CATION CHANNEL BLOCKED BY EXTRACELLULAR CA2+ IN THE APICAL MEMBRANE OF THE CHICK EMBRYONIC ECTODERM

In the chick embryo (20 h incubation, gastrula stage), the apical memb rane of the ectodermal cells shows a high density of a non-selective c ation channel which is blocked by very low extracellular Ca2+ concentr ations. Properties of this channel were studied at the single-channel level using the patch-clamp technique in the cell-attached mode. With 1 mmol/l Ca2+ in the pipette, only outward current was present and the channel conductance measured at +120 mV was 25.5 pS. In the absence o f Ca2+, also inward current through the channel was observed. The cond uctances measured at -50 mV were 49.5 pS with Na+ as the charge carrie r, 72.5 pS with K+, 49.1 pS with Cs+, and 18.5 pS with Li+. The conduc tance measured at +80 mV was around 23 pS in all four cases. The rever sal potential was similar (around 25 mV) for all four ions, which indi cates a poor selectivity of the channel. In the absence of Ca2+ and th e presence of 1 mmol/l ethylenebis(oxonitrilo)tetraacetate (EGTA), the kinetics of the channel were characterized by bursts of the order of seconds. During a burst, the channel flickered between one open and on e closed level. The open time was constant between -30 mV and -80 mV, while the closed time decreased with hyperpolarization. The open time varied according to the permeant ion (K+<Na+ = Cs+<Li+). Extracellular Ca2+ blocked the inward current in a voltage-dependent manner. The K- d values, 1 mu mol/l at - 30 mV and 3.2 mu mol/l at -80 mV, indicate t hat Ca2+ ions exit the channel toward the intracellular side. A weak v oltage dependency of the association rate constant suggests that the C a2+-binding site is close to the outside mouth. Extracellular Ca2+ was much less efficient at blocking the outward current (K-d about 1 mmol /l at 80 mV). Tetracaine, but not uraniumdioxide, decreased the openin g probability of the channel. The embryonic channel shows similarities with the Ca2+-blockable, poorly selective channel described in the ep ithelium of toad urinary bladder.