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W R Giles

Publications and source records attributed to W R Giles.

94 records · Page 6Linked to original sources
PubMed1985

A transient outward current in isolated cells from the crista terminalis of rabbit heart.

Voltage-clamp experiments were carried out with the objective of identifying and characterizing the time- and voltage-dependent properties of a transient outward current recorded in single myocytes from the crista terminalis region of the rabbit heart. A collagenase enzymic dispersion procedure similar to that described by Desilets & Horackova (1982) was used to obtain these viable individual myocytes. Transmembrane ionic currents were recorded using a single micro-electrode voltage-clamp technique. In experiments aimed at studying a tetrodotoxin-resistant transient inward current, (ICa); a transient outward current was consistently recorded following blockade of ICa with Cd2+ (5 X 10(-4) M). The time and voltage dependence of the activation and inactivation of this current were measured. Its steady-state inactivation curve spans the voltage range -70 to -10 mV, and it is activated between -20 and +10 mV. The reversal potential of this transient outward current is approximately -75 mV in [K+]O 5 mM, suggesting that it is carried mainly by K+. This transient outward current can be inhibited completely by external application of 4-aminopyridine (4-AP, 3 mM). The time- and voltage-dependent properties, the reversal potential, and the sensitivity to 4-AP of this transient outward current are all very similar to those of a transient outward current first identified in molluscan neurones. Hence, we have labelled it, IA. Selective inhibition of IA and knowledge of its voltage- and time-dependent properties yield specific predictions concerning its role in the action potential of isolated crista terminalis cells. Consistent with these predictions, a decrease in stimulus rate is found to decrease the duration of the action potential and vice versa; and application of effective doses of 4-AP results in a substantial lengthening of the action potential. These results are discussed in terms of the possible physiological role of IA in subsidiary or follower pace-maker tissue, and the anatomical and physiological heterogeneity of the sino-atrial node region of the rabbit heart.

4-Aminopyridine↗
PubMed1985

Voltage clamp of bull-frog cardiac pace-maker cells: a quantitative analysis of potassium currents.

Spontaneously active single cells have been obtained from the sinus venosus region of the bull-frog, Rana catesbeiana, using an enzymic dispersion procedure involving serial applications of trypsin, collagenase and elastase in nominally 0 Ca2+ Ringer solution. These cells have normal action potentials and fire spontaneously at a rate very similar to the intact sinus venosus. A single suction micro-electrode technique (Hamill, Marty, Neher, Sakmann & Sigworth, 1981; Hume & Giles, 1983) has been used to record the spontaneous diastolic depolarizations or pace-maker activity as well as the regenerative action potentials in these cells. This electrophysiological activity is completely insensitive to tetrodotoxin (TTX; 3 X 10(-6) M) and is very similar to that recorded from an in vitro sinus venosus preparation. The present experiments were aimed at identifying the transmembrane potassium currents, and analysing their role(s) in the development of the pace-maker potential and the repolarization of the action potential. Depolarizing voltage-clamp steps from the normal maximum diastolic potential (-75 mV) elicit a time- and voltage-dependent activation of an outward current. The reversal potential of this current in normal Ringer solution [( K+]0 2.5 mM) is near -95 mV; and it shifts by 51 mV per tenfold increase in [K+]0, which strongly suggests that this current is carried by K+. We therefore labelled it IK. The reversal potential of IK did not shift in the positive direction following very long (20 s) depolarizing clamp steps to +20 mV, indicating that 'extracellular' accumulation of [K+]0 does not produce any significant artifacts. The fully activated instantaneous current-voltage (I-V) relationship for IK is approximately linear over the range of potentials -130 to -30 mV. Thus, the ion transfer mechanism of IK may be described as a simple ohmic conductance in this range of potentials. Positive relative to -30 mV, however, the I-V exhibits significant inward rectification. A Hodgkin-Huxley analysis of the kinetics of IK, including a demonstration that the envelope of tails quantitatively matches the time course of the onset of IK during a prolonged depolarizing clamp step has been completed. The steady-state activation variable (n infinity) of IK spans the voltage range approximately -40 to +10 mV. It is well-fitted by a Boltzmann distribution function with half-activation at -20 mV. The time course of decay of IK is a single exponential. However, the activation or onset of IK shows clear sigmoidicity in the range of potentials from the activation threshold (-40 mV) to 0 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMedDate not supplied

Mathematical model of an adult human atrial cell: the role of K+ currents in repolarization.

We have developed a mathematical model of the human atria myocyte based on averaged voltage-clamp data recorded from isolated single myocytes. Our model consists of a Hodgkin-Huxley-type equivalent circuit for the sarcolemma, coupled with a fluid compartment model, which accounts for changes in ionic concentrations in the cytoplasm as well as in the sarcoplasmic reticulum. This formulation can reconstruct action potential data that are representative of recordings from a majority of human atrial cells in our laboratory and therefore provides a biophysically based account of the underlying ionic currents. This work is based in part on a previous model of the rabbit atrial myocyte published by our group and was motivated by differences in some of the repolarizing currents between human and rabbit atrium. We have therefore given particular attention to the sustained outward K+ current (I[sus]), which putatively has a prominent role in determining the duration of the human atrial action potential. Our results demonstrate that the action potential shape during the peak and plateau phases is determined primarily by transient outward K+ current, I(sus) and L-type Ca2+ current (I[Ca,L]) and that the role of I(sus) in the human atrial action potential can be modulated by the baseline sizes of I(Ca,L), I(sus) and the rapid delayed rectifier K+ current. As a result, our simulations suggest that the functional role of I(sus) can depend on the physiological/disease state of the cell.

Action Potentials↗