CHARACTERIZATION OF A WHOLE-CELL CA2-BLOCKABLE MONOVALENT CATION CURRENT IN ISOLATED ECTODERMAL CELLS OF CHICK-EMBRYO()

The presence of a Ca2+-blockable monovalent cation current is demonstr ated in isolated ectodermal cells of the chick embryo using the whole- cell patch-clamp method. In the absence of any stimulation, the whole- cell current is time independent and rectifies outwardly at membrane p otentials higher than +40 mV. The outward current is neither carried b y Cl- channels nor by K+ channels. Application of a Ca2+-free solution containing 1 mmol/l ethylenediaminetetraacetic acid (EDTA) elicits a large inward current and increases the outward current. The inward cur rent can be carried by extracellular Li+, Na+, K+ and Cs+, but not N-m ethyl-D-glucamine. The Ca2+-blockable monovalent cation channel discri minates very poorly among these cations. The estimated number of chann els per cell is around 2000. Extracellular protons block the inward Na + current in the absence of extracellular Ca2+. The apparent negative logarithm of the dissociation constant for proton (pK(H)) at - 100 mV is 5.8. Among 12 potential channel modulators, including verapamil and nifedipine, only quinine decreases the current. Quinine blocks this c urrent with a dissociation constant, K-d, equal to 0.18 mmol/l, indepe ndent of the membrane potential. This study demonstrates the presence of a whole-cell Ca2+-blockable monovalent cation current in dissociate d chick ectodermal cells with permeation properties similar to those o bserved at the single-channel level. Contrary to studies made of other tissues, we did not observe any blocking effect of verapamil and nife dipine on the Ca2+-blockable monovalent cation current.