Properties of the apamin-sensitive component of Ca2+-dependent K+ current in smooth muscle cells of the guinea pig Taenia coli

With the help of a standard voltage-clamp technique, we investigated transmembrane ion currents in isolated smooth muscle cells of the guinea pig taenia coli. In Ca2+-dependent K+ current, we identified and studied the properties of an apamin-sensitive voltage-independent component carried through the channels of low conductance tin many publications called small conductance, I-SK(Ca)). This component did not show the temporal inactivation; I-SK(Ca) was insensitive to the action of 4 mM tetraethylammonium but was completely blocked by 500 nM of apamin. It was shown that I-SK(Ca) is very sensitive to changes in the intracellular Ca2+ concentration ([Ca2+](i)): a decrease in [Ca2+](i) up to 50 nM resulted in the almost complete blockade of the current. The entry of Ca ions into a cell from the external solution through the voltage-operated Ca2+ channels of L-type was not an obligatory condition for activation of I-SK(Ca). The current-voltage relationship for I-SK(Ca) had a maximum within the voltage range of +20 to +50 mV.