Biomedical subjects

T L Croxton

Publications and source records attributed to T L Croxton.

8 recordsLinked to original sources

PubMed1992

A possible relationship between KCl symport and basolateral K(+)-conductance in Necturus gallbladder epithelial cells.

1. Apical membrane potential (Va), transepithelial potential (VT), fractional apical voltage ratio (FVa = delta Va/delta VT), tissue resistance (RT), and intracellular Cl- (aiCl) and K+ (aiK) activities were measured in isolated gallbladders maintained between oxygenated bicarbonate-free physiological media (23 degrees C, pH 7.2 or 8.2) in a divided chamber. The basolateral membrane potential (Vb) was calculated from the measured values of Va and VT. 2. Cl- removal from the serosal medium (which should accelerate coupled basolateral KCl exit) significantly depolarized Vb, decreased aiCl, decreased FVa, increased RT, and attenuated the depolarization of Vb (delta Vb) induced by high K+ added to the serosal side. These changes are consistent with a decrease in the K(+)-conductance of the basolateral membrane (gbK). 3. Addition of furosemide (an inhibitor of KCl cotransport) to the serosal medium induced significant increases in Vb, FVa, and high K(+)-induced delta Vb, indicating an increase in gbK. 4. In the presence of serosal furosemide, Cl- removal from the serosal medium did not significantly alter Vb, aiCl or delta Vb from their corresponding values when serosal Cl- was present. 5. Serosal furosemide had no significant effect on aiK and aiCl measured with double-barreled ion-selective microelectrodes. 6. These results suggest the possibility of a reciprocal relationship between gbK and the rate of basolateral KCl cotransport. This may contribute to the maintenance of aiK in gallbladder epithelial cells.

Animals↗

PubMed1992

Calibration of ion-selective microelectrodes.

Measurements of intracellular and extracellular ion activities with ion-selective microelectrodes generally involve calibration of the electrodes in solutions of known composition and fitting of the calibration data with a theoretical expression. The Nicolsky equation is frequently used to describe the variation of electrode potential with primary ion activity in the presence of a constant amount of interference. In this report, we review the estimation of primary ion activities in calibration solutions and discuss the practical use of the Nicolsky equation. We describe a specialized computer program, developed in this laboratory, for routine input and editing of calibration data, fitting of data with the Nicolsky equation, and calculation of experimental ion activities from the fitted curve. Earlier versions of this program have proven helpful in several investigations in this and in other laboratories.

Calibration↗

PubMed1989

Liquid junction potentials calculated from numerical solutions of the Nernst-Planck and Poisson equations.

We present numerical solutions for the one-dimensional Nernst-Planck and Poisson system of equations for steady-state electrodiffusion. Commonly used approximate solutions to these equations invoke assumptions of local electroneutrality (Planck approximation) or constant electric field (Goldman approximation). Calculations were performed to test the ranges over which these approximate theories are valid. For a dilutional junction of a 1:1 electrolyte, separated from adjoining perfectly stirred solutions by sharp boundaries, the Planck approximation is valid for values of kappa dL greater than 10, where 1/kappa d is the Debye length of the more dilute solution. The Goldman approximation is valid for kappa cL less than 0.1 where 1/kappa c is the Debye length of the more concentrated solution. These results suggest that the modeling of electrodiffusive flows in and near membrane ion channels may require numerical solutions of this set of equations rather than the use of either limiting case.

Cell Membrane↗

PubMed1989

Patch clamp recording from anterior pituitary cells identified by reverse hemolytic plaque assay.

The study of hormone secretion by anterior pituitary cells is complicated by the presence of multiple cell types. For unambiguous interpretation of data it is necessary to identify the cells from which measurements are made. We have described a reliable experimental approach involving the identification of cultured cells of a particular type with a reverse hemolytic plaque assay. The electrical characteristics of individual identified cells can then be studied using patch clamp recording. This electrophysiological approach is well suited to the study of complex systems in cultured cells. Although this combined approach requires some expertise in a variety of techniques, it is workable and should yield valuable information regarding the role of ion channels in the cellular control of hormone secretion by the anterior pituitary.

