311:423-441). However, this theory does not account for SR channel properties in mixtures containing thallous ion. SR channel conductance is less than predicted in mixed salt solutions of thallium with either potassium or ammonium (J. Fox, 1983, Biochim. Biophys. Acta 736:241-245), yet is greater than expected in mixtures of lithium and thallium. In a simple single-ion pore, the ratio of the products of the single-salt binding constants and maximum conductances is equal to the permeability ratio calculated from zero-current potential experiments under near equilibrium conditions. This is not found for the SR channel when thallous ion is present. SR channel properties in the presence of thallous ion can, however, be explained by a model which postulates the existence of two external modulatory sites on the channel, without implying double-occupancy in the permeation pathway. When thallous ion is bound to a modulatory site the maximum conductance of the channel to all permeating ions is altered (thallous included). Two other models (a three-barrier, two-internal-site pore which allows multiple occupancy, and a pore with fluctuating barriers) are discussed, but are found to be unable to fit our conductance data at different concentrations.

Animals↗
PubMed1984

Coupling between fluxes in one-particle pores with fluctuating energy profiles. A theoretical study.

We show theoretically that extending pore models to allow for fluctuations between configurations with different energy profiles results in the prediction of coupling between fluxes and forces of different species diffusing through singly occupied pores. Considering the case of a one-site, two-barrier pore capable of existing in two states, and using Eyring rate theory to describe the translocation of two permeant species, the flux of each is found to be linked to the driving force of the other via cross coefficients that are given as explicit functions of concentrations and potential, and that obey Onsager's relations when the system is near equilibrium. Conditions for the existence of coupling are that both states of the channel be permeable to both diffusing species and that the peaks of the two energy barrier shift by different amounts during the state transition of the pore. Some implications of this model on phenomena of biological interest are discussed briefly.

Ion Channels↗
PubMed1981

Relationship of the Southern California Sensory Integration Tests, the Southern California Postrotary Nystagmus Test, and clinical observations accompanying them to evaluations in otolaryngology, ophthalmology, and audiology: two descriptive case studies.

A preliminary investigation was conducted into the relationship of the Southern California Sensory Integration Tests (SCSIT), the Southern California Postrotary Nystagmus Test (SCPNT), and clinical observations accompanying these tests to evaluations in otolaryngolgoy, ophthalmology, and audiology, ophthalmology, and audiology. The subjects were two children with vestibularly based sensory integrative dysfunction. The results revealed that there was no agreement between the results of the SCPNT and the otolaryngological evaluation. There was some agreement between the ophthalmology evaluation and the clinical observations accompanying the SCSIT. Both subjects scored poorly in two areas of auditory processing. Possible reasons for these results are discussed as well as implications for occupational therapy reached and practice.

Audiology↗
PubMed1980

A model for the effects of potential and external K+ concentration on the Cs+ blocking of inward rectification.

Inward rectification in starfish egg cell membranes is blocked by external Cs+, the degree of blocking being an increasing function of hyperpolarizing potentials and of the external concentration of K+. While the effect of K+ and the particular functional dependence of blocking on potential are difficult to reconcile with a one-ion pore model, both features can be accounted for assuming that the pore has at least two sites and tat a considerable fraction of the blocked pores is simultaneously occupied by a Cs+ ion in the inner site and by a K+ in the outer one.

Animals↗
PubMed1978

A model for anomalous rectification: electrochemical-potential-dependent gating of membrane channels.

A model is presented for "anomalous rectification" based upon electrical measurements on the egg cell membrane of the starfish. The objective is to postulate a plausible molecular mechanism which yields an expression for the conductance similar to that deduced empirically by Hagiwara and Takahashi (1974), i.e.,: formula: (see text), where B, deltaVh and v are constant, cK is the external K+ concentration, and deltaV (= V - V0) is the displacement of the membrane potential from its resting value. It is shown that a similar dependence of the conductance on deltaV is expected for a particular class of models in which the K+ ions are also implicated in "gating". To give a specific example, we consider the case in which the formation of ion-permeable pores requires a voltage-induced orientation of membrane-bound, electrically-charged groups and subsequent complexation of these groups with the external cations. Furthermore, the proportionality between GK and CK1/2, when the internal K+ concentration is constant, is accounted for by conventional descriptions of the ionic fluxes using Eyring's rate reaction theory. In terms of the present model, B and deltaVh are explicit functions of the internal K+ concentrations and are thus constant only as long as this is unvaried. The particular value of v required to fit the data (v congruent to 8.4 mV) is rationalized by the assumption that each of the orientable groups carries three negative elementary charges. In addition, the predictions of the present model are compared with those deduced from an alternative viewpoint, which is related to Armstrong's "blocking particle hypothesis", in that the probability for opening and closing of the pore is assumed to depend on whether the pore is occupied or empty. Differences and similarities between the two models, as well as ways to discriminate between them, are discussed.

