A23187—Channel behaviour: Fluorescence study

Jyothi, G.; Surolia, A.; Easwaran, K. R. K.

doi:10.1007/bf02716816

Cited by 10 publications

(11 citation statements)

References 18 publications

(13 reference statements)

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“…The above analysis is predicated on the assumption that electrogenic Ca2+ transport would occur through a carriermediated mechanism. Recent work by Easwaran and coworkers has raised the possibility that A23187 can form ion-conducting channels, particularly during longer timeframe experiments (Balasubramanian et al, 1992;Jyothi et al, 1994). Although not a carrier-mediated mechanism, the movement of Ca2+ through channels would also be electrogenic.…”

Section: Do Ca2+ Ionophores Transport Electrogenically?mentioning

confidence: 99%

Effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin suggest problems with common applications of these compounds in biological systems

Erdahl

Chapman

Taylor

et al. 1995

Biophysical Journal

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Phospholipid vesicles loaded with Quin-2 and 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF) have been used to investigate the effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin. At an external pH of 7.0, a delta pH (inside basic) of 0.4-0.6 U decreases the rate of Ca2+ transport into the vesicles by severalfold under some conditions. The apparent extent of transport is also decreased. In contrast, raising the pH by 0.4-0.6 U in the absence of a delta pH increases both of these parameters, although by smaller factors. The relatively large effects of a delta pH on the transport properties of Ca2+ ionophores seem to reflect a partial equilibration of the transmembrane ionophore distribution with the H+ concentration gradient across the vesicle membrane. This unequal distribution of ionophore can cause a very slow or incomplete ionophore-dependent equilibration of delta pCa with delta pH. A second factor of less certain origin retards full equilibration of delta pCa when delta pH = 0. These findings call into question several ionophore-based methods that are used to investigate the regulatory activities of Ca2+ and other divalent cations in biological systems. Notable among these are the null-point titration method for determining the concentration of free cations within cells and the use of ionophores plus external cation buffers to calibrate intracellular cation indicators. The present findings also indicate that the transport mode of Ca2+ ionophores is more strictly electroneutral than was thought, based upon previous studies.

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Section: Do Ca2+ Ionophores Transport Electrogenically?mentioning

confidence: 99%

Effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin suggest problems with common applications of these compounds in biological systems

Erdahl

Chapman

Taylor

et al. 1995

Biophysical Journal

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“…3 of Jyoti et al (1994) shows quite the opposite behavior. Thus, contrary to the claim made by Jyoti et al (1994), even their data do not support the channel mechanism. 2.…”

Section: Comparison With Mechanisms Of Cal-mediated Monovalent Metal mentioning

confidence: 91%

“…In the second kinetic study, the ⌬pH was created by mixing vesicle solutions at pH ϳ 7.5 with a buffer of slightly different pH in a stopped-flow instrument (Jyoti et al, 1994). In this work, the rate-limiting step of ⌬pH decay was not identified and the observed nonlinear dependence of ⌬pH decay rate on [Cal] 0 was used to argue that the ion transport is through channels formed by CAL aggregates in the membrane.…”

Section: Comparison With Mechanisms Of Cal-mediated Monovalent Metal mentioning

confidence: 99%

“…In the decay traces shown in Fig. 1 a of Jyoti et al (1994) the "base lines" [the value of a 0 exp(Ϫk app t) at infinitely long time t] are uncertain since the data have not been recorded at longer intervals of time. The drift in the amplifiers used for recording the data could also cause base line errors.…”

Section: Comparison With Mechanisms Of Cal-mediated Monovalent Metal mentioning

confidence: 99%

“…In the present work we have tested such hypotheses by kinetic measurements using M ϩ ϭ Li ϩ , Na ϩ , K ϩ , and Cs ϩ . A recent controversy about the existence of CAL oligomers and the channel mechanism in CAL facilitated transmembrane H ϩ /M ϩ transport (Balasubramanian et al, 1992;Jyoti et al, 1994;Prabhananda and Kombrabail, 1994;Thomas et al, 1997) has also been examined. Our experimental strategy is based on the following.…”

Section: Introductionmentioning

confidence: 99%

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Evidence for Dimer Participation and Evidence Against Channel Mechanism in A23187-Mediated Monovalent Metal Ion Transport Across Phospholipid Vesicular Membrane

Prabhananda

Kombrabail

1998

Biophysical Journal

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The decay of the pH difference (DeltapH) across soybean phospholipid vesicular membrane by ionophore A23187 (CAL)-mediated H+/M+ exchange (M+ = Li+, Na+, K+, and Cs+) has been studied in the pH range 6-7.6. The DeltapH in these experiments were created by temperature jump. The observed dependence of DeltapH relaxation rate 1/tau on the concentration of CAL, pH, and the choice of M+ in vesicle solutions lead to the following conclusions. 1) The concentrations of dimers and other oligomers of A23187 in the membrane are small compared to the total concentration of A23187 in the membrane, similar to that in chloroform solutions reported in the literature. 2) In the H+ transport cycle leading to DeltapH decay, the A23187-mediated H+ translocation across the membrane is a fast step, and the rate-limiting step is the A23187-mediated M+ translocation. 3) Even though the monomeric Cal-H is the dominant species translocating H+, Cal-M is not the dominant species translocating M+ (even at concentrations higher than [Cal-H]), presumably because its dissociation rate is much higher than its translocation rate. 4) The pH dependence of 1/tau shows that the dimeric species Cal2LiLi, Cal2NaNa, Cal2KH, and Cal2CsH are the dominant species translocating M+. The rate constant associated with their translocation has been estimated to be approximately 5 x 10(3) s-1. With this magnitude for the rate constants, the dimer dissociation constants of these species in the membrane have been estimated to be approximately 4, 1, 0.05, and 0.04 M, respectively. 5) Contrary to the claims made in the literature, the data obtained in the DeltapH decay studies do not favor the channel mechanism for the ion transport in this system. 6) However, they support the hypothesis that the dissociation of the divalent metal ion-A23187 complex is the rate limiting step of A23187-mediated divalent metal ion transport.

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Evidence against Formation of A23187 Dimers and Oligomers in Solution: Photo-Induced Degradation of Ionophore A23187

Thomas

Wang

Pfeiffer

et al. 1997

Archives of Biochemistry and Biophysics

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A23187—Channel behaviour: Fluorescence study

Cited by 10 publications

References 18 publications

Effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin suggest problems with common applications of these compounds in biological systems

Effects of pH conditions on Ca2+ transport catalyzed by ionophores A23187, 4-BrA23187, and ionomycin suggest problems with common applications of these compounds in biological systems

Evidence for Dimer Participation and Evidence Against Channel Mechanism in A23187-Mediated Monovalent Metal Ion Transport Across Phospholipid Vesicular Membrane

Evidence against Formation of A23187 Dimers and Oligomers in Solution: Photo-Induced Degradation of Ionophore A23187

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