Inward membrane currents in mammalian myocardium
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Voltage clamp experiments (intracellular electrode for voltage control, current applied through a sucrose gap) were carried out in cat myocardium with an improved clamp circuit. The improvements were: a) reduction of series resistance by clamping the extracellular bath to earth potential and b) monitoring the longitudinal core resistancer ig of the fibres and the shunt resistancer eg in the gap.
Ratios ofr ig /r eg <1 (usually due to increase ofr ig because of decoupling of the cells in the gap) cause artifactual current wave forms of the configuration of a more or less suppressed “action potential”. The artifactual component of the recorded current flows through the shunt resistance.
In most preparations, shorter than 1 mm, cable properties do not permit space clamp (tested with a second intracellular electrode). Both the rapid sodium current and a slow inward current differing in threshold, amplitude and time course “escape” and produce voltage gradients making measurement of membrane current incorrect. In these preparations the configuration of the current trace depends on the position of the intracellular electrode in the voltage control circuit.
In preparations of suitable geometry, smallr ig /r eg ratio and small series resistance space clamp is achieved. Under this condition the two inward currents are clearly separated from each other.
The threshold of the slow inward current is at −35 mV. The maximum amplitude of the slow inward current amounts to 1–5 μA (preparation length and diameter about 0.5 mm). A plot of h∞ vs. membrane potential shows inactivation of the slow inward current between −50 mV and −20 mV.
Following a depolarization step the slow inward current is turned on with a delay of a few milliseconds. Its activation time amounts to about 20 ms at threshold and 6–8 ms at zero potential. Its inactivation time constant amounts to 60–70 ms between −25 mV and +10 mV, being much longer outside this range.
Key wordsVoltage Clamp Rapid and Slow Inward Current Mammalian Myocardium
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