Animals↗
PubMed1977

Anomalous permeabilities of the egg cell membrane of a starfish in K+-Tl+ mixtures.

The electrical properties of "inward" rectifying egg cell membranes of the starfish mediastera aequalis have been studied in the presence of K(+)-Tl(+) mixtures. When the ratio of the external concentrations of these ions is changed while their sum is kept constant, both the conductance and the zero-current membrane potential go through a minimum, showing clear discrepancies from theoretical results based on conventional electrodiffusion models (E.g., Goldman's equation). By contrast, when the ration of the two concentrations is fixed and their sum varied, the potential follows an ideal Nernst slope, consistent with Goldman's equation. The membrane conductance which, according to previous studies on similar membranes, is to be viewed as a function of the displacement of the membrane potential from its resting value deltaV, shows marked differences between the cases in which K(+) or Tl(+) are the predominant ions: when K(+) is the predominant permeant ion in solution, the addition of small amounts of Tl(+) inhibits the current, while corresponding blocking effects of K(+) on the current are not observed when Tl(+) is the predominant permeant ion. Also, the time course of the conductance during voltage clamp is different in the two cases, being much faster in Tl(+) than in K(+) solution for comparable values of deltaV. Most of the above features are accounted for by a model in which it is assumed that the ionic channels have external binding sites for cations and that their permeability properties depend on the species of the cation bound (K(+)or Tl(+) in the present experiments).

Animals↗
PubMed1976

Influence of molecular variations of ionophore and lipid on the selective ion permeability of membranes: II. A theoretical model.

The steady-state electrical properties induced by neutral carriers of ions in lipid bilayer membranes and the time dependence of the membrane current for low applied voltages are described theoretically in terms of a model which allows for a voltage dependence of the interfacial reactions, as well as for a trapezoid shape of the internal free energy barrier for translocation of the complex. The basic features of the model are closely related to those of others presented previously (J.E. Hall, C.A. Mead & G. Szabo, 1973, J Membrane Biol. 11:75; S.B. Hladky, 1974, Biochim, Biophys. Acta 352:71, S.B. Hladky, 1975, Biochim, Biophys. Acta 375: 327; Eisenman, Krasne & Ciani, 1975, Ann. N.Y. Acad, Sci. 264:34), but the analysis of its consequences on the steady-state and nonsteady-state electrical characteristics is given here in greater detail and is extended to provide the expression for the zero-current potential in ionic gradients. It is shown that parameters, such as the width of the trapezoidal barrier, the plane of the reaction and the ratio constant of translocation across the membrane interior to the rate of constant of dissociation of the complex, can be deduced from steady-state analysis, whereas the individual values of these constants and the distance between the equilibrium positions of the complexes are deducible from relaxation measurements.

Biological Transport, Active↗
PubMed1975

Effects of unstirred layers on the steady-state zero-current conductance of bilayer membranes mediated by neutral carriers of ions.

Some effects of diffusion polarization and chemical reactions on the steady-state zero-current conductance of lipid bilayers mediated by neutral carriers of ions have been studied theoretically and experimentally. Assuming that ion permeation across the interfaces occurs via a heterogeneous reaction between ions in the solution and carriers in the membrane, the relationship between the conductance and the aqueous concentration of carriers is shown to be linear only in a limited range of sufficiently low concentrations. At higher carrier concentrations, which for the most strongly bound cations are within the range of the experimentally accessible values, the conductance is expected to become limited by diffusion of the carried ion in the unstirred layers and therefore reach an upper limiting value independent of the membrane properties. This expectation has been successfully verified for glyceryl-monooleate membranes in the presence of the ions K+, Rb+ and NH+4 and carriers such as valinomycin and trinactin. The experimental results support, at least for the present system, the generally accepted view that complexation between ions and the macrocyclic antibiotics occurs at the membrane surface; it is shown, in fact, that for a different mechanism, such as that by which the complexes would form in the aqueous solutions and cross the interfaces as lipid-soluble ions, the same type of saturation would be expected to be observable only for unrealistically high values of the rate constants of the ion-carrier association. A previously proposed criterion to distinguish between these two mechanisms, based on the dependence of the conductance on the ion concentration, is discussed from the viewpoint of this more comprehensive model.

Ammonia↗