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 membrane of the ectodermal cells shows a high density of a non-selective cation channel which is blocked by very low extracellular Ca2+ concentrations. 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 of Ca2+, also inward current through the channel was observed. The conductances measured at -50 mV were 49.5 pS with Na+ as the charge carrier, 72.5 pS with K+, 49.1 pS with Cs+, and 18.5 pS with Li+. The conductance measured at +80 mV was around 23 pS in all four cases. The reversal potential was similar (around 25 mV) for all four ions, which indicates a poor selectivity of the channel. In the absence of Ca2+ and the 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 one 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 Kd values, 1 mumol/l at -30 mV and 3.2 mumol/l at -80 mV, indicate that Ca2+ ions exit the channel toward the intracellular side. A weak voltage dependency of the association rate constant suggests that the Ca(2+)-binding site is close to the outside mouth. Extracellular Ca2+ was much less efficient at blocking the outward current (Kd about 1 mmol/l at 80 mV). Tetracaine, but not uraniumdioxide, decreased the opening probability of the channel. The embryonic channel shows similarities with the Ca(2+)-blockable, poorly selective channel described in the epithelium of toad urinary bladder.
Li
J Q
JQ
Institut de Physiologie, Faculté de médecine, Université de Lausanne, Switzerland.
Prod'hom
B
B
Kucera
P
P
D000818:Animals; D002118:Calcium / Q000502:physiology*; D002414:Cations, Monovalent / Q000737:chemistry; D002462:Cell Membrane / Q000502:physiology; D002642:Chick Embryo; D004475:Ectoderm / Q000378:metabolism* / Q000502:physiology; D004533:Egtazic Acid / Q000494:pharmacology; D004848:Epithelium / Q000502:physiology; D016623:Ion Pumps / Q000378:metabolism* / Q000502:physiology; D007700:Kinetics; D018408:Patch-Clamp Techniques; D010539:Permeability; D017974:Uranium Compounds
D002414:Cations, Monovalent; D016623:Ion Pumps; D017974:Uranium Compounds; D004533:Egtazic Acid; D002118:Calcium