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Calcium-activated chloride current in rabbit ventricular myocytes
1Department of Physiology and Biophysics, University of Vermont, Burlington 05405.
Abstract:
We have used the whole-cell patch-clamp technique to examine the ionic basis for a transient outward current in rabbit ventricular myocytes. High concentrations of intracellular calcium buffer prevented the current, isoproterenol increased it, and cadmium, nisoldipine, ryanodine, or caffeine blocked it. These data are consistent with a current that is calcium activated, by the calcium transient that causes contraction. The current was not blocked by external 4-aminopyridine or tetraethylammonium, and it was still present if external potassium was omitted and internal potassium was replaced by cesium. The current was absent when intracellular and extracellular chloride concentrations were drastically reduced, even when intracellular and extracellular potassium concentrations were normal. The current was blocked by the anion transport blockers 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) and responded to extracellular chloride changes as expected for a chloride current. We used SITS and DIDS to define the voltage dependence of the transient outward current. The current first appeared at voltages positive to the threshold of the calcium current and declined as voltage approached the calcium reversal potential. Tail-current experiments suggested that the current rectified strongly in the outward direction. We propose that the 4-aminopyridine-resistant transient outward current of rabbit ventricular myocytes is a calcium-activated chloride current.
Insights
Rabbit ventricular myocytes exhibit a transient outward current that is activated by calcium. This current is identified as a calcium-activated chloride current, distinct from potassium currents.
Area of Science:
- Cardiovascular Physiology
- Ion Channel Function
- Cellular Electrophysiology
Background:
- Rabbit ventricular myocytes possess a transient outward current crucial for cardiac function.
- The precise ionic basis of this current has remained incompletely understood, impacting our knowledge of cardiac electrophysiology.
Purpose of the Study:
- To elucidate the ionic mechanism underlying the transient outward current in rabbit ventricular myocytes.
- To characterize the properties and activators/inhibitors of this specific ion current.
Main Methods:
- Whole-cell patch-clamp electrophysiology was employed to record ionic currents.
- Pharmacological agents (cadmium, nisoldipine, ryanodine, caffeine, SITS, DIDS) and ionic manipulations (calcium buffering, potassium and chloride concentration changes) were used to probe current characteristics.
Main Results:
- The current was calcium-dependent, increased by isoproterenol, and blocked by calcium-related agents.
- It was resistant to 4-aminopyridine and tetraethylammonium, and persisted when potassium was replaced by cesium.
- The current was abolished by reducing chloride concentrations and blocked by anion transport inhibitors SITS and DIDS, confirming its chloride nature.
- Voltage-dependence studies indicated outward rectification, with the current appearing positive to calcium current threshold.
Conclusions:
- The 4-aminopyridine-resistant transient outward current in rabbit ventricular myocytes is a calcium-activated chloride current (ICaCl).
- This finding contributes to a deeper understanding of ion transport and electrophysiological regulation in cardiac cells.