A newly identified Ca2+ dependent K+ channel in the smooth muscle membrane of single cells dispersed from the rabbit portal vein

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Abstract
We found a new type of Ca2+-dependent K+ channel in smooth muscle cell membranes of single cells of the rabbit portal vein. A slope conductance of the current was 180 pS when 142 mM K+ solution was exposed to both sides of the membrane (this channel was named the KM channel, in comparison to the known KL and KS channels from the same membrane patch; Inoue et al. 1985). This KM channel was less sensitive to the cytoplasmic Ca2+ concentration, [Ca2+]i, but was sensitive to the extracellular Ca2+, [Ca2+]o, e.g. in the outside-out membrane patch, lowering the [Ca2+]o in the bath markedly reduced the open probability of this channel, and also in cell-attached configuration, lowering of the [Ca2+]o using the internally perfused patch clamp electrode device reduced the opening of KM channel. TEA+ (1–10 mM) reduced the amplitude of the elementary current through the KM channel applied from each side of the membrane, but this agent inhibited the KM channel to a greater extent when applied to the inner than to the outer surface of the membrane. Furthermore, this KM channel had a weak voltage dependency, and the open probability of the channel remained much the same within a wide range of potential (from −60 mV to +60 mV). Whereas most Ca2+-dependent K+ channels are regulated mainly by [Ca2+]i and possess a voltage dependency, these properties of the KM channel differed from other Ca2+-dependent K+ channels. The elucidation of this KM channel should facilitate explanations of the actions of external Ca2+ or TEA+ on the membrane potential, in the smooth muscles of the rabbit portal vein.