Animals↗

PubMed1988

A model of the gating of ion channels.

The gating of ion channels in biological membranes has usually been described in terms of Markov transitions between a few discrete open or closed states. Such models predict that the distributions of open and closed durations decay as a sum of exponential terms. Recent experimental data have indicated that certain channels are not easily described by these models. We show that distributions of open and closed times similar to those seen experimentally are predicted by a model that involves only one open and closed state but that assumes the activation energy of the gating process to be stochastic. This model involves only a few parameters and these have direct physical interpretations. Measurements of the correlation between the durations of successive open or closed events is shown to provide an experimental method for distinguishing between this and other models.

Ion Channels↗

PubMed1988

Gonadotropin-releasing hormone induces oscillatory membrane currents in rat gonadotropes.

Electrophysiological studies were performed to characterize membrane currents of rat gonadotropes under basal conditions and after exposure to secretagogues. Gonadotropes were identified in primary cultures of rat anterior pituitaries by a reverse hemolytic plaque assay. Giga-seal patch clamp recording with the cell-attached configuration was used to monitor membrane currents in these cells. Spontaneous spikes in basal current were seen. These were blocked by methoxyverapamil and probably reflect Ca2+-dependent action potentials. Brief GnRH stimulation induced slow oscillatory changes in membrane current that evolved into a series of large amplitude inward pulses after about 8 min. Treatment with TRH had no effect, and depolarization with K+ led to delayed inward currents without any oscillatory behavior. Under conditions of Ca2+ channel blockade, GnRH stimulation did not induce pulses of inward current, but did lead to oscillatory activation of a small conductance ion channel apparently selective for K+. Taken together these results suggest that GnRH induces oscillations in intracellular Ca2+ and that these oscillations are controlled by biochemical processes.

Animals↗

PubMed1987

Measurement of intracellular chloride activity in mouse liver slices with microelectrodes.

Steady-state membrane potential (Vm) and intracellular Cl- activity (aCli) were measured with double-barreled Cl(-)-selective microelectrodes in mouse liver slices. In bathing solutions (33.8 degrees C) containing pyruvate, glutamate, fumarate, and glucose, Vm and aCli were -27.6 +/- 1.0 mV and 32.6 +/- 1.5 mM, respectively. This apparent value of aCli exceeded the level required for passive distribution of this ion (aCleq = 26.4 +/- 1.3 mM) by 6.2 +/- 1.0 mM. This difference was essentially unchanged in experiments where (i) Na+ was replaced by choline, (ii) HCO3- was removed, and (iii) Cl- was replaced by gluconate. These data argue against the presence of Na+- or HCO3(-)-coupled Cl- transport mechanisms in the plasma membrane of mouse liver cells. This implies that aCli is in fact at equilibrium and interference with the response of Cl(-)-selective microelectrodes by intracellular anions is responsible for the apparent difference between aCli and aCleq. We found that Cl(-)-selective microelectrodes containing Corning 477315 ligand are sensitive to taurocholate, a representative bile salt. Their selectivity to taurocholate is about 60-times their selectivity towards Cl-. This suggests that interference of bile acids at concentrations normally present in hepatocytes with determinations of aCli can account for the apparent difference aCli-aCleq.

Animals↗

PubMed1987

A microcomputer interface for a digital audio processor-based data recording system.

An inexpensive interface is described that performs direct transfer of digitized data from the digital audio processor and video cassette recorder based data acquisition system designed by Bezanilla (1985, Biophys. J., 47:437-441) to an IBM PC/XT microcomputer. The FORTRAN callable software that drives this interface is capable of controlling the video cassette recorder and starting data collection immediately after recognition of a segment of previously collected data. This permits piecewise analysis of long intervals of data that would otherwise exceed the memory capability of the microcomputer.

Biophysical Phenomena